Workbench: Xray: Optimize and fix implementation.

There was a method explained in the Weighted Blended Order-Independent
Transparency paper to support hardware that does not support per render
target blending function.

So now only 2 geometry passes are required instead of 3 (one being the
outline/depth fill pass).

This also fix how the blending is done. There was some premult confusion
in the implementation.

1 files changed, 9 insertions, 3 deletions

diff --git a/source/blender/draw/engines/workbench/shaders/workbench_forward_transparent_accum_frag.glsl b/source/blender/draw/engines/workbench/shaders/workbench_forward_transparent_accum_frag.glsl
index f1eacd281b1..53903a4af76 100644
--- a/source/blender/draw/engines/workbench/shaders/workbench_forward_transparent_accum_frag.glsl
+++ b/source/blender/draw/engines/workbench/shaders/workbench_forward_transparent_accum_frag.glsl
@@ -26,7 +26,7 @@ layout(std140) uniform material_block {
 };
 
 layout(location=0) out vec4 transparentAccum;
-
+layout(location=1) out float revealageAccum; /* revealage actually stored in transparentAccum.a */
 
 void main()
 {
@@ -70,7 +70,13 @@ void main()
 
 	vec3 shaded_color = diffuse_light * diffuse_color.rgb + specular_color;
 
-	vec4 premultiplied = vec4(shaded_color.rgb * alpha, alpha);
-	transparentAccum = calculate_transparent_accum(premultiplied);
+	/* Based on :
+	 * McGuire and Bavoil, Weighted Blended Order-Independent Transparency, Journal of
+	 * Computer Graphics Techniques (JCGT), vol. 2, no. 2, 122–141, 2013
+	 */
+	/* Listing 4 */
+	float weight = calculate_transparent_weight(alpha);
+	transparentAccum = vec4(shaded_color * weight, alpha);
+	revealageAccum = weight;
 }