EEVEE: Fix volumetric resolve in large scenes.

On NVIDIA volumetric resolve failed for large production scenes.
The result would remove most color from the final render. The cause
seems to be a faulty driver.

This change ported the fragment shader to a compute shader which
would select a different compiler branch and didn't show the error.

1 files changed, 38 insertions, 0 deletions

diff --git a/source/blender/draw/engines/eevee/shaders/volumetric_resolve_comp.glsl b/source/blender/draw/engines/eevee/shaders/volumetric_resolve_comp.glsl
new file mode 100644
index 00000000000..2b0139ff923
--- /dev/null
+++ b/source/blender/draw/engines/eevee/shaders/volumetric_resolve_comp.glsl
@@ -0,0 +1,38 @@
+
+#pragma BLENDER_REQUIRE(volumetric_lib.glsl)
+
+/* Based on Frosbite Unified Volumetric.
+ * https://www.ea.com/frostbite/news/physically-based-unified-volumetric-rendering-in-frostbite */
+
+/* Step 4 : Apply final integration on top of the scene color. */
+
+uniform sampler2D inSceneDepth;
+
+layout(local_size_x = 1, local_size_y = 1) in;
+
+#ifdef TARGET_IMG_FLOAT
+layout(binding = 0, rgba32f) uniform image2D target_img;
+#else
+layout(binding = 0, rgba16f) uniform image2D target_img;
+#endif
+
+void main()
+{
+  ivec2 co = ivec2(gl_GlobalInvocationID.xy);
+  vec2 uvs = co / vec2(textureSize(inSceneDepth, 0));
+  float scene_depth = texture(inSceneDepth, uvs).r;
+
+  vec3 transmittance, scattering;
+  volumetric_resolve(uvs, scene_depth, transmittance, scattering);
+
+  /* Approximate volume alpha by using a monochromatic transmittance
+   * and adding it to the scene alpha. */
+  float alpha = dot(transmittance, vec3(1.0 / 3.0));
+
+  vec4 color0 = vec4(scattering, 1.0 - alpha);
+  vec4 color1 = vec4(transmittance, alpha);
+
+  vec4 color_in = imageLoad(target_img, co);
+  vec4 color_out = color0 + color1 * color_in;
+  imageStore(target_img, co, color_out);
+}