Fixed color grading curves applying a slight shift in darks

Still a very tiny shift in the highlights (can be seen using the
waveform) but it's far less pronunced that it was in the darks before.

Author: Chman

PostProcessing/Resources/Shaders/ColorGrading.cginc

@@ -193,16 +193,20 @@ half RotateHue(half value, half low, half hi)
 //
 half3 YrgbCurve(half3 c, sampler2D curveTex)
 {
+    const float kHalfPixel = (1.0 / 128.0) / 2.0;
+
     // Y
-    half mr = tex2D(curveTex, half2(c.r, 0.75)).a;
-    half mg = tex2D(curveTex, half2(c.g, 0.75)).a;
-    half mb = tex2D(curveTex, half2(c.b, 0.75)).a;
-    c = saturate(half3(mr, mg, mb));
+    c += kHalfPixel.xxx;
+    float mr = tex2D(curveTex, float2(c.r, 0.75)).a;
+    float mg = tex2D(curveTex, float2(c.g, 0.75)).a;
+    float mb = tex2D(curveTex, float2(c.b, 0.75)).a;
+    c = saturate(float3(mr, mg, mb));
 
     // RGB
-    half r = tex2D(curveTex, half2(c.r, 0.75)).r;
-    half g = tex2D(curveTex, half2(c.g, 0.75)).g;
-    half b = tex2D(curveTex, half2(c.b, 0.75)).b;
+    c += kHalfPixel.xxx;
+    float r = tex2D(curveTex, float2(c.r, 0.75)).r;
+    float g = tex2D(curveTex, float2(c.g, 0.75)).g;
+    float b = tex2D(curveTex, float2(c.b, 0.75)).b;
     return saturate(half3(r, g, b));
 }
 

PostProcessing/Resources/Shaders/LutGen.shader

@@ -84,7 +84,7 @@ Shader "Hidden/Post FX/Lut Generator"
         #endif
 
             // YRGB curves (done in linear/LDR for now)
-            color = saturate(YrgbCurve(color, _Curves));
+            color = YrgbCurve(color, _Curves);
 
             return color;
         }