T73434: Improve Weight Paint Overlay Drawing.

Master multiplied the weight paint on top of the rendered image. This
reduced readability.

This patch removes the multiplication for weight painting and adds a
hint of the geometry below the overlay.

Reviewed By: Mets, pablodp606, campbellbarton

Maniphest Tasks: T73434

Differential Revision: https://developer.blender.org/D12170

6 files changed, 52 insertions, 31 deletions

diff --git a/source/blender/draw/engines/overlay/overlay_paint.c b/source/blender/draw/engines/overlay/overlay_paint.c
index d52640ed174..60a90616d29 100644
--- a/source/blender/draw/engines/overlay/overlay_paint.c
+++ b/source/blender/draw/engines/overlay/overlay_paint.c
@@ -92,18 +92,26 @@ void OVERLAY_paint_cache_init(OVERLAY_Data *vedata)
     case CTX_MODE_PAINT_WEIGHT: {
       opacity = is_edit_mode ? 1.0 : pd->overlay.weight_paint_mode_opacity;
       if (opacity > 0.0f) {
-        state = DRW_STATE_WRITE_COLOR | DRW_STATE_DEPTH_EQUAL;
-        state |= pd->painting.alpha_blending ? DRW_STATE_BLEND_ALPHA : DRW_STATE_BLEND_MUL;
+        state = DRW_STATE_WRITE_COLOR | DRW_STATE_DEPTH_EQUAL | DRW_STATE_BLEND_ALPHA;
         DRW_PASS_CREATE(psl->paint_color_ps, state | pd->clipping_state);
 
-        sh = OVERLAY_shader_paint_weight();
+        const bool do_shading = draw_ctx->v3d->shading.type != OB_WIRE;
+
+        sh = OVERLAY_shader_paint_weight(do_shading);
         pd->paint_surf_grp = grp = DRW_shgroup_create(sh, psl->paint_color_ps);
         DRW_shgroup_uniform_block(grp, "globalsBlock", G_draw.block_ubo);
         DRW_shgroup_uniform_bool_copy(grp, "drawContours", draw_contours);
-        DRW_shgroup_uniform_bool_copy(grp, "useAlphaBlend", pd->painting.alpha_blending);
         DRW_shgroup_uniform_float_copy(grp, "opacity", opacity);
         DRW_shgroup_uniform_texture(grp, "colorramp", G_draw.weight_ramp);
 
+        /* Arbitrary light to give a hint of the geometry behind the weights. */
+        if (do_shading) {
+          float light_dir[3];
+          copy_v3_fl3(light_dir, 0.0f, 0.5f, 0.86602f);
+          normalize_v3(light_dir);
+          DRW_shgroup_uniform_vec3_copy(grp, "light_dir", light_dir);
+        }
+
         if (pd->painting.alpha_blending) {
           state = DRW_STATE_WRITE_DEPTH | DRW_STATE_DEPTH_LESS_EQUAL;
           DRW_PASS_CREATE(psl->paint_depth_ps, state | pd->clipping_state);
@@ -257,17 +265,10 @@ void OVERLAY_paint_draw(OVERLAY_Data *vedata)
 
   OVERLAY_PassList *psl = vedata->psl;
   OVERLAY_FramebufferList *fbl = vedata->fbl;
-  DefaultFramebufferList *dfbl = DRW_viewport_framebuffer_list_get();
 
   if (DRW_state_is_fbo()) {
-    if (pd->painting.alpha_blending) {
-      GPU_framebuffer_bind(pd->painting.in_front ? fbl->overlay_in_front_fb :
-                                                   fbl->overlay_default_fb);
-    }
-    else {
-      /* Paint overlay needs final color because of multiply blend mode. */
-      GPU_framebuffer_bind(pd->painting.in_front ? dfbl->in_front_fb : dfbl->default_fb);
-    }
+    GPU_framebuffer_bind(pd->painting.in_front ? fbl->overlay_in_front_fb :
+                                                 fbl->overlay_default_fb);
   }
 
