Temporal Auto-Exposure with Hardware Blending#

Traditional auto-exposure pipelines often involve multiple passes and textures:

Textures

  1. Previous average brightness.

  2. Current average brightness.

Passes

  1. Store the previous average brightness.

  2. Generate the current average brightness.

  3. Smooth the average brightnesses and compute auto-exposure.

This document demonstrates how to optimize this process using hardware blending:

Textures

  1. Combined average brightnesses (previous + current).

Passes

  1. Generate and smooth the average brightnesses.

  2. Compute auto-exposure.

Source Code#
/*
   Automatic exposure shader using hardware blending.
*/

/*
   Vertex shaders.
*/

struct APP2VS
{
   float4 HPos : POSITION;
   float2 Tex0 : TEXCOORD0;
};

struct VS2PS
{
   float4 HPos : POSITION;
   float2 Tex0 : TEXCOORD0;
};

VS2PS VS_Quad(APP2VS Input)
{
   VS2PS Output;
   Output.HPos = Input.HPos;
   Output.Tex0 = Input.Tex0;
   return Output;
}

/*
   Pixel shaders.

   ---

   AutoExposure(): https://knarkowicz.wordpress.com/2016/01/09/automatic-exposure/
*/

float3 GetAutoExposure(float3 Color, float2 Tex)
{
   float LumaAverage = exp(tex2Dlod(SampleLumaTex, float4(Tex, 0.0, 99.0)).r);
   float Ev100 = log2(LumaAverage * 100.0 / 12.5);
   Ev100 -= _ManualBias; // Optional manual bias.
   float Exposure = 1.0 / (1.2 * exp2(Ev100));
   return Color * Exposure;
}

float4 PS_GenerateAverageLuma(VS2PS Input) : COLOR0
{
   float4 Color = tex2D(SampleColorTex, Input.Tex0);
   float3 Luma = max(Color.r, max(Color.g, Color.b));

   // OutputColor0.rgb = Highest brightness from red/green/blue components.
   // OutputColor0.a = Weight for temporal blending.
   float Delay = 1e-3 * _Frametime;
   return float4(log(max(Luma.rgb, 1e-2)), saturate(Delay * _SmoothingSpeed));
}

float3 PS_Exposure(VS2PS Input) : COLOR0
{
   float4 Color = tex2D(SampleColorTex, Input.Tex0);
   return GetAutoExposure(Color.rgb, Input.Tex0);
}

technique AutoExposure
{
   // Pass0: Renders to a self-blending texture.
   // NOTE: Ensure no other shader overwrites this texture.
   pass GenerateAverageLuma
   {
         // Enable hardware blending.
         BlendEnable = TRUE;
         BlendOp = ADD;
         SrcBlend = SRCALPHA;
         DestBlend = INVSRCALPHA;

         VertexShader = VS_Quad;
         PixelShader = PS_GenerateAverageLuma;
   }

   // Pass1: Applies auto-exposure using the texture from Pass0.
   pass ApplyAutoExposure
   {
         VertexShader = VS_Quad;
         PixelShader = PS_Exposure;
   }
}

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