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

5 files changed, 31 insertions, 7 deletions

diff --git a/source/blender/draw/modes/paint_weight_mode.c b/source/blender/draw/modes/paint_weight_mode.c
index f0dd1994e09..c4608dfd397 100644
--- a/source/blender/draw/modes/paint_weight_mode.c
+++ b/source/blender/draw/modes/paint_weight_mode.c
@@ -91,7 +91,7 @@ typedef struct PAINT_WEIGHT_PrivateData {
 static void PAINT_WEIGHT_engine_init(void *UNUSED(vedata))
 {
 	if (!e_data.weight_face_shader) {
-		e_data.weight_face_shader = GPU_shader_get_builtin_shader(GPU_SHADER_SIMPLE_LIGHTING_SMOOTH_COLOR_ALPHA);
+		e_data.weight_face_shader = GPU_shader_get_builtin_shader(GPU_SHADER_MULTIPLY_AND_BLEND_PREPROCESSING);
 	}
 
 	if (!e_data.wire_overlay_shader) {
@@ -129,15 +129,11 @@ static void PAINT_WEIGHT_cache_init(void *vedata)
 		/* Create a pass */
 		psl->weight_faces = DRW_pass_create(
 		        "Weight Pass",
-		        DRW_STATE_WRITE_COLOR | DRW_STATE_DEPTH_EQUAL | DRW_STATE_BLEND);
+		        DRW_STATE_WRITE_COLOR | DRW_STATE_DEPTH_EQUAL | DRW_STATE_MULTIPLY);
 
 		stl->g_data->fweights_shgrp = DRW_shgroup_create(e_data.weight_face_shader, psl->weight_faces);
 
-		static float light[3] = {-0.3f, 0.5f, 1.0f};
-		static float world_light = 1.0f;
-		DRW_shgroup_uniform_vec3(stl->g_data->fweights_shgrp, "light", light, 1);
-		DRW_shgroup_uniform_float(stl->g_data->fweights_shgrp, "alpha", &v3d->overlay.weight_paint_mode_opacity, 1);
-		DRW_shgroup_uniform_float(stl->g_data->fweights_shgrp, "global", &world_light, 1);
+		DRW_shgroup_uniform_float(stl->g_data->fweights_shgrp, "opacity", &v3d->overlay.weight_paint_mode_opacity, 1);
 	}
 
 	{
diff --git a/source/blender/gpu/CMakeLists.txt b/source/blender/gpu/CMakeLists.txt
index 20360cc62cb..5c010a58137 100644
--- a/source/blender/gpu/CMakeLists.txt
+++ b/source/blender/gpu/CMakeLists.txt
@@ -140,6 +140,7 @@ data_to_c_simple(shaders/gpu_shader_diag_stripes_frag.glsl SRC)
 data_to_c_simple(shaders/gpu_shader_simple_lighting_frag.glsl SRC)
 data_to_c_simple(shaders/gpu_shader_simple_lighting_smooth_color_frag.glsl SRC)
 data_to_c_simple(shaders/gpu_shader_simple_lighting_smooth_color_alpha_frag.glsl SRC)
+data_to_c_simple(shaders/gpu_shader_multiply_and_blend_preprocessing.glsl SRC)
 data_to_c_simple(shaders/gpu_shader_flat_color_frag.glsl SRC)
 data_to_c_simple(shaders/gpu_shader_flat_color_alpha_test_0_frag.glsl SRC)
 data_to_c_simple(shaders/gpu_shader_flat_id_frag.glsl SRC)
diff --git a/source/blender/gpu/GPU_shader.h b/source/blender/gpu/GPU_shader.h
index 21d5b5871fa..d14164e930e 100644
--- a/source/blender/gpu/GPU_shader.h
+++ b/source/blender/gpu/GPU_shader.h
@@ -118,6 +118,7 @@ typedef enum GPUBuiltinShader {
 	GPU_SHADER_SIMPLE_LIGHTING_FLAT_COLOR,
 	GPU_SHADER_SIMPLE_LIGHTING_SMOOTH_COLOR,
 	GPU_SHADER_SIMPLE_LIGHTING_SMOOTH_COLOR_ALPHA,
+	GPU_SHADER_MULTIPLY_AND_BLEND_PREPROCESSING,
 
 	/* for simple 2D drawing */
 	/**
diff --git a/source/blender/gpu/intern/gpu_shader.c b/source/blender/gpu/intern/gpu_shader.c
index 0f9e0b2008e..9feaed80c2f 100644
--- a/source/blender/gpu/intern/gpu_shader.c
+++ b/source/blender/gpu/intern/gpu_shader.c
@@ -64,6 +64,7 @@ extern char datatoc_gpu_shader_simple_lighting_frag_glsl[];
 extern char datatoc_gpu_shader_simple_lighting_flat_color_frag_glsl[];
 extern char datatoc_gpu_shader_simple_lighting_smooth_color_frag_glsl[];
 extern char datatoc_gpu_shader_simple_lighting_smooth_color_alpha_frag_glsl[];
+extern char datatoc_gpu_shader_multiply_and_blend_preprocessing_glsl[];
 extern char datatoc_gpu_shader_flat_color_frag_glsl[];
 extern char datatoc_gpu_shader_flat_color_alpha_test_0_frag_glsl[];
 extern char datatoc_gpu_shader_flat_id_frag_glsl[];
@@ -706,6 +707,9 @@ static const GPUShaderStages builtin_shader_stages[GPU_NUM_BUILTIN_SHADERS] = {
 	[GPU_SHADER_SIMPLE_LIGHTING_SMOOTH_COLOR_ALPHA] =
 		{ datatoc_gpu_shader_3D_normal_smooth_color_vert_glsl,
 		  datatoc_gpu_shader_simple_lighting_smooth_color_alpha_frag_glsl },
+	[GPU_SHADER_MULTIPLY_AND_BLEND_PREPROCESSING] =
+		{ datatoc_gpu_shader_3D_normal_smooth_color_vert_glsl,
+		  datatoc_gpu_shader_multiply_and_blend_preprocessing_glsl },
 
 	[GPU_SHADER_2D_IMAGE_MASK_UNIFORM_COLOR] =
 		{ datatoc_gpu_shader_3D_image_vert_glsl,
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;
+}