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.

4 files changed, 27 insertions, 35 deletions

diff --git a/source/blender/draw/engines/workbench/shaders/workbench_common_lib.glsl b/source/blender/draw/engines/workbench/shaders/workbench_common_lib.glsl
index 8908891d7e6..0b494aa019f 100644
--- a/source/blender/draw/engines/workbench/shaders/workbench_common_lib.glsl
+++ b/source/blender/draw/engines/workbench/shaders/workbench_common_lib.glsl
@@ -65,11 +65,11 @@ void fresnel(vec3 I, vec3 N, float ior, out float kr)
 	// kt = 1 - kr;
 }
 
-vec4 calculate_transparent_accum(vec4 premultiplied) {
-	float a = min(1.0, premultiplied.a) * 8.0 + 0.01;
+float calculate_transparent_weight(float alpha) {
+	/* Eq 10 */
+	float a = min(1.0, alpha) * 8.0 + 0.01;
 	float b = -gl_FragCoord.z * 0.95 + 1.0;
-	float w = clamp(a * a * a * 1e8 * b * b * b, 1e-2, 3e2);
-	return premultiplied * w;
+	return alpha * clamp(a * a * a * 1e8 * b * b * b, 1e-2, 3e3);
 }
 
 vec3 view_vector_from_screen_uv(vec2 uv, vec4 viewvecs[3], mat4 proj_mat)
diff --git a/source/blender/draw/engines/workbench/shaders/workbench_forward_composite_frag.glsl b/source/blender/draw/engines/workbench/shaders/workbench_forward_composite_frag.glsl
index f335e1a15bd..1d9f37274bd 100644
--- a/source/blender/draw/engines/workbench/shaders/workbench_forward_composite_frag.glsl
+++ b/source/blender/draw/engines/workbench/shaders/workbench_forward_composite_frag.glsl
@@ -2,9 +2,7 @@ out vec4 fragColor;
 
 uniform usampler2D objectId;
 uniform sampler2D transparentAccum;
-#ifdef WORKBENCH_REVEALAGE_ENABLED
 uniform sampler2D transparentRevealage;
-#endif
 uniform vec2 invertedViewportSize;
 
 layout(std140) uniform world_block {
@@ -16,29 +14,24 @@ void main()
 	ivec2 texel = ivec2(gl_FragCoord.xy);
 	vec2 uv_viewport = gl_FragCoord.xy * invertedViewportSize;
 	uint object_id = texelFetch(objectId, texel, 0).r;
-	vec4 transparent_accum = texelFetch(transparentAccum, texel, 0);
-#ifdef WORKBENCH_REVEALAGE_ENABLED
-	float transparent_revealage = texelFetch(transparentRevealage, texel, 0).r;
-#endif
-	vec4 color;
-	vec4 bg_color;
+
+	/* Listing 4 */
+	vec4 trans_accum = texelFetch(transparentAccum, texel, 0);
+	float trans_revealage = trans_accum.a;
+	trans_accum.a = texelFetch(transparentRevealage, texel, 0).r;
 
 #ifdef V3D_SHADING_OBJECT_OUTLINE
 	float outline = calculate_object_outline(objectId, texel, object_id);
 #else /* V3D_SHADING_OBJECT_OUTLINE */
 	float outline = 1.0;
 #endif /* V3D_SHADING_OBJECT_OUTLINE */
-	bg_color = vec4(background_color(world_data, uv_viewport.y), 0.0);
-	if (object_id == NO_OBJECT_ID) {
-		color = bg_color;
-	} else {
-#ifdef WORKBENCH_REVEALAGE_ENABLED
-		color = vec4((transparent_accum.xyz / max(transparent_accum.a, EPSILON)) * (1.0 - transparent_revealage), 1.0);
-		color = mix(bg_color, color, clamp(1.0 - transparent_revealage, 0.0, 1.0));
-#else
-		color = vec4(transparent_accum.xyz / max(transparent_accum.a, EPSILON), 1.0);
-#endif
-	}
-
-	fragColor = vec4(mix(world_data.object_outline_color.rgb, color.xyz, outline), 1.0);
+	vec3 bg_color = background_color(world_data, uv_viewport.y);
+
+	/* TODO: Bypass the whole shader if there is no xray pass and no outline pass. */
+	vec3 trans_color = trans_accum.rgb / clamp(trans_accum.a, 1e-4, 5e4);
+	vec3 color = mix(trans_color, bg_color, trans_revealage);
+
+	color = mix(world_data.object_outline_color.rgb, color, outline);
+
+	fragColor = vec4(color, 1.0);
 }
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;
 }
 
diff --git a/source/blender/draw/engines/workbench/shaders/workbench_forward_transparent_revealage_frag.glsl b/source/blender/draw/engines/workbench/shaders/workbench_forward_transparent_revealage_frag.glsl
deleted file mode 100644
index c591425b950..00000000000
--- a/source/blender/draw/engines/workbench/shaders/workbench_forward_transparent_revealage_frag.glsl
+++ /dev/null
@@ -1,7 +0,0 @@
-layout(location=0) out float transparentRevealage;
-uniform float alpha = 0.5;
-void main()
-{
-	transparentRevealage = alpha;
-}
-