OpenGL ES Programming Tips¶
This topic is for readers who have some experience programming OpenGL® ES and want to improve the performance of their OpenGL ES application. It aims at providing recommendations on getting the most out of the API and hardware resources without diving into too many architectural details.
Some of the recommendations in this topic are incompatible with each other. One must consider the trade-offs between CPU load, memory, bandwidth, shader processing power, precision, and image quality. Premature assumptions about the effectiveness of trade-offs should be avoided. The only definite answers come from benchmarking and looking at rendered images!
The items below are not ordered according to importance or potential performance increase. The identifiers in parentheses exist only for reference.
Inefficient management of GL state leads to increased CPU load that may limit the amount of useful work the CPU could be doing elsewhere. Reducing the number of times rendering is paused due to GL state change will increase the chance of realizing the potential throughput of the GPU. The main point in this section is: do not modify or query GL state unless absolutely necessarily.
Do not set any state redundantly (S1)¶
All relevant GL states should be initialized during application
initialization and not in the main render loop. For instance,
glViewport() finds its
way into the application render loop even though the values passed to
these functions are always constant. Other times they are set
unconditionally in the loop, just in case their values have changed per
frame. Only call these functions when the values actually do need to
change. Additionally, do not automatically set state back to some
predefined value (e.g., the GL defaults). That idiom might be useful
while developing your application as it makes it easier to re-order
pieces of rendering code, but it should not be done in production code
without a very good reason.
Avoid querying any GL state in the render loop (S2)¶
When a GL context is created, the state is initially well-defined. Every state variable has a default value that is described in the OpenGL ES specification (“State Tables”). Except when compiling shaders, determining available extensions, or the application needs to query implementation specific constants, there should be no need to query any GL state. These queries can almost always be done in initialization. Well-written applications check for GL errors in debug builds. If no errors are reported as a result of changing state, it is assumed that the changes are now part of the new GL state. For these two reasons, the current state is always known, and you should almost never need to query any GL state in a loop. If an application frequently calls functions that begin with glIs* or glGet*, these calls should be tracked down and eliminated.
Do not repeat per object state when binding (S4)¶
Recall that some state is bound to the object it affects. As that state is stored in the object, you do not need to repeat it when you rebind the object. A very common mistake is setting the texture parameters for filtering and wrapping every time a texture object is bound. Another common mistake is updating uniform variables that have not changed value since the last time the particular shader program was used. In particular, when batching opportunities are limited, repeating per object state generates enormously inefficient GL code that can easily have a measurable impact on frame rate.
Enable backface culling whenever possible (S5)¶
Always enable face culling whenever the back-faces are not visible. Rendering the back-faces of primitives is often not necessary.
The default GL state has backface culling disabled, so this is one state that should almost always be set during application initialization and be left enabled for the application lifetime.
The amount of geometry, as well as the way it is transferred to the GL, can have a very large impact on both CPU and GPU load. If the application sends geometry inefficiently or too frequently, that alone can create a bottleneck on the CPU side that does not give the GPU enough data to work efficiently. Geometry must be submitted in sizable chunks to realize the potential of the GPU. At the same time, geometry should be minimally defined, stored on the server side, and it should be accessed in a way to get the most out of the two GPU caches that exist before and after vertices are transformed.
Use indexed primitives (G1)¶
The vertex processing engine contains a cache where previously transformed vertices are stored. It is called Post-TnL vertex cache. Taking full advantage of this cache can lead to very large performance improvement when vertex processing is the bottleneck. To fully utilize it, it is necessary for the GPU to be able to recognize previously transformed vertices. This can only be accomplished by specifying indexed primitives for the geometry. However, for any non-trivial geometry the optimal order of indices will not be obvious to the programmer. If some geometry is complex, and the application bottleneck is vertex processing, then look into computing a vertex order that maximizes the hit ratio of the Post TnL cache. The topic has been thoroughly studied for years and even simple greedy algorithms can provide a substantial performance boost. Good results have been reported with the algorithm described in Linear-Speed Vertex Cache Optimisation, by Tom Forsyth, RAD Game Tools (28 September 2006).
There is a free implementation of the algorithm in a library called
The number of vertex attributes and the size of each attribute may determine the efficiency of this cache—it has storage for a fixed number of bytes or active attributes, not a fixed number of vertices. A lot of attribute data per vertex increases the risk of cache misses, resulting in potentially redundant transformations of the same vertices.
Reduce vertex attribute size and components (G2)¶
It is important to use an appropriate attribute size and minimize the
number of components to avoid wasting memory bandwidth and to increase
the efficiency of the cache that stores pre-transformed vertices. This
cache is called pre-TnL vertex cache. For instance, you rarely need to
specify attributes in 32‑bit
FLOATs. It might be possible to define the
object-space geometry using 3
BYTEs per vertex for a simple object, or three
SHORTs for a more complex or larger object. If the geometry requires
floating-point representation, half-floats (available in extension
OES_vertex_half_float.txt) may be sufficient. Per-vertex colors are
accurately stored with 3 x
BYTEs with a flag to normalize in
VertexAttributePointer. Texture coordinates can sometimes be represented
SHORTs with a flag to normalize (if not tiling).
The exception case that normalizing texture coordinates is not necessary if they are only used to sample a cube map texture.
Vertex normals can often be represented with three
SHORTs (in a few cases,
such as for cuboids, even as 3
BYTEs), and these should be normalized.
A normal can even be represented with two components if the direction
(sign) of the normal is implicit, given its length is known to be 1. The
remaining coordinate can be derived in a vertex shader (e.g.
if memory or bandwidth (rather than vertex processing) is a likely bottleneck.
