Metal: GLSL shader compatibility 3rd pass

Undefined behaviour for divergent control-flow fixes, replacement for partial vector references, and resolution of a number of calculation precision issues occuring on macOS.

Authored by Apple: Michael Parkin-White

Ref: T96261

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D14437

3 files changed, 11 insertions, 3 deletions

diff --git a/source/blender/draw/engines/eevee/shaders/lightprobe_filter_visibility_frag.glsl b/source/blender/draw/engines/eevee/shaders/lightprobe_filter_visibility_frag.glsl
index d25ef23a706..681e69ae384 100644
--- a/source/blender/draw/engines/eevee/shaders/lightprobe_filter_visibility_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/lightprobe_filter_visibility_frag.glsl
@@ -15,7 +15,6 @@ uniform float visibilityRange;
 uniform float visibilityBlur;
 
 uniform float sampleCount;
-uniform float;
 
 out vec4 FragColor;
 
diff --git a/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl b/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl
index 9f1afc4767c..4ff42892f7d 100644
--- a/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl
@@ -44,8 +44,13 @@ void main()
   volumeObjectLocalCoord = (volumeObjectToTexture * vec4(volumeObjectLocalCoord, 1.0)).xyz;
 
   if (any(lessThan(volumeObjectLocalCoord, vec3(0.0))) ||
-      any(greaterThan(volumeObjectLocalCoord, vec3(1.0))))
+      any(greaterThan(volumeObjectLocalCoord, vec3(1.0)))) {
+    /* Note: Discard is not an explicit return in Metal prior to versions 2.3.
+     * adding return after discard ensures consistent behaviour and avoids GPU
+     * side-effects where control flow continues with undefined values. */
     discard;
+    return;
+  }
 #endif
 
 #ifdef CLEAR
diff --git a/source/blender/draw/engines/eevee/shaders/volumetric_integration_frag.glsl b/source/blender/draw/engines/eevee/shaders/volumetric_integration_frag.glsl
index 11f57c0a82e..527bbd18896 100644
--- a/source/blender/draw/engines/eevee/shaders/volumetric_integration_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/volumetric_integration_frag.glsl
@@ -70,7 +70,11 @@ void main()
     vec3 Tr = exp(-s_extinction * s_len);
 
     /* integrate along the current step segment */
-    Lscat = (Lscat - Lscat * Tr) / max(vec3(1e-8), s_extinction);
+    /* Note: Original calculation carries precision issues when compiling for AMD GPUs
+     * and running Metal. This version of the equation retains precision well for all
+     * macOS HW configurations. */
+    Lscat = (Lscat * (1.0f - Tr)) / max(vec3(1e-8), s_extinction);
+
     /* accumulate and also take into account the transmittance from previous steps */
     finalScattering += finalTransmittance * Lscat;