NVIDIA HBAO+ 2.4.

Overview

HBAO+ is a SSAO algorithm designed to achieve high efficiency on DX11 GPUs. The algorithm is based on HBAO [Bavoil and Sainz 2008], with the following differences:

1. To minimize cache trashing, HBAO+ does not use any randomization texture. Instead, the algorithm uses an Interleaved Rendering approach, generating the AO in multiple passes with a unique jitter value per pass [Bavoil and Jansen 2013].
2. To avoid over-occlusion artifacts, HBAO+ uses a simpler AO approximation than HBAO, similar to “Scalable Ambient Obscurance” [McGuire et al. 2012] [Bukowski et al. 2012].
3. To minimize flickering, the HBAO+ is always rendered in full resolution, from full-resolution depths.

Package

doc/—this documentation page.

lib/—header file, import libraries and DLLs, for Win32, Win64 and Mac OS X.

samples/—source for sample applications demonstrating NVIDIA HBAO+.

Getting Started

1. INITIALIZE THE LIBRARY:

   GFSDK_SSAO_CustomHeap CustomHeap;
   CustomHeap.new_ = ::operator new;
   CustomHeap.delete_ = ::operator delete;
   
   GFSDK_SSAO_Status status;
   GFSDK_SSAO_Context_D3D11* pAOContext;
   status = GFSDK_SSAO_CreateContext_D3D11(pD3D11Device, &pAOContext, &CustomHeap);
   assert(status == GFSDK_SSAO_OK); // HBAO+ requires feature level 11_0 or above

2. SET INPUT DEPTHS:

   GFSDK_SSAO_InputData_D3D11 Input;
   Input.DepthData.DepthTextureType = GFSDK_SSAO_HARDWARE_DEPTHS;
   Input.DepthData.pFullResDepthTextureSRV = pDepthStencilTextureSRV;
   Input.DepthData.ProjectionMatrix.Data = GFSDK_SSAO_Float4x4(pProjectionMatrix);
   Input.DepthData.ProjectionMatrix.Layout = GFSDK_SSAO_ROW_MAJOR_ORDER;
   Input.DepthData.MetersToViewSpaceUnits = SceneScale;

3. SET AO PARAMETERS:

   GFSDK_SSAO_Parameters_D3D11 Params;
   Params.Radius = 2.f;
   Params.Bias = 0.1f;
   Params.PowerExponent = 2.f;
   Params.Blur.Enable = true;
   Params.Blur.Radius = GFSDK_SSAO_BLUR_RADIUS_8;
   Params.Blur.Sharpness = 4.f;
   Params.Output.BlendMode = GFSDK_SSAO_OVERWRITE_RGB;

4. RENDER AO:

   status = pAOContext->RenderAO(pD3D11Context, &Input, &Params, pOutputColorRTV);
   assert(status == GFSDK_SSAO_OK);

Data Flow

Input Requirements

• The library has entry points for DX11 and GL3.2.
• Requires a depth texture to be provided as input, along with associated projection info.
• Optionally, can also take as input a GBuffer normal texture associated with the input depth texture:
  • Can add normal-mapping details to the AO.
  • Can be used to fix normal reconstruction artifacts with dithered LOD dissolves.
  • But makes the integration more complex. We recommend starting with input normals disabled.
• Optionally, can also take as input a viewport rectangle associated with the input textures:
  • Defines a sub-area of the input & output full-resolution textures to be sourced and rendered to.
  • The library re-allocates its internal render targets if the Viewport.Width or Viewport.Height changes for a given AO context.

MSAA Support

• If the input textures are MSAA then:
  • If Output.MSAAMode is set to PER_PIXEL_AO, the library internally uses only sample0 and renders per-pixel AO (default & recommended path).
  • If Output.MSAAMode is set to PER_SAMPLE_AO, the library does one full pass per MSAA subsample, which is effectively super-sampling the AO (slow path).
• In practice, we have found the PER_PIXEL_AO strategy to not cause any objectionable artifacts, even when using HBAO+ with TXAA.

HBAO+ Pipeline

CoarseAO & DetailAO

• Coarse AO
  • For each pixel, 32 occlusion samples are taken in a variable-radius disk, with a minimum of 4 full-resolution pixels between the center pixel and the sample coordinates.
  • This occlusion contribution is weighted by the CoarseAO parameter, and is always computed.
• Detail AO
  • Optionally, 4 occlusion samples can be taken to the left/right/top/bottom of the center pixel, to capture close-range occlusion details.
  • This occlusion contribution is weighted by the DetailAO parameter, and is skipped if DetailAO == 0.f.

Parameters

AO Radius

Definition

For a given AO receiver point P and AO Radius R, sample point S is ignored if ||P-S|| > R

Impact on search area

The AO radius is a multiplier for the screen-space search radius, and the number of samples is fixed. So if the AO radius is too large, under-sampling artifacts may appear.

MetersToViewSpaceUnits

If you are not sure what to set this value to, you can:

• Set the AO Radius parameter to 1.0 and
• Increase MetersToViewSpaceUnits until the AO looks like it’s being cast up to 1 meter away

MetersToViewSpaceUnits is used internally

• To convert the AO radius parameter from meters to view-space units
• To adjust the blur sharpness parameter

DetailAO

• The DetailAO may cause over-darkening on alpha-tested vegetation (grass typically)
• You may want to set DetailAO = 0.f in this case
  • As a bonus, this improves the performance because the DetailAO computation is fully skipped

Power Exponent

• The PowerExponent parameter controls the darkness of the final AO: FinalAO = pow(AO, PowerExponent).
• Typical PowerExponent values are in the range [2.0, 3.0].

AO Bias

• The AO Bias parameter can be used to hide low-tessellation artifacts
• Can also help reducing false-occlusion artifacts near screen borders
• It weights the AO contributions more strongly for samples towards the normal direction

AO Blur

• Optional cross-bilateral filter
  • To remove jittered-sampling artifacts (noise)
  • To hide under-sampling artifacts (banding) & reduce jittering
• 4 Modes
  • No Blur
  • Blur Radius 2
  • Blur Radius 4
  • Blur Radius 8

Blur Sharpness

The greater the sharpness parameter, the more the blur preserves edges

Blur Sharpness Profile

Optionally, a larger blur sharpness can be used for a user-defined foreground depth range

Integration Time Estimates

• Initial integration (for a rendering engineer)
  • <1 man-day with no input normals
  • 1-2 man-days with input normals
• Initial parameter tuning
  • <1 man-hour
  • Tuning the parameter should be quick once the input data are correctly fed into the library
  • The same HBAO+ parameters may be used globally across the whole game

Additional Links

[Bavoil et al. 2008] “Image-Space Horizon-Based Ambient Occlusion”

[McGuire et al. 2012] “Scalable Ambient Obscurance”

[Bukowski et al. 2012] “Scalable High-Quality Motion Blur and Ambient Occlusion”

[Bavoil and Jansen 2013] “Particle Shadows & Cache-Efficient Post-Processing”