An optimal OpenGL ES application will take advantage of any
characteristics specific to the geometry. For instance, a smooth sphere
uses the normalized vertex coordinates as normal—these are trivially
computed in a vertex shader. It is important to benchmark intermediate
results to ensure the vertex processing engine is not already saturated.
Finally remember, if some attribute for a primitive or a number of
primitives is constant for the same draw call, then disable the
particular vertex attribute index and set the constant value with
VertexAttrib*() instead of replicating the data.
Pack vertex attributes (G3)¶
Vertex attributes normally have different sets of attributes that are completely unrelated. Unlike uniform and varying variables in shader programs, vertex attributes do not get automatically packed, and the number of vertex attributes is a limited resource. Failure to pack these attributes together may lead to limitations sooner than expected. It is more efficient to pack the components into fewer attributes even though they may not be logically related. For instance, if each vertex comes with two sets of texture coordinates for multi-texturing, these can often be combined these into one attribute with four components instead of two attributes with two components. Unpacking and swizzling components is rarely a performance consideration.
Choose an appropriate vertex attribute layout (G4)¶
There are two commonly used ways of storing vertex attributes:
Arrays of structures
Structures of arrays
An array of structures stores the attributes for a given vertex sequentially with an appropriate offset for each attribute and a non-zero stride. The stride is computed from the number of attribute components and their sizes. An array of structures is the preferred way of storing vertex attributes due to more efficient memory access. If the vertex attributes are constant (not updated in the render loop) there is no question that an array of structures is the preferred layout.
In contrast, a structure of arrays stores the vertex attributes in separate buffers using the same offset for each attribute and a stride of zero. This layout forces the GPU to jump around and fetch from different memory locations as it assembles the needed attributes for each vertex. The structure of arrays layout is therefore less efficient than an array of structures in most cases. The only time to consider a structure of arrays layout is if one or more attributes must be updated dynamically. Strided writes in array of structures can be expensive relative to the number of bytes modified. In this scenario, the recommendation is to partition the attributes such that constant and dynamic attributes can be read and written sequentially, respectively. The attributes that remain constant should be stored in an array of structures. The attributes that are updated dynamically should be stored in smaller separate buffer objects (or perhaps just a single buffer if the attributes are updated with the same frequency).
Use consistent winding (G5)¶
The geometry winding (clockwise or counter-clockwise) should be
determined up front and defined in code. The geometry face that is
culled by GL can be changed with
glFrontFace(), but having to switch
back and forth between winding for different geometry batches during
rendering is not optimal for performance and can be avoided in most
Always use vertex and index buffer objects (G6)¶
Recall that vertices for geometry can either be sourced from application
memory every time it is rendered or from buffers in graphics memory
where it has been stored previously. The same applies to vertex array
indices. To achieve good performance, you should never continuously
source the data from application memory with
objects should always be used to store both geometry and indices. Check
that no code is calling
glDrawArrays(), and that no code is calling
glDrawElements() without a buffer bind.
The default buffer usage flag when allocating buffer objects is STATIC_DRAW. In many cases this will lead to fastest access.
STATIC_DRAW does not mean one can never write to the buffer (although any writing to a buffer should always be avoided as much as possible). A
STATIC_DRAW flag may in fact be the appropriate usage flags, even if the buffer contents are updated every few frames. Only after careful benchmarking and arriving at conclusive results should changing the usage flag to one of the alternatives (
STREAM_DRAW) be considered.
Batch geometry into fewer buffers and draw calls (G7)¶
There are only so many draw calls, or batches of geometry, that can be submitted to GL before the application becomes CPU bound. Each draw call has an overhead that is more or less fixed. Therefore, it is very important to increase the sizes of batches whenever possible. There does not need to be a one-to-one correspondence between a draw call and a buffer—a large vertex buffer can store geometry with a similar layout for multiple models. One or more index buffers can be used to select the subset of vertices needed from the vertex buffer. A common mistake is to have too many small buffers, leading to too many draw calls and thus high CPU load. If the number of draw calls issued in any given frame goes into many hundreds or thousands, then it is time to consider combining similar geometry in fewer buffers and use appropriate offsets when defining the attribute data and creating the index buffer.
Unconnected geometry can be stitched together with degenerate triangles
(alternatively, by using extension
available). Degenerate triangles are triangles where two or more
vertices are coincident leading to a null surface. These are trivially
rejected and ignored by the GPU. The benefit from stitching together
geometry with degenerate triangles, such that fewer and larger buffers
are needed, tends to outweigh the minor overhead of sending degenerates
triangles down the pipeline. If geometry batches are being broken up to
bind different textures, then look at combining several images into
fewer textures (T5).
Use the smallest possible data type for indices (G8)¶
When the geometry uses relatively few vertices, an index buffer should
specify vertices using only
UNSIGNED_BYTE instead of
an even larger integer type if the ES2 implementation supports it).
Count the number of unique vertices per buffer and choose the right data
type. When batching geometry for several unrelated models into fewer
buffer objects (G7), then a larger data type for the indices may be
required. This is not a concern compared to the larger performance
benefits of batching.
Avoid allocating new buffers in the rendering loop (G9)¶
If the application frequently updates the geometry, then allocate a set
of sufficiently large buffers when the application initializes. A
glBufferData() call with a
NULL data pointer will reserve the amount of
memory you specify. This eliminates the time spent waiting for an
allocation to complete in the rendering loop. Reusing preallocated
buffers also helps to reduce memory fragmentation.
Writing to a buffer object that is being used by the GPU can introduce bubbles in the pipeline where no useful work is being done. To avoid reducing throughput when updating buffers, consider cycling between multiple buffers to minimize the possibility of updating the buffer from which content is currently being rendered.
