HDR: Improve ACES tone mapping curve

The previous one tended to over-expose the final image.

Shaders/HDR/exposure-include.frag

@@ -9,12 +9,11 @@ float log10(float x)
     return one_over_log10 * log(x);
 }
 
-// Custom exposure curve based on the one proposed on
+// Exposure curve from 'Perceptual Effects in Real-time Tone Mapping'.
-// 'Perceptual Effects in Real-time Tone Mapping'.
 // http://resources.mpi-inf.mpg.de/hdr/peffects/krawczyk05sccg.pdf
 float keyValue(float L)
 {
-    return 0.82 - 2.0 / (log10(L + 0.84) + 2.6);
+    return 1.03 - 2.0 / (log10(L + 1.0) + 2.0);
 }
 
 vec3 applyExposure(vec3 color, float avgLuminance, float threshold)

Shaders/HDR/postprocess.frag

@@ -15,6 +15,32 @@ uniform bool debug_ev100;
 
 vec3 applyExposure(vec3 color, float avgLuminance, float threshold);
 
+/**
+ * ACES tone mapping
+ * From 'Baking Lab' by MJP and David Neubelt
+ * Original by Stephen Hill
+ * https://github.com/TheRealMJP/BakingLab/blob/master/BakingLab/ACES.hlsl
+ * Licensed under the MIT license
+ */
+// sRGB => XYZ => D65_2_D60 => AP1 => RRT_SAT
+const mat3 ACESInputMat = mat3(
+    0.59719, 0.07600, 0.02840,
+    0.35458, 0.90834, 0.13383,
+    0.04823, 0.01566, 0.83777);
+// ODT_SAT => XYZ => D60_2_D65 => sRGB
+const mat3 ACESOutputMat = mat3(
+     1.60475, -0.10208, -0.00327,
+    -0.53108,  1.10813, -0.07276,
+    -0.07367, -0.00605,  1.07602);
+
+vec3 ACESFitted(vec3 color)
+{
+    vec3 v = ACESInputMat * color;
+    vec3 a = v * (v + 0.0245786) - 0.000090537;
+    vec3 b = v * (0.983729 * v + 0.4329510) + 0.238081;
+    return clamp(ACESOutputMat * (a / b), 0.0, 1.0);
+}
+
 vec3 getDebugColor(float value)
 {
     float level = value*3.14159265/2.0;
@@ -40,16 +66,6 @@ vec3 debugEV100(vec3 hdr, float avgLuminance)
     return getDebugColor(norm);
 }
 
-const float a = 2.51;
-const float b = 0.03;
-const float c = 2.43;
-const float d = 0.59;
-const float e = 0.14;
-vec3 ACESFilm(vec3 x)
-{
-    return clamp((x*(a*x+b))/(x*(c*x+d)+e), 0.0, 1.0);
-}
-
 vec3 encodeSRGB(vec3 linearRGB)
 {
     vec3 a = 12.92 * linearRGB;
@@ -71,7 +87,7 @@ void main()
     }
 
     // Tonemap
-    vec3 color = ACESFilm(exposedHdrColor);
+    vec3 color = ACESFitted(exposedHdrColor);
     // Gamma correction
     color = encodeSRGB(color);