Weight Paint: Multiply overlay on the mesh

Use the multiply blending mode for the weight paint overlay.
To support the opacity slider, we need a new shader. Otherwise this combination of multiplication and mixing does not seem to be supported by glBlendFunc.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D3727

1 files changed, 22 insertions, 0 deletions

diff --git a/source/blender/gpu/shaders/gpu_shader_multiply_and_blend_preprocessing.glsl b/source/blender/gpu/shaders/gpu_shader_multiply_and_blend_preprocessing.glsl
new file mode 100644
index 00000000000..88d84aeaef6
--- /dev/null
+++ b/source/blender/gpu/shaders/gpu_shader_multiply_and_blend_preprocessing.glsl
@@ -0,0 +1,22 @@
+uniform float opacity;
+
+in vec4 finalColor;
+out vec4 fragColor;
+
+/* Blend Mode goal:
+ *     First multiply the foreground and background and then mix the result
+ *     of that with the background based on a opacity value.
+ *
+ *     result = background * foreground * opacity + background * (1 - opacity)
+ *            = background * (foreground * opacity + (1 - opacity))
+ *                           <------------------------------------>
+ *                                 computed in this shader
+ *
+ * Afterwards the background and the new foreground only have to be multiplied.
+ */
+
+void main()
+{
+	fragColor = finalColor * opacity + (1 - opacity);
+	fragColor.a = finalColor.a;
+}