Cull early and often (G10)¶
The GPU will not rasterize primitives when all of its vertices fall outside the viewport. It also avoids processing hidden fragments when the depth test or stencil test fails (P4). However, this does not mean that the GPU should do all the work in deciding what is visible. In the case of vertices, they need to be loaded, assembled and processed in the vertex shader, before the GPU can decide whether to cull or clip parts of the geometry. Testing a single, simple bounding volume that encloses the geometry against the current view frustum on the CPU side is a lot faster than testing hundreds of thousands of vertices on the GPU. If an application is limited by vertex processing, this is definitely the place to begin optimizing. Spheres are the most efficient volumes to test against and the volume of choice if geometry is rotational symmetrical. For some geometry, spheres tend to lead to overly conservative visibility acceptance. A rectangular cuboid (box) is only a slightly more expensive test but can be made to fit more tightly on most geometry. Hierarchical culling can often be employed to reduce the number of tests necessary on the CPU. Efficient and advanced culling algorithms have been heavily researched and published for many years. You can find a good introduction in Visibility in Computer Graphics, by Jiří Bittner and Peter Wonka.
Writing efficient shaders is critical to achieving good performance. One should treat shaders like pieces of code that run in the inner-most loops on a CPU. There is a very high cost to littering these with conditionals or recomputing loop invariants. Before optimizing an expensive vertex shader, make sure geometry that is entirely outside the view frustum is being culled on the CPU. Before optimizing an expensive fragment shader, make sure the application is not generating an excess number of fragments with it.
When optimizing shaders, any source code that does not contribute to its output variables is optimized out by the compiler. This feature can be exploited to gain knowledge about whether the shader is part of the current bottleneck by multiplying the output variable with a null vector to reduce the workload and then measure if frame rate improves. Conversely, at the final stages of optimization one can quickly measure if there is headroom for increasing workload to offload computations to shader unit or to improve image quality by adding meaningless but expensive ALU instructions, or texture sampling, to the output variables.
Move computations up the pipeline (P1)¶
As the rendering pipeline is traversed from the CPU to the vertex processor and then to the fragment processor, the required workload tends to increase a few orders of magnitude each time. Computations constant per model, per primitive or per vertex do not belong in the fragment processor and should be moved up to the vertex processor or earlier. Per draw call computations do not belong in the vertex processor and should be moved to the CPU. For instance, if lighting is done in eye-space, the light vector should be transformed into eye-space and stored in a uniform rather than repeating this for each vertex or, even worse, per fragment. The light vector should naturally be stored pre-normalized. Usually the light vector computations are constant for the draw call, so they do not belong in any shader.
Do not write large or generalized shaders (P2)¶
It is critical to resist the temptation to write shader programs that
take different code paths depending on whether one or more constant
variable have a particular value. Uniforms are intended as constants for
one (or hopefully many) primitives—they are not substitutes for calling
glUseProgram(). Shaders should be minimal and specialized to the task
they perform. It is much better to have many small shaders that run fast
than a few large shaders that all run slow. Code re-use (when source
shaders are supported) should be handled at the application level using
glShaderSource(). If the advice here of not writing generalized shaders
goes against the conflicting goal of minimizing shader and state
changes, smaller and more specialized shaders are generally preferred.
Additionally, be careful with writing shader functions intended for
concatenation into the final shader source code—shared functions tend
to be overly generic and make it harder to exploit possible shortcuts.
Take advantage of application specific knowledge (P3)¶
Application specific knowledge can be used to simplify or avoid
computations. Math shortcuts should be pursued everywhere because there
are optimizations that the shader compiler or the GPU cannot make. For
instance, rendering screen-aligned primitives is common for 2D user
interface and post-processing effects. In this case, the entire
modelview transformation is avoided by defining the vertices in NDC
(normalized device coordinates). A full-screen quad has its vertex
coordinates in the [-1.0,1.0] range so these can be passed directly from
the vertex attribute to
gl_Position. The types of matrix transformations
applied in the application when creating the modelview matrix should be
tracked and exploited when possible. For instance, an orthonormal matrix
(e.g. no non-uniform scaling) leads to an opportunity to avoid
computations when transforming normals with the inverse-transpose
Optimize for depth and stencil culling (P4)¶
The GPU can quickly reject fragments based on depth or stencil testing before the fragment shader is executed. The depth complexity of a scene is the number of times each fragment gets written. Depth complexity can be measured by incrementing values in a stencil buffer. A high depth complexity in a 3D scene can be a result of rendering opaque objects in a non-optimal order. The worst case is rendering back-to-front (aka painter’s algorithm) because it leads to a large number of fragments being overdrawn. An application with high depth complexity should ensure that opaque objects are rendered sorted front-to-back order with depth testing enabled. Straightforward rendering of 2D user interfaces also leads to a high depth complexity that can often be decreased with the same technique but also by using the stencil buffer to mask fragments. Applications that are heavily fragment limited can be sped up significantly with clever use of these techniques—sometimes up to a factor of 10 or more.
If vertex processing is not a bottleneck, it is worthwhile to run
experiments that prime the depth buffer in a first pass. Disable all
color writes with
glColorMask() on the first pass. The fragments in the
depth buffer can then serve as occluders in a second pass when color
writes are enabled, and the expensive fragment shaders are executed.
Disable depth writes with
glDepthMask() in the second pass since there
is no point in writing it twice.
