Name
EXT_unpack_subimage
Name Strings
GL_EXT_unpack_subimage
Contact
Peter Pipkorn, NVIDIA Corporation (ppipkorn 'at' nvidia.com)
Contributors
Daniel Kartch, NVIDIA Corporation (dkartch 'at' nvidia.com)
Nicolai de Haan, NVIDIA Corporation (nicolaid 'at' nvidia.com)
Status
First Draft
Version
Last Modifed Date: Mar 25, 2011
Author Revision: 0.1
Number
TBD
Dependencies
The extension is written against the OpenGL ES 2.0 specification.
The extension references the OpenGL 2.0 specification.
Overview
This OpenGL ES 2.0 extension adds support for GL_UNPACK_ROW_LENGTH,
GL_UNPACK_SKIP_ROWS and GL_UNPACK_SKIP_PIXELS as valid enums to
PixelStore. The functionality is the same as in OpenGL. These are useful
for updating textures with a sub-rectangle of pixel data coming from a larger
image in host memory.
IP Status
None
Issues
1. Can't this be done with repeated calls to TexSubImage2D/TexSubImage3D?
Yes, it is possible to unpack pixels from a sub-rectangle in host
memory by by calling these functions for one line at a time, but
this could add unnecessary burden on the CPU system. Specifying
GL_UNPACK_ROW_LENGTH makes it possible to unpack sub-rectangles of
pixels with lower overhead.
2. Should the corresponding PACK enums be added?
No, it should be done in a separate extension. There is no dependency
between the PACK enums and the UNPACK enums.
3. Are these UNPACK_SKIP_* tokens strictly necessary?
No. The same functionality can be achieved by advancing the pixel
pointer to host memory appropriately before issuing an unpacking
function call. They are included here for both completeness and
for convenience.
New Procedures and Functions
None
New Tokens
Accepted by the <pname> parameters of PixelStorei,
GetIntegerv, and GetFloatv:
GL_UNPACK_ROW_LENGTH 0x0CF2
GL_UNPACK_SKIP_ROWS 0x0CF3
GL_UNPACK_SKIP_PIXELS 0x0CF4
(These are the same values as the corresponding enums in OpenGL)
Additions to the OpenGL ES 2.0 Specification
Modifications to Table 3.1 (PixelStore parameters)
Add the following entries:
Parameter Name Type Initial Value Valid Range
============== ==== ============= ===========
UNPACK_ROW_LENGTH integer 0 [0,Infinity)
UNPACK_SKIP_ROWS integer 0 [0,Infinity)
UNPACK_SKIP_PIXELS integer 0 [0,Infinity)
Modifications to 3.6.2 Transfer of Pixel Rectangles, in the Unpacking section:
Change
"The number of groups in a row is width;"
to
"If the value of UNPACK_ROW_LENGTH is not positive, then the number
of groups in a row is <width>; otherwise the number of groups
is UNPACK_ROW_LENGTH."
After the sentence
"If the number of bits per element is not 1, 2, 4 or 8 times the
number of bits in a GL ubyte, then k = nl for all values of a."
insert:
"There is a mechanism for selecting a sub-rectangle of groups from
a larger containing rectangle. This mechanism relies on three
integer parameters: UNPACK_ROW_LENGTH, UNPACK_SKIP_ROWS, and
UNPACK_SKIP_PIXELS. Before obtaining the first group from memory,
the pointer supplied to TexImage2D is effectively advanced by
(UNPACK_SKIP_PIXELS)n + (UNPACK_SKIP_ROWS)k elements. Then <width>
groups are obtained from contiguous elements in memory (without
advancing the pointer), after which the pointer is advanced by k
elements. <height> sets of <width> groups of values are obtained
this way. See figure 3.6."
Before Table 3.5 Packed pixel formats insert
Figure 3.8 from the OpenGL 2.0 specification (a visual description
of UNPACK_ROW_LENGTH, UNPACK_SKIP_ROWS, and UNPACK_SKIP_PIXELS)
Errors
None
New State
Modifications to Table 6.12 Pixels in section 6.2 State Tables:
Get Value Type Get Cmnd Initial Description Sec. Attribute
Value
==================== ==== =========== ======= ============================ ===== ===========
UNPACK_ROW_LENGTH Z+ GetIntegerv 0 Value of UNPACK_ROW_LENGTH 3.6.1 pixel-store
UNPACK_SKIP_ROWS Z+ GetIntegerv 0 Value of UNPACK_SKIP_ROWS 3.6.1 pixel-store
UNPACK_SKIP_PIXELS Z+ GetIntegerv 0 Value of UNPACK_SKIP_PIXELS 3.6.1 pixel-store
Revision History
03/25/2011 0.1
- Original draft.
