浅析UE4中的EyeAdaption

EyeAdaptation In UE4

Eye Adaptation - UE4 Documentation中，UE4是采用基于Histogram的方法来计算画面的平均中灰值的，并通过LowPercent和HighPercent来过滤一些极黑或者极亮的
像素块。通过Min Brightness和Max Brightness来限制中灰值L的取值范围。Speed Up和Speed Down来控制明到暗和暗到明的过度变化。并允许通过Auto Exposure Bias
来曝光补偿。

Auto Exposure Mathematic Model

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在[EyeAdaption - Overview]中，我们总结了EyeAdaptation的Mathematic Model。通过阅读UE4的Source Code，整体的EyeAdaption的过程大致一样，
只是在计算平均亮度值之一块有较大的差异。

Auto Exposure in UE4

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在UE4中，计算平均中灰值的方式是基于histogram的，并且多了个曝光补偿因子来控制最终的曝光值。

浅析UE4中的EyeAdaption

UE4的EyeAdaption的代码基本上是按照Auto Exposure in UE4编写。因此，接下来只是单纯的把对应的mathematic equation和相关的Code放到一起，
并简单的在UE4的Code中简单注释公式和代码相关变量的关系。

Eye Adaptation参数

/// PostProcessHistogramCommon.usf

// Lmin: EyeAdaptationMin, LMax: EyeAdaptationMax
// [0] .x:ExposureLowPercent/100, .y:EyeAdaptationHighPercent/100, .z:EyeAdaptationMin, .w:EyeAdaptationMax

// temporal adaptation's *delta time*: DeltaWorldTime, temporal adaptation's *speed*: EyeAdaptionSpeedUp/Down
// [1] .x:exp2(ExposureOffset), .y:DeltaWorldTime, .zw: EyeAdaptionSpeedUp/Down

// 基于Histogram的Average Luminance的参数
// [2] .x:Histogram multiply, .y:Histogram add, .z:HistogramMinIntensity w:unused
float4 EyeAdaptationParams[3];

Eye Adaptation

/// PostProcessTonemap.usf

// Exposure存储在EyeAdaptation纹理里，vEyeAdapationVS.x即Exposure
vEyeAdapationVS.x = EyeAdaptation.Load(int3(0, 0, 0)).r;
OutColor = TonemapCommonPS(UV, vEyeAdapationVS, GrainUV, FringeUV, FullViewUV, SvPosition);

// TonemapCommonPS
float ExposureScale = InExposureScaleVignette.x;
...
// LinearColor, 即Color(i,j)
LinearColor *= ExposureScale;
...

从EyeAdaptation纹理中得到当前的Exposure值，并把当前的LinearColor乘以该Exposure值来调整画面的明暗。

Temporal Adaptation

/// PostProcessEyeAdaptation.usf

float ComputeEyeAdaptation(float OldExposure, float TargetExposure, float FrameTime)
{
    float Diff = TargetExposure - OldExposure;
    
    // dark adaption & light adaption 的 *speed*参数
    const float EyeAdaptionSpeedUp   = EyeAdaptationParams[1].z;
    const float EyeAdaptionSpeedDown = EyeAdaptationParams[1].w;
    
    // Diff的正负表示从明到暗或者暗到明的过程
    float AdaptionSpeed = (Diff > 0) ? EyeAdaptionSpeedUp : EyeAdaptionSpeedDown;
    
    float Factor = 1.0f - exp2(-FrameTime * AdaptionSpeed);
    
    // Temporal Exposure *L*: OldExposure + Diff * Factor
    // clamp(L, LMin, LMax), LMin：EyeAdaptationParams[0].z， LMax: EyeAdaptationParams[0].w
    return clamp(OldExposure + Diff * Factor, EyeAdaptationParams[0].z, EyeAdaptationParams[0].w);
}

Exposure Calculation

/// PostProcessEyeAdaptation.usf

float2 MainEyeAdaptationCommon()
{
    float2 OutColor = 0;
    // 曝光补偿
    float ExposureOffsetMultipler = EyeAdaptationParams[1].x;
    
    // 从Histogram中计算画面的中灰值
    float TargetExposure = ComputeEyeAdaptationExposure(PostprocessInput0);
    
    // Lold
    float OldExposureScale = PostprocessInput0.Load(int3(0, 1, 0)).r;
    float OldExposure = ExposureOffsetMultipler / ( OldExposureScale != 0 ? OldExposureScale : 1.0f );
    
    // delta time in temporal adaptation
    float FrameTime = EyeAdaptationParams[1].y;
    
    // eye adaptation changes over time
    float SmoothedExposure = ComputeEyeAdaptation(OldExposure, TargetExposure, FrameTime);
    
    // KeyValue = 1.0f, smoothedExposure = clamp(L, Lmin, Lmax)
    float SmoothedExposureScale = 1.0f / max(0.0001f, SmoothedExposure);
    float TargetExposureScale =   1.0f / max(0.0001f, TargetExposure);
    
    // Exposure
    OutColor.r = ExposureOffsetMultipler * SmoothedExposureScale;
    OutColor.g = ExposureOffsetMultipler * TargetExposureScale;
    
    return OutColor;
}

*1.0/max(0.0001f,SmoothedExposure)*避免除以一个很小的数值。

Average Luminance

/// PostProcessHistogramCommon.usf

float ComputeEyeAdaptationExposure(Texture2D HistogramTexture)
{
    float HistogramSum = ComputeHistogramSum(HistogramTexture);
    // low percent: EyeAdaptationParams[0].x, high percent: EyeAdaptationParams[0].y
    float UnclampedAdaptedLuminance = ComputeAverageLuminaneWithoutOutlier(HistogramTexture, HistogramSum * EyeAdaptationParams[0].x, HistogramSum * EyeAdaptationParams[0].y);
    // Lmin: EyeAdaptationParams[0].z, Lmax: EyeAdaptationParams[0].w
    float ClampedAdaptedLuminance = clamp(UnclampedAdaptedLuminance, EyeAdaptationParams[0].z, EyeAdaptationParams[0].w);
    
    return ClampedAdaptedLuminance;
}

可以这么理解，把画面的所有像素放到一个数组里，根据像素的Lum值大小排序，那么HistogramSum就是该数组的大小，[histogramSum * EyeAdaptationParams[0].x, HistogramSum * EyeAdaptationParams[0].y]表示对
画面平均亮度值有贡献的区域。即平均亮度值是该区域的亮度和除以区域所包含的像素个数。

基于Histogram的方法的Average Luminance算法涉及的内容比较多，在这里只是一带而过。

小结

EyeAdaptation关键点就是如何计算画面的平均亮度值。

Average Luminance Calculation Using a Compute Shader(with code in blog)中，先将画面转到lum空间，通过Dx的Mimap或者Compute Shader计算出平均亮度值。
UE4也是通过Compute Shader计算出画面的Histogram，通过Histogram计算出平均亮度值。

Reference

• Eye Adpation(Auto-Exposure)
• Automatic exposure
• Average Luminance Calculation Using a Compute Shader
• PostProcessEyeAdaptation.usf
• PostProcessTonemap.usf
• PostProcessHistogramCommon.usf