Do not discard fragments, or modify depth, unless absolutely necessary (P5)¶
Some operations prevent the hardware from enabling its automatic
optimization that rejects fragments early in the pipeline (early-Z). In
particular, the discard operation that discards fragments based on some
criteria will disable early-Z on some platforms. It is critical to limit
the use of discarding as much as possible (e.g., alpha testing)—unless
depth writing can be disabled. Another example is found in the
GL_NV_fragdepth extension available on some platforms, where the depth
value can be written from the fragment shader. This operation also
forces the GPU to opt out of Early-Z reject in order to ensure correct
Avoid conditionals in shaders when possible (P6)¶
Fragments are processed in chunks and both branches of a conditional may need to be evaluated before the result of the false branch can be discarded by the GPU. Be careful with assuming that conditionals skip computations and reduce the workload. This warning is particularly relevant to fragment shaders. Benchmarking shaders can determine if conditionals in the vertex or fragment shaders actually end up decreasing the workload. Some conditional code can be rewritten in terms of algebra and/or built-in functions. For instance, the dot product between a normal and a light vector may be negative in which case the result is not needed in a lighting equation. Instead of:
if (nDotL > 0.0) ...
The value can be clamped with:
clamp(nDotL, 0.0, 1.0)
And unconditionally used in the result (the negative value results in a
zero-product). Clamp may be faster than max and/or min for the 0.0 and
1.0 cases, but as always benchmarking will have the final say in the
matter. Another reason to make an effort of avoiding conditionals in
fragment shaders is that mipmapped textures return undefined results
when executed in a block statement that is conditional on run-time
values. Although the GLSL functions
texture*Lod() can be used to bias or
specify the mipmap LOD, it is expensive to manually derive the mipmap
LOD. In addition, these LOD biasing samplers may not run as fast as the
Use appropriate precision qualifiers (P7)¶
Recall that the default precision in vertex shaders is highp, and that fragment shaders have no default precision until explicitly set. Precision qualifiers can be valuable hints to the compiler to reduce register pressure and may improve performance for several reasons. Low precision may run twice as fast in hardware as high precision. These optimizations can be approached by initially using highp and gradually reducing the precision to lowp until rendering artifacts appear; if it looks good enough, then it is good enough. As a rule of thumb, vertex position, exponential and trigonometry functions need highp. Texture coordinates may need anything from lowp to highp depending on texture width and height. Many application-defined uniform variables, interpolated colors, normals and texture samples can usually be represented using lowp. However, floating-point texture samplers need more than low precision—this is one of several reasons to minimize the use of floating-point textures (T6).
Use the built-in functions and variable (P8)¶
The built-ins have a higher chance of compiling optimally and may even
be implemented in hardware. For instance, do not write shader code to
determine the forward primitive face or compute the reflection vector in
terms of dot-products and algebra; instead, use the built-in variable
gl_FrontFacing or the built-in function
Consider encoding complex functions in textures (P9)¶
Shaders normally contain both arithmetic (ALU) and texture operations. A batch of ALU operations may hide the latency of fetching samples from texture because they occur in parallel. If a shader is the primary bottleneck, and when the ALU operations significantly outnumber the texture operations, it is worthwhile to investigate if some of these operations can be encoded in textures. Sub-expressions in shaders can sometimes be stored in LUTs (look-up-tables). LUTs can sometimes be implemented in textures with sufficient precision and accessed as 1D or 2D textures with NEAREST filtering.
The old trick of using cubemaps to normalize vectors is most likely a performance loss on discrete GPUs. If you pursue this idea, then make sure to benchmark to determine if you have improved or worsened the performance!
Limit the amount of indirect texturing (P10)¶
Indirect texturing can sometimes be useful, but when the result of a texture operation depends on another texture operation, the latency of texture sampling is difficult to hide. It also tends to lead to scattered reads that minimize the benefit of the texture cache. Indirect texturing can sometimes be reduced, or avoided, at the expense of memory. Whether that trade-off makes sense should of course be analyzed and benchmarked.
Do not let GLSL syntax obscure math optimizations (P11)¶
The GLSL shading language conveniently overloads arithmetic operators
for vectors and matrices. Care must be taken to not miss optimization
opportunities due to this syntax simplification. For instance, a
rotation matrix can, but should not, be defined as homogeneous 4x4 matrix
just because the other operand is a vec4 vector. A generalized rotation
matrix should be described only as a 3 x 3 matrix and applied to vec3
vectors. And a rotation around basic vectors can be done even more
mat3*vec3 by directly accessing the relevant vector
and matrix components with cos and sin. Take advantage of any specific
application knowledge to reduce the number of needed scalar
Only normalize vectors when it is necessary (P12)¶
Normalization of vectors is required to efficiently calculate the angle between them, or perhaps more typically, the diffuse component in many commonly used lighting equations. In theory, normalization is required for geometry normals after having transformed them with the normal matrix. In practice, it may not matter depending on the composition of the matrix. It is a common mistake to normalizing vectors where it is not necessary, or at least not visually discernible. As a result, the application may run slower for no good reason. For instance, consider the case of interpolating vertex normals in order to compute lighting per fragment (i.e., Phong shading). If the normal matrix only rotates, there is little reason to normalize the normal vectors before interpolating.
Barycentric interpolation will not preserve the unit length of these vectors. So normals that are interpolated in varying variables do must be normalized to ensure the dot-product with the light vector obeys the cosine emission law.
Textures consume the largest amount of the available memory and bandwidth in many applications. One of the best places to look for improvements when short on memory or bandwidth to optimize texture size, format and usage. Careless use of texturing can degrade the frame rate and can even result in inferior image quality.
Use texture compression whenever possible (T1)¶
Texture compression brings several benefits that result from textures
taking up less memory: compressed textures use less of the available
memory bandwidth, reduces download time, and increases the efficiency of
the texture cache. The
texture_compression_latc extensions both provide block-based lossy
texture compression that can be decompressed efficiently in hardware.
