[client] egl: correct CAS processing for HDR

Convert HDR scRGB input to normalized linear Rec.2020 before CAS, then
convert the filtered result back to scRGB. Use the content metadata to
select an appropriate normalization peak while leaving SDR unchanged.

This follows AMD's documented recommendation for applying CAS to HDR
content and prevents HDR highlights from being clamped and dimmed.

client/renderers/EGL/desktop.c

@@ -42,6 +42,13 @@
 #include "postprocess.h"
 #include "filters.h"
 
+enum
+{
+  HDR_PEAK_LUMINANCE_MIN     = 80,
+  HDR_PEAK_LUMINANCE_DEFAULT = 10000,
+  HDR_PEAK_LUMINANCE_MAX     = 10000
+};
+
 struct DesktopShader
 {
   EGL_Shader * shader;
@@ -482,12 +489,24 @@ bool egl_desktopRender(EGL_Desktop * desktop, unsigned int outputWidth,
 
   *fullFrame = false;
   const bool hdr = desktop->hdr && !desktop->useSpice;
+  uint32_t hdrPeak = 0;
+  if (hdr)
+  {
+    hdrPeak = max(desktop->format.hdrMaxDisplayLuminance,
+        desktop->format.hdrMaxContentLightLevel);
+    if (!hdrPeak)
+      hdrPeak = HDR_PEAK_LUMINANCE_DEFAULT;
+    else
+      hdrPeak = clamp(hdrPeak,
+          HDR_PEAK_LUMINANCE_MIN, HDR_PEAK_LUMINANCE_MAX);
+  }
+
   const bool processFrame = atomic_exchange(&desktop->processFrame, false) ||
     egl_postProcessConfigModified(desktop->pp);
   if (processFrame &&
       egl_postProcessRun(desktop->pp, tex, desktop->mesh,
         width, height, outputWidth, outputHeight, dma,
-        hdr && desktop->hdrPQ) &&
+        hdr && desktop->hdrPQ, (float)hdrPeak) &&
       egl_postProcessNeedsFullFrame(desktop->pp))
   {
     /* The filter output may have changed everywhere, but this only applies to

client/renderers/EGL/filter.h

@@ -33,6 +33,8 @@ typedef struct EGL_FilterRects
   EGL_DesktopRects * rects;
   GLfloat * matrix;
   int width, height;
+  /* zero for SDR, otherwise the HDR signal peak in cd/m² */
+  float hdrPeak;
 }
 EGL_FilterRects;
 

client/renderers/EGL/filter_ffx_cas.c

@@ -29,12 +29,15 @@
 #include "basic.vert.h"
 #include "ffx_cas.frag.h"
 
+#define CAS_SCRGB_REFERENCE_WHITE 80.0f
+
 typedef struct EGL_FilterFFXCAS
 {
   EGL_Filter base;
 
   EGL_Shader     * shader;
   EGL_Uniform    * uConsts;
+  EGL_Uniform    * uHDRScale;
   EGL_Effect     * effect;
   EGL_EffectPass * pass;
   bool             enable;
@@ -133,6 +136,7 @@ static bool egl_filterFFXCASInit(EGL_Filter ** filter)
   }
 
   this->uConsts   = egl_shaderGetUniform(this->shader, "uConsts"  );
+  this->uHDRScale = egl_shaderGetUniform(this->shader, "uHDRScale");
   egl_filterFFXCASLoadState(&this->base);
 
   *filter = &this->base;
@@ -270,6 +274,10 @@ static EGL_Texture * egl_filterFFXCASRun(EGL_Filter * filter,
 {
   EGL_FilterFFXCAS * this = UPCAST(EGL_FilterFFXCAS, filter);
 
+  const float hdrScale = rects->hdrPeak > 0.0f ?
+    CAS_SCRGB_REFERENCE_WHITE / rects->hdrPeak : 0.0f;
+  egl_uniform1f(this->uHDRScale, hdrScale);
+
   return egl_effectRun(this->effect, rects, texture);
 }
 

client/renderers/EGL/postprocess.c

@@ -876,7 +876,8 @@ static bool configureChain(EGL_PostProcess * this, EGL_PixelFormat pixFmt,
 
 bool egl_postProcessRun(EGL_PostProcess * this, EGL_Texture * tex,
     EGL_DesktopRects * rects, int desktopWidth, int desktopHeight,
-    unsigned int targetX, unsigned int targetY, bool useDMA, bool hdrPQ)
+    unsigned int targetX, unsigned int targetY, bool useDMA, bool hdrPQ,
+    float hdrPeak)
 {
   if (targetX == 0 && targetY == 0)
     DEBUG_FATAL("targetX || targetY == 0");
@@ -922,6 +923,7 @@ bool egl_postProcessRun(EGL_PostProcess * this, EGL_Texture * tex,
     .matrix  = this->matrix,
     .width   = desktopWidth,
     .height  = desktopHeight,
+    .hdrPeak = hdrPeak,
   };
 
   EGL_Texture * texture = tex;

client/renderers/EGL/postprocess.h

@@ -47,7 +47,8 @@ bool egl_postProcessNeedsFullFrame(EGL_PostProcess * this);
  * targetX/Y is the final target output dimension hint if scalers are present */
 bool egl_postProcessRun(EGL_PostProcess * this, EGL_Texture * tex,
     EGL_DesktopRects * rects, int desktopWidth, int desktopHeight,
-    unsigned int targetX, unsigned int targetY, bool useDMA, bool hdrPQ);
+    unsigned int targetX, unsigned int targetY, bool useDMA, bool hdrPQ,
+    float hdrPeak);
 
 /* Return the current output texture and whether it remains PQ encoded. */
 EGL_Texture * egl_postProcessGetOutput(EGL_PostProcess * this,

client/renderers/EGL/shader/ffx_cas.frag

@@ -5,12 +5,14 @@ precision highp float;
 precision highp int;
 
 #include "compat.h"
+#include "hdr.h"
 
 in  vec2  fragCoord;
 out vec4  fragColor;
 
 uniform sampler2D sampler1;
 uniform uvec4     uConsts[2];
+uniform float     uHDRScale;
 
 #define A_GPU 1
 #define A_GLSL 1
@@ -19,7 +21,11 @@ uniform uvec4     uConsts[2];
 
 vec3 imageLoad(ivec2 point)
 {
-  return texelFetch(sampler1, point, 0).rgb;
+  vec3 color = texelFetch(sampler1, point, 0).rgb;
+  if (uHDRScale > 0.0)
+    color = max(bt709to2020(color * uHDRScale), vec3(0.0));
+
+  return color;
 }
 
 AF3 CasLoad(ASU2 p)
@@ -39,4 +45,9 @@ void main()
   CasFilter(
     fragColor.r, fragColor.g, fragColor.b,
     point, uConsts[0], uConsts[1], true);
+
+  if (uHDRScale > 0.0)
+    fragColor.rgb = bt2020to709(fragColor.rgb) / uHDRScale;
+
+  fragColor.a = 1.0;
 }