Temporal AA

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以前记录的旧文档，先简单整理一下，免得遗失

jitter sample

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View.ViewMatrices.HackAddTemporalAAProjectionJitter(FVector2D(SampleX * 2.0f / View.ViewRect.Width(), SampleY * -2.0f / View.ViewRect.Height()));
  • Halton(2, 3) sequence
  • Box-Muller transform

Reprojection

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ViewUniformShaderParameters.ClipToPrevClip = InvViewProj * PrevViewProj;
  • 当前帧屏幕坐标（ScreenPos）通过viewproj矩阵，reproject到上一帧（history）的屏幕坐标，这针对静止模型或只有相机运动的情况
  • 通过矩阵的Reprojection在模型存在运动时就失效了（动画：位移、骨骼动画等），需要通过motion vector（velocity buffer）来判断是否存在模型的运动，如果存在就使用motion vector来采样历史帧的屏幕坐标
  • motion vector可以

细节

  1. history buffer的维护 history buffer是由View.PrevViewInfo.TemporalAAHistory和View.ViewState->PrevFrameViewInfo.TemporalAAHistory维护，保存在FTemporalAAHistory的RT[]成员中。在AddGen4MainTemporalAAPasses中，通过AddTemporalAAPass的InputHistory(View.PrevViewInfo.TemporalAAHistory/OutputHistory(View.ViewState->PrevFrameViewInfo.TemporalAAHistory)参数传入/传出，这俩参数的类型都是FTemporalAAHistory。
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const FTemporalAAHistory& InputHistory = View.PrevViewInfo.TemporalAAHistory;
FTemporalAAHistory& OutputHistory = View.ViewState->PrevFrameViewInfo.TemporalAAHistory;

const FTAAOutputs TAAOutputs = ::AddTemporalAAPass(
    GraphBuilder,
    View,
    TAAParameters,
    InputHistory,
    &OutputHistory);

AddTemporalAAPass中，通过InputHistory传入当前帧的history scene buffer（TAAStandalone.usf, HistoryBuffer）；并创建NewHistoryTexture[4]，作为TAA Main pass（TAAStandalone.usf/MainCS, OutComputeTex）的输出，是下一帧的history scene buffer；

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// HistoryBuffer_0
for (int32 i = 0; i < FTemporalAAHistory::kRenderTargetCount; i++)
{
    if (InputHistory.RT[i].IsValid())
    {
        PassParameters->HistoryBuffer[i] = GraphBuilder.RegisterExternalTexture(InputHistory.RT[i]);
    }
    else
    {
        PassParameters->HistoryBuffer[i] = BlackDummy;
    }
}

// OutComputeTex_0
for (int32 i = 0; i < FTemporalAAHistory::kRenderTargetCount; i++)
{
    PassParameters->OutComputeTex[i] = GraphBuilder.CreateUAV(NewHistoryTexture[i]);
}

NewHistoryTexture是FRDGTexture类型，作为当前帧的输出，如何成为下一帧的history buffer？FTemporalAAHistory的RT[]是pooled render target，RDG通过提取FRDGTexture的方式延长资源的生命周期，将NewHistoryTexture和OutputHistory->RT[]绑定，在下一帧PreVisibilityFrameSetup中将View.ViewState->PrevFrameViewInfo赋值给View.PrevViewInfo，完成current buffer到history buffer的蜕变，最终走向落幕。 > The pooled render target pointer can also be extracted from a FRDGTexture. This allows you to preserve contents of a resource across graph invocations. However, the extraction is deferred until the graph has executed; this is because the resource may be allocated during execution based on the lifetime of the resource in the graph,The API therefore exposes QueueTextureExtraction, which allows you to provide a pointer which will be filled upon execution of the graph.

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{
    View.PrevViewInfo = ViewState->PrevFrameViewInfo;
}

// Replace previous view info of the view state with this frame, clearing out references over render target.
if (!View.bStatePrevViewInfoIsReadOnly)
{
    ViewState->PrevFrameViewInfo = NewPrevViewInfo;
}

  1. ScreenPosToHistoryBufferUV

  2. sample position的处理

  3. blend factor UE4的TAAStandalone.usf实现RGBToYCoCg中，转换矩阵乘了4，得到的luminance也是真实的4倍，根据[1]3.3.1介绍，blend factor为\(w(c) = \frac{1}{1 + L(c)}\)，具体实践也相应的体现了4倍。 

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    float HdrWeight4(float3 Color, float Exposure)
    {
    	return rcp(Luma4(Color) * Exposure + 4.0); // Luma4得到4倍的亮度，在最终WeightedLerpFactors的计算中，4被约掉
    }

  1. 1.A Survey of Temporal Antialiasing Techniques ↩︎