The S3TC extension gives 8:1 or 4:1 compression ratio and is suitable
for color images with 3 or 4 channels (with or without alpha) with
relatively low-frequency data. Photographs and other images that
compress satisfactory with JPEG are great candidates for S3TC. Images
with hard and crisp edges are poorer candidates for S3TC and may appear
slightly blurred and noisy. The LATC extension yields a 2:1 compression
ratio, but improves on the quality, and can be useful for high
resolution normal maps. The third coordinate is derived in the fragment
shader—be sure to benchmark if the application can afford this trade-off
between memory and computations! Unlike S3TC, the channels in LATC are
compressed separately, and quantization is less hard. Using texture
compression does not always result in lower perceived image quality, and
with these extensions one can experiment with increasing the texture
resolution for the same memory. There are off-line tools to compress
textures (even if a GL extension supports compressing them on-the-fly).
Search the NVIDIA developer website for “Texture Tools.”
Use mipmaps when appropriate (T2)¶
Mipmaps should be used when there is not an obvious one-to-one mapping between texels and frame buffer pixels. If texture minification occurs in a scene and there are no mipmaps to access, texture cache utilization will be poor due to the sparse sampling. If texture minification occurs more often than not, then the texture size may be too large to begin with. Coloring the mipmap levels differently can provide a visual clue to the amount of minification that is occurring. When reducing the texture size, it may also be worthwhile to perform experiments to see if some degree of magnification is visually acceptable and if it improves frame rate.
glGenerateMipmap() is convenient, it should not be the only
option for generating a mipmap chain. This function emphasizes execution
speed over image quality by using a simple box filter. Generating
mipmaps off-line using more advanced filters (e.g. Lanczos/Sinc) will
often yield improved image quality at no extra cost. However,
glGenerateMipmap() may be preferable when generating textures
dynamically due to speed. One of the only situations where you do not
want to use mipmaps is if there is always a one-to-one mapping between
texels and pixels. This is sometimes the case in 3D, but more often the
case for 2D user interfaces. Recall that a mipmapped texture takes up
33% more storage than un-mipmapped, but they can provide much better
performance and even better image quality through reduced aliasing.
Use the smallest possible textures size (T3)¶
Always use the smallest possible texture size for any content that gives acceptable image quality. The appropriate size of a texture should be determined by the size of the frame buffer and the way the textured geometry is projected onto it. But even when you have a one-to-one mapping between texels and frame buffer pixels, there may be cases where a smaller size can be used. For instance, when blending a texture on the existing content in the entire frame buffer, the texture does not necessarily have to be the same width and height as the frame buffer. It may be the case that a significantly smaller texture that is magnified will produce results that are good enough. The bandwidth that is saved from using a smaller and more appropriately sized texture can instead be spent where it actually contributes to better image quality or performance.
Use the smallest possible texture format and data type (T4)¶
If hardware accelerated texture compression cannot be used for some
textures, then consider using a texture format with fewer components
and/or fewer bits per component. Textures for user interface elements
sometimes have hard edges or color gradients that result in inferior
image quality when compressed. The S3TC algorithm make assumptions that
changes are smooth and colors values can be quantized. If these
assumptions do not fit a particular image, but the number of unique
colors is still low, then experiment with storing these in a packed
texture format using 16 bit/texel (e.g.
the colors are remapped with less accuracy it may not be noticeable in
the final application. Grayscale images should be stored as LUMINANCE
and tinted images can sometimes be stored the same way with the added
cost of a dot product with the tint color. If normal maps do not
compress satisfactory with the LATC format, then it may be possible to
store two of the normals coordinates in uncompressed
derive the third in a shader assuming the direction (sign) of the normal
is implicit (as is the case of a heightmap terrain).
When optimizing uncompressed textures, the exception case that 24-bit (RGB) textures are not necessarily faster to download or smaller in memory than 32-bit (RGBA) on most GPUs. In this case, it may be possible to use the last component for something useful. For instance, if there already is an 8-bit grayscale texture that is needed at the same time as an opaque color texture, that single component texture can be stored in the unused alpha component of a 32-bit (RGBA). The component could define a specular/reflectance map that describe where and to what degree light is reflected. This is useful for terrain satellite imagery or land cover textures with water, snow, or ice areas, or for car textures with their metal and glass surfaces, or for textures for buildings with glass windows.
Store multiple images in each texture object (T5)¶
There is no requirement that there is a one-to-one mapping between an image and a texture object. Textures objects can contain multiple distinct images. These are sometimes referred to as a “texture atlas” or a “texture page.” The geometry defines texture coordinates that only reference a subset of the texture. Texture atlases are useful for minimizing state changes and enables larger batches when rendering. For example, residential houses and office buildings and factories might all use distinct texture images. But the geometry and vertex layout for each is most likely identical so these could share the same buffer object. If the distinct images are stored in a texture atlas instead of as separate textures, then these different kinds of buildings can all be rendered more efficiently in the same draw call (G7). The texture object could be a 2D texture, a cubemap texture or an array texture.
Note that if mipmapping is enabled, the sub-textures in an atlas must have a border wide enough to ensure that smaller mipmaps are not generated using texels from neighboring images. And if texture tiling (
MIRRORED_REPEAT) is needed for a sub-image then it may be better to store it outside the texture atlas.
Emulating either wrapping mode in a shader by manipulating texture coordinates is possible, but not free. A cubemap texture can sometimes be useful since wrapping and filtering apply per face, but the texture coordinates used must be remapped to a vec3 which may be inconvenient. If all the sub-images have the same or similar size, format and type (e.g. image icons), the images are a good candidate for the array texture extension if supported. Array textures may be more appropriate here than a 2D texture atlas where mipmapping and wrapping restrictions have to be taken into consideration.
Float textures are always expensive (T6)¶
Textures with a floating-point format should be avoided whenever possible. If these textures are simply being used to represent a larger range of values, it may be possible to replace these with fixed point textures and scaling instructions. For instance, unsigned 16-bit integers cannot even accurately be represented by half-precision floats (FP16). These would have to be stored using single precision (FP32) leading to twice the memory and bandwidth requirements. It might be better to store these values in two components using 8 bits (LA8) and spend ALU instructions to unpack them in a shader.
