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

#. 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].

#. 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].

#. To minimize flickering, the HBAO+ is always rendered in full resolution, from full-resolution depths.

#. To reduce halo artifacts behind foreground objects, HBAO+ can take as input a second depth layer, using the multi-layer SSAO approach from [Bavoil and Sainz 2009] and [McGuire et al. 2013].

.. image:: \_static\hbao-plus-in-tom-clancys-splinter-cell-blacklist-2.jpg

Package
##############

``doc/``\—this documentation page.

``lib/``\—header file, import libraries and DLLs, for Win32 and Win64.

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

Getting Started
###############

#. 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

#. SET INPUT DEPTHS::

    GFSDK_SSAO_InputData_D3D11 Input;
    Input.DepthData.DepthTextureType = GFSDK_SSAO_HARDWARE_DEPTHS;
    Input.DepthData.pFullResDepthTextureSRV = pDepthStencilTextureSRV;
    Input.DepthData.pFullResDepthTexture2ndLayerSRV = pDepthStencilTexture2ndLayerSRV; // Required only if Params.DualLayerAO=true
    Input.DepthData.ProjectionMatrix.Data = GFSDK_SSAO_Float4x4(pProjectionMatrix);
    Input.DepthData.ProjectionMatrix.Layout = GFSDK_SSAO_ROW_MAJOR_ORDER;
    Input.DepthData.MetersToViewSpaceUnits = SceneScale;

#. SET AO PARAMETERS::

    GFSDK_SSAO_Parameters Params;
    Params.Radius = 2.f;
    Params.Bias = 0.1f;
    Params.PowerExponent = 2.f;
    Params.Blur.Enable = true;
    Params.Blur.Radius = GFSDK_SSAO_BLUR_RADIUS_4;
    Params.Blur.Sharpness = 16.f;
    Params.DualLayerAO = true;

#. SET RENDER TARGET::

    GFSDK_SSAO_Output_D3D11 Output;
    Output.pRenderTargetView = pOutputColorRTV;
    Output.Blend.Mode = GFSDK_SSAO_OVERWRITE_RGB;

#. RENDER AO::

    status = pAOContext->RenderAO(pD3D11Context, Input, Params, Output);
    assert(status == GFSDK_SSAO_OK);
	
Data Flow
#############
	
Input Requirements
++++++++++++++++++++

* The library has entry points for D3D11, D3D12 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.

* Optionally, can also take as input a second depth texture (second depth layer), to add information about the scene geometry that is hidden behind the first depth layer.

  * This enables removing halo artifacts behind foreground characters, and that with a single call to RenderAO.

  * The second depth layer can simply be a copy of the scene depth buffer before the dynamic geometry is rendered into it.

MSAA Support
++++++++++++++++++++

* Requirements

  * The input depth & normal textures are required to have matching dimensions and MSAA sample count.

  * The output render target can have arbitrary dimensions and MSAA sample count.

* Per-Pixel AO Algorithm

  * If the input textures are MSAA, only sample 0 is used to render the AO.

  * If the output render target is MSAA, a per-pixel AO value is written to all samples.

  * In practice, we have found this strategy to not cause any objectionable artifacts, even when using HBAO+ with TXAA.

HBAO+ Pipeline
++++++++++++++++++++

.. image:: \_static\pipeline_without_input_normals.png

.. image:: \_static\pipeline_with_input_normals.png

Occlusion Samples
++++++++++++++++++++

* 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.

* The 8 nearest samples to the center pixel are weighted by the NearAO parameter.

* The other samples are weighted by the FarAO parameter.

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.

.. image:: \_static\AO_Radius_1.png

.. image:: \_static\AO_Radius_4.png

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

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

.. image:: \_static\AOBias_0_0.png 

.. image:: \_static\AOBias_0_3.png 

Foreground AO
++++++++++++++++++++

* When this feature is enabled, the screen-space AO kernel radius is:

  * inversely proportional to ViewDepth for ViewDepth > ForegroundViewDepth

  * uniform in screen-space for ViewDepth <= ForegroundViewDepth (instead of getting arbitrarily large)

* To tune the ForegroundViewDepth parameter

  * Start with ForegroundViewDepth = 0.f

  * Increase the ForegroundViewDepth to get less closer-scale occlusion on the foreground objects.

.. image:: \_static\ForegroundAO_OFF.png

.. image:: \_static\ForegroundAO_ON.png

Background AO
++++++++++++++++++++

* When this feature is enabled, the screen-space AO kernel radius is:

  * inversely proportional to ViewDepth for ViewDepth < BackgroundViewDepth

  * uniform in screen-space for ViewDepth >= BackgroundViewDepth (instead of falling off to zero).

* To tune the BackgroundViewDepth parameter

  * Start from a very large BackgroundViewDepth value (e.g. the camera's ZFar)

  * Decrease the BackgroundViewDepth to get larger-scale occlusion in the background.

.. image:: \_static\BackgroundAO_OFF.png

.. image:: \_static\BackgroundAO_ON.png

AO Blur
++++++++++++++++++++

* Optional cross-bilateral filter

  * To remove jittered-sampling artifacts (noise)
  
  * To hide under-sampling artifacts (banding) & reduce jittering
  
* 3 Modes

  * No Blur
  
  * Blur Radius 2
  
  * Blur Radius 4
  
.. image:: \_static\No_Blur.png   

.. image:: \_static\Blur_Radius_4.png 

Blur Sharpness
++++++++++++++++++++

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

.. image:: \_static\Blur_Sharpness_0.png 

.. image:: \_static\Blur_Sharpness_8.png  

Dual Layer
++++++++++++++++++++

The dual layer feature increases the overall quality of the AO, by removing halo artifacts behind foreground objects. To use this feature, the application needs to provide a second depth texture via Input.DepthData.pFullResDepthTexture2ndLayerSRV, and set Params.DualLayerAO = true.

Here are screenshots from our DX11 sample app, which is drawing

* the shader-ball object into a first depth buffer

* the shader-ball and background box into a second depth buffer

and feeding these 2 depth layers as input to RenderAO.

.. image:: \_static\DualLayer_Disabled.png

.. image:: \_static\DualLayer_Enabled.png

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" <http://www.nvidia.com/object/siggraph-2008-HBAO.html>`_

`[Bavoil and Sainz 2009] "Multi-Layer Dual-Resolution Screen-Space Ambient Occlusion" <https://dl.acm.org/citation.cfm?id=1598035>`_

`[McGuire et al. 2012] "Scalable Ambient Obscurance" <http://graphics.cs.williams.edu/papers/SAOHPG12/>`_

`[Bukowski et al. 2012] "Scalable High-Quality Motion Blur and Ambient Occlusion" <http://graphics.cs.williams.edu/papers/VVSIGGRAPH12/>`_

`[Bavoil and Jansen 2013] "Particle Shadows & Cache-Efficient Post-Processing" <https://developer.nvidia.com/gdc-2013>`_

`[McGuire et al. 2013] "Lighting Deep G-Buffers: Single-Pass, Layered Depth Images with Minimum Separation Applied to Indirect Illumination" <http://research.nvidia.com/publication/lighting-deep-g-buffers-single-pass-layered-depth-images-minimum-separation-applied>`_

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