Floating-point textures may not support anything better than nearest filtering.
Prefer power-of-two (POT) textures in most cases (T7)¶
Although Non-Power-of-Two (NPOT) textures are supported in ES2 they come with a CLAMP_TO_EDGE restriction on the wrapping mode (unless relaxed by an extension). More importantly, they cannot be mipmapped (unless relaxed by an extension). For that reason, POT textures should be used when there is not significant memory and bandwidth to be saved from using NPOT. However, an NPOT texture may be padded internally to accommodate alignment restrictions in hardware and that the amount of memory saved might not be quite as large as the width and height suggests. As a rule of thumb, only large (i.e., hundreds of texels) NPOT textures will effectively save a significant amount of memory over POT textures.
Update textures sparingly (T8)¶
Writing to GPU resources can be expensive—it applies to textures as well. If texture updates are required, then determine if they really need to be updated per frame or if the same texture can be reused for several frames. For environment maps, unless the lighting or the objects in the environment have been transformed (e.g., moved and/or rotated) sufficiently to invalidate the previous map, the visual difference may not be noticeable, but the performance improvement can be. The same applies to the depth texture(s) used for shadow mapping algorithms.
Update textures efficiently (T9)¶
When updating an existing texture, use
glTexSubImage*() instead of
redefining its entire contents with TexImage when possible.
glTexSubImage*() it is important to specify the same texture format and data type with which that the texture object was defined; otherwise, there may be an expensive conversion as texels are being updated.
If the application is only updating from a sub-rectangle of pixels in
client memory, then remember that the driver has no knowledge about the
stride of pixels in your image. When the width of the image rectangle
differs from the texture width, this normally requires a loop through
single pixel rows calling
glTexSubImage*() repeatedly while updating the
client memory offsets with pointer arithmetic. In this case, the
unpack_subimage extension can be used (if supported) to set the
UNPACK_ROW_LENGTH pixelstore parameter to update the entire region with
Partition rendering based on alpha blending/testing (T10)¶
It is sometimes possible to improve performance by splitting up the rendering based on whether alpha blending or alpha testing is required. Use separate draw calls for opaque geometry so these can be rendered with maximum efficiency with blending disabled. Perform other draw calls for transparent geometry with alpha blending enabled, taking into account the draw ordering that transparent geometry requires. As always, benchmarks should be run to determine if this improves or reduces the frame rate since you will be batching less by splitting up draw calls.
Filter textures appropriately (T11)¶
Do not automatically set expensive texture filters and enable anisotropic filtering. Remember that nearest-neighbor filtering always fetches one texel, bilinear filtering fetches up to four texels and trilinear fetches up to eight texels. However, it can be incorrect to draw assumptions about the performance cost based on this. Bilinear filtering may not cost four times as much as nearest filtering, and trilinear can be more or less than twice as expensive as bilinear. Even though textures have mipmaps, it does not automatically mean trilinear filtering should be used. That decision should be made entirely from observing the images from the running application. Only then can a judgment be made if any abrupt changes between mipmap levels are visually disturbing enough to justify the cost of interpolating the mipmaps with trilinear filtering. The same applies to anisotropic filtering, which is significantly more expensive and bandwidth intensive than bilinear or trilinear filtering. If the angle between the textured primitives and the projection plane (e.g. near plane) is never very large, there is nothing to be gained from anisotropic sampling, and there a potential for lower performance. Therefore, an application should start off with the simplest possible texture filtering and only enable more expensive filtering after users have inspected the output images. It might be worthwhile to benchmark the changes and take notes along the way. This will provide a better indication of the relative cost of filtering method and if concessions must be made if the performance budget is exceeded.
Try to exploit texture tiling (T12)¶
It is common for images to contain the same repeated pattern of pixels.
Or an image might repeat a few patterns that are close enough in
similarity that they could be replaced with a single pattern without
impacting image quality. Tiling textures saves on memory and bandwidth.
Some image processing applications can identify repeated patterns and
can crop them so they can be tiled infinitely without seams when using
textures with the REPEAT wrap mode. Sometimes even a quarter of a tile
or shingle may be sufficient to store while using
Consider if tiling variation can be restored or achieved with
multi-texturing, using for instance a less expensive grayscale texture
that repeats at a different frequency to modulate the texels from the
Use frame buffer objects (FBO) for dynamically generated textures (T13)¶
OpenGL ES comes with functions for copying previously rendered pixels
from a frame buffer into a texture
glCopyTexSubImage() ). These functions should be avoided whenever
possible for performance reasons. It is better to bind a frame buffer
object with a texture attachment and render directly to the texture.
Make sure you check for frame buffer completeness.
Not all pixel formats are color-renderable. Formats with three or four components in 16 or 32 bits are color-renderable in OpenGL ES 2.0, but LUMINANCE and/or ALPHA may require a fallback to
This topic contains miscellaneous OpenGL ES programming tips.
Avoid reading back the frame buffer contents (M1)¶
Reading back the frame buffer flushes the GL pipeline and limits the
amount CPU/GPU parallelism. Reading frequently or in the middle of a
frame stalls the GPU and limits the throughput with lower frame rate as
a result. If the buffer contents must be read back (perhaps for picking
3D objects in a complex scene), it should be done minimally and
scheduled at the beginning of the next frame. In the special case that
the application is reading back into a sub-rectangle of pixels in client
pack_subimage extension (if supported) is very useful.
PACK_ROW_LENGTH pixel store parameter will reduce the loop
overhead that will otherwise be necessary (T9).
Avoid clearing buffers needlessly (M2)¶
If the application always covers the entire color buffer for each frame,
then bandwidth can be saved by not clearing it. It is a common mistake
glClear(GL_COLOR_BUFFER_BIT) when it is not necessary. If only
part of the color buffer is modified, then constrain pixel operations to
that region by enabling scissor testing and define a minimal scissor box
for the region. The same applies to depth and stencil buffers if full
screen testing is not needed.
Disable blending when it is not needed (M3)¶
Most blending operations require a read and a write to the frame buffer.
Memory bandwidth is often doubled when rendering with blending is enabled. The number of blended fragments should be kept to a minimum—it can drastically speed up the GL application.
Minimize memory fragmentation (M4)¶
Buffer objects and
glTexImage*() functions are effectively graphics
memory allocations. Reusing existing buffer objects and texture objects
will reduce memory fragmentation. If geometry or textures are generated
dynamically, the application should allocate a minimal pool of objects
for this purpose during application initialization. It may be that two
buffers or textures used in a round-robin fashion are optimal for
reducing the risk that the GPU is waiting on the resource. Also, recall
that sampling a texture that is being rendered to, at the same time, is
undefined. This can be another reason to alternate between objects. For
more information, see Memory Fragmentation
in this topic.
There is no “Memory Fragmentation” heading, and the targeted “Video Memory Overview” label does not seem relevant. Perhaps this is a reference to “Avoiding Memory Fragmentation.”
Optimizing OpenGL ES Applications¶
Optimization is an iterative process. It can be time consuming, especially without prior experience determining where bottlenecks tend to occur. Effort should be directed towards the critical areas instead of starting a random place in the rendering code. When the graphics application is complex it may be difficult to know where to start or exactly where optimizations will yield the best return.
Partition the analysis into manageable chunks¶
Many rendering applications are complex and consist of hundreds of objects. But usually they consist of logically separate rendering code. For example, a rendered image may consist of roads, buildings, landmarks, points of interest, sky, clouds, buildings, water, terrain, icons, and a 2D user interface. It is helpful to write the GL application such that rendering of each type of object can be disabled easily. This allows easy identification of the most expensive objects when benchmarking and therefore makes optimizing the rendering code more manageable.
Become familiar with bottlenecks in the graphics pipeline¶
It is important to begin optimizations by identifying the performance bottlenecks at the different stages in the graphics pipeline. Because the work introduced in the beginning of the pipeline normally affects the work needed at later stages, it often makes sense to work backwards from the end of the pipeline. An introduction to identifying graphics bottlenecks can be found in GPU Gems, in Chapter 28, Graphics Pipeline Performance (Cem Cebenoyan, NVIDIA).
Avoiding Memory Fragmentation¶
Memory Fragmentation generally is a bad thing. This is especially true for computer graphics applications. In addition to avoiding system memory fragmentation, a graphics application should strive to avoid video memory fragmentation as well.
Fortunately, controlling video memory fragmentation has techniques very similar to those used to avoid system memory fragmentation. Since system memory fragmentation control is fairly well known, this document will only treat system memory issues in passing and focuses on video memory techniques.
Video Memory Overview¶
Video memory is much more heterogeneous than system memory.
NVIDIA video memory allocation algorithms have to take the following into account:
There are multiple types of video memory types. The number and names of the types vary by GPU model, but GPUs generally have at least two; linear, which is essentially unformatted, and one or more GPU specific types. The GPU tracks different types of memory, and will access and write them differently. The types are important because GPU native types can be faster for a given set of operations; in some GPU architectures, the difference is small, on the order of 10-15%. On others, it can be quite large, more than 100% faster than linear memory.
Video memory is often banked, especially for mipmapped textures. In most architecture, alternating mipmap levels for a given texture must be put in separate banks. This separation is mandatory in most NVIDIA GPUs.
In addition to the restrictions above, different memory regions have different alignment restrictions, to match host pages, improve DMA performance, or speed up frame buffer scanout. These alignment requirements may be orthogonal to the memory types, adding further complication.
The allocator may have other special restrictions that enhance performance, such as distributing allocations to a sequence of different banks to improve allocation speed.
These extra constraints complicate the video memory allocator and make allocations much more sensitive to reductions in available video memory. This is the major reason why NVIDIA does not support multiple independent heaps in video memory, instead requiring the application to allocate in such a way as to minimize fragmentation.
Allocating and Freeing Video Memory¶
This topic describes considerations for allocating and freeing video memory.
When using OpenGL ES/EGL, there is only a small set of APIs that actually lead to long-term video memory buffer allocation:
glBufferData(enum target, sizeiptr size, const void *data, enum usage)
glTexImage2D(enum target, int level, int internalFormat, sizei width, sizei height, int border, enum format, enum type, const void *pixels)
glCopyTexImage2D(enum target, int level, enum internalformat, int x, int y, sizei width, sizei height, int border)
glCopyTexImage2D() allocates only when it copies to a null.
eglCreateWindowSurface(EGLDisplay dpy, EGLConfig config, NativeWindowType win, const EGLint *attrib_list)
eglCreatePbufferSurface(EGLDisplay dpy, EGLConfig config, const EGLint *attrib_list)
eglCreatePixmapSurface(EGLDisplay dpy, EGLConfig config, NativePixmapType pixmap, const EGLint *attrib_list)
A similar set of APIs free allocated video memory buffers, whether they are textures, VBOs, or surfaces:
glDeleteBuffers(sizei n, uint *buffers)
glDeleteTextures(sizei n const uint *textures)
eglDestroySurface(EGLDisplay dpy, EGLSurface surface)
glDeleteTextures() works only if the texture object is greater than zero; the default texture can’t be explicitly deleted (although it can be replaced with a texture containing one or two dimensions of zero, which accomplishes the same thing).
Conceptually, these calls can be thought of as
VBOs and texture maps, respectively. The same techniques for avoiding
fragmentation can also be applied.
Updating a Subregion of a Buffer¶
In many cases, avoiding fragmentation means placing multiple objects into the same shared buffer, or reusing a buffer by deleting or overwriting an older object with a newer one. OpenGL ES provides a method for updating an arbitrary section of allocated VBOs, textures, and surfaces:
glBufferSubData(enum target, intptr offset, sizeiptr size, const void *data, enum usage)
glTexSubImage2D(enum target, int level, int xoffset, int yoffset, sizei width, sizei height, enum format, enum type, const void *pixels)
glCopyTexSubImage2D(enum target, int level, int xoffset, int yoffset, int x, int y, sizei width, sizei height)
glScissor(int left, int bottom, sizei width, sizei height);
glViewport(int x, int y, sizei w, sizei h)
glCopyTexSubImage2D() update a subregion of the
target texture image. In the first case, the source comes from an
application buffer; in the second, from a rendering surface.
glViewport() limit rendering to a subregion of a
rendering surface. The first specifies the region of the buffer that
glClear() will affect; the second updates the transforms to limit
rendered OpenGL ES primitives to the specified subregion.
Using a Buffer Subregion¶
Completing the functionality needed to reuse allocated buffers is the ability to use an arbitrary subregion of a texture, VBO, or surface:
glDrawArrays(enum mode, int first, sizei count)
glReadPixels(int x, int y, sizei width, sizei height, enum format, enum type, void *data)
glCopyTexImage2D(enum target, int level, int x, int y, sizei width, sizei height)
glCopyTexSubImage2D(enum target, int level, int xoffset, int yoffset, int x, int y, sizei width, sizei height)
glDrawArrays() allows the application to choose a contiguous
subset of a VBO.
For textures, there’s no explicit call to limit texturing source to a particular subregion. But texture coordinates and wrapping modes can be specified in order to render an arbitrary subregion of texture object.
glReadPixels() can be used to read from a subregion of a
display, when copying data back to an application-allocated buffer.
glCopyTexSubImage2D() also restrict themselves to
copying from a subregion of the display surface when transferring data
to a texture map. The only area that’s problematic is controlling the
direct display of window surface back buffer. OpenGL ES and EGL have no
way to show only a subregion of the backbuffer surface, but the native
windowing systems may have this functionality.
Best Practices for Video Memory Management¶
The following is a list of good practices when allocating video memory to avoid or minimize fragmentation:
1. Allocate large buffers early¶
Ideally allocate large buffers at the start of the program. On average, allocating large surfaces gets more difficult as more allocations occur. When more allocations occur, free space is broken into smaller pieces.
2. Combine many small allocations into a smaller number of larger allocations¶
Small allocations can disproportionately reduce available free space. Not only does the allocator have a fixed overhead per allocation, regardless of size, but small allocations tend to break up large areas of free space into smaller pieces.
The ability to load a subregion of a VBO or texture map, and the ability
to render that subregion independently, makes it possible to combine
VBOs and textures together. For textures, a large texture can be used to
hold a grid of smaller images. For VBOs, multiple vertex arrays can be
combined end to end into a larger one. Besides reducing fragmentation,
combining related images into a single texture, and related vertex
arrays into a single VBO often improves rendering time, since it reduces
the number of
glBindTexture() calls required to render
a set of related objects.
3. Reduce the variation in size of allocated buffers ideally to a single size¶
Allocating buffers of varying sizes, especially sizes that aren’t small multiples of each other, is disruptive of memory space and causes fragmentation. The ability to load and render a subset of a VBO or texture means that data loaded doesn’t have to match the size of the allocated buffer; as long as it’s smaller, it will work.
This approach does waste space, in that some of the allocated buffer isn’t used, but this waste is often offset by the saving in reduced fragmentation and fixed allocation overhead. This approach can often be combined with approach (2) (combining multiple objects into one allocated buffer) to reduce total wastage. Generally, it’s safe to ignore wastage iF it’s a small percentage of the allocated buffer size (e.g. less than 5%).
This approach is particularly good for dynamically allocated data, since fixed size blocks can often be freed and reallocated with little or no fragmentation. If wastage is excessive, a set of buffer sizes can be chosen (often a consecutive set of power of two sizes), and the smallest free buffer that will contain the data is used.
4. Reuse, rather than free and reallocate, buffers whenever possible¶
The ability to reload a previously allocated buffer with new data for both VBOs and textures makes this technique practical. Reuse is particularly important for large buffers; it is often better to create a large buffer at program start, and reuse it during mode switches, etc., even if it requires allocating a larger buffer to handle all possible cases.
5. Minimize dynamic allocation¶
If possible, take memory allocation and freeing out of the inner loop of your application. The ability to reuse buffers makes this practical, even for algorithms that require dynamic allocation. Even with reuse, however, it’s still better to organize the code to minimize allocations and frees, and to move the remaining ones out of the main code path as much as possible.
6. Try to group dynamic allocations¶
If dynamic allocation is mandatory, try to group similar allocations and frees together. Ideally, an allocation of a buffer is followed by freeing it before another allocation happens. This rarely can be done in practice, but combining a group of related allocations is often nearly as effective.
Again, allocations and frees should be replaced whenever possible. Grouping them is a last resort.
Graphics Driver CPU Usage¶
In some cases, the reported graphics driver CPU usage may be high, but in fact the yield is related to other CPUs. To reduce the reported CPU usage, set the environment variable as follows:
$ export __GL_YIELD=USLEEP
The NVIDIA® Jetson™ system on a chip (SoC) includes an extremely powerful and flexible 3D GPU whose power is well matched to the OpenGL ES APIs.