   if (psl->paint_depth_ps) {
diff --git a/source/blender/draw/engines/overlay/overlay_private.h b/source/blender/draw/engines/overlay/overlay_private.h
index 03bfaf56f24..68f60bee779 100644
--- a/source/blender/draw/engines/overlay/overlay_private.h
+++ b/source/blender/draw/engines/overlay/overlay_private.h
@@ -736,7 +736,7 @@ GPUShader *OVERLAY_shader_paint_face(void);
 GPUShader *OVERLAY_shader_paint_point(void);
 GPUShader *OVERLAY_shader_paint_texture(void);
 GPUShader *OVERLAY_shader_paint_vertcol(void);
-GPUShader *OVERLAY_shader_paint_weight(void);
+GPUShader *OVERLAY_shader_paint_weight(bool shading);
 GPUShader *OVERLAY_shader_paint_wire(void);
 GPUShader *OVERLAY_shader_particle_dot(void);
 GPUShader *OVERLAY_shader_particle_shape(void);
diff --git a/source/blender/draw/engines/overlay/overlay_shader.c b/source/blender/draw/engines/overlay/overlay_shader.c
index 7a7ae9a921b..edf9148c8c0 100644
--- a/source/blender/draw/engines/overlay/overlay_shader.c
+++ b/source/blender/draw/engines/overlay/overlay_shader.c
@@ -211,7 +211,7 @@ typedef struct OVERLAY_Shaders {
   GPUShader *paint_point;
   GPUShader *paint_texture;
   GPUShader *paint_vertcol;
-  GPUShader *paint_weight;
+  GPUShader *paint_weight[2];
   GPUShader *paint_wire;
   GPUShader *particle_dot;
   GPUShader *particle_shape;
@@ -1334,13 +1334,14 @@ GPUShader *OVERLAY_shader_paint_vertcol(void)
   return sh_data->paint_vertcol;
 }
 
-GPUShader *OVERLAY_shader_paint_weight(void)
+GPUShader *OVERLAY_shader_paint_weight(const bool shading)
 {
+  int index = shading ? 1 : 0;
   const DRWContextState *draw_ctx = DRW_context_state_get();
   const GPUShaderConfigData *sh_cfg = &GPU_shader_cfg_data[draw_ctx->sh_cfg];
   OVERLAY_Shaders *sh_data = &e_data.sh_data[draw_ctx->sh_cfg];
-  if (!sh_data->paint_weight) {
-    sh_data->paint_weight = GPU_shader_create_from_arrays({
+  if (!sh_data->paint_weight[index]) {
+    sh_data->paint_weight[index] = GPU_shader_create_from_arrays({
         .vert = (const char *[]){sh_cfg->lib,
                                  datatoc_common_globals_lib_glsl,
                                  datatoc_common_view_lib_glsl,
@@ -1349,10 +1350,10 @@ GPUShader *OVERLAY_shader_paint_weight(void)
         .frag = (const char *[]){datatoc_common_globals_lib_glsl,
                                  datatoc_paint_weight_frag_glsl,
                                  NULL},
-        .defs = (const char *[]){sh_cfg->def, NULL},
+        .defs = (const char *[]){sh_cfg->def, shading ? "#define FAKE_SHADING\n" : "", NULL},
     });
   }
-  return sh_data->paint_weight;
+  return sh_data->paint_weight[index];
 }
 
 GPUShader *OVERLAY_shader_paint_wire(void)
diff --git a/source/blender/draw/engines/overlay/shaders/paint_weight_frag.glsl b/source/blender/draw/engines/overlay/shaders/paint_weight_frag.glsl
index 0020d76ed6a..8009713d655 100644
--- a/source/blender/draw/engines/overlay/shaders/paint_weight_frag.glsl
+++ b/source/blender/draw/engines/overlay/shaders/paint_weight_frag.glsl
@@ -1,12 +1,12 @@
 
 in vec2 weight_interp; /* (weight, alert) */
+in float color_fac;
 
 out vec4 fragColor;
 
 uniform float opacity = 1.0;
 uniform sampler1D colorramp;
 
-uniform bool useAlphaBlend = false;
 uniform bool drawContours = false;
 
 float contours(float value, float steps, float width_px, float max_rel_width, float gradient)
@@ -68,6 +68,13 @@ vec4 contour_grid(float weight, float weight_gradient)
   return grid * clamp((weight_gradient - flt_eps) / flt_eps, 0.0, 1.0);
 }
 
+vec4 apply_color_fac(vec4 color_in)
+{
+  vec4 color = color_in;
+  color.rgb = max(vec3(0.005), color_in.rgb) * color_fac;
+  return color;
+}
+
 void main()
 {
   float alert = weight_interp.y;
@@ -75,12 +82,13 @@ void main()
 
   /* Missing vertex group alert color. Uniform in practice. */
   if (alert > 1.1) {
-    color = colorVertexMissingData;
+    color = apply_color_fac(colorVertexMissingData);
   }
   /* Weights are available */
   else {
     float weight = weight_interp.x;
     vec4 weight_color = texture(colorramp, weight, 0);
+    weight_color = apply_color_fac(weight_color);
 
     /* Contour display */
     if (drawContours) {
@@ -93,14 +101,9 @@ void main()
     }
 
     /* Zero weight alert color. Nonlinear blend to reduce impact. */
-    color = mix(weight_color, colorVertexUnreferenced, alert * alert);
+    vec4 color_unreferenced = apply_color_fac(colorVertexUnreferenced);
+    color = mix(weight_color, color_unreferenced, alert * alert);
   }
 
-  if (useAlphaBlend) {
-    fragColor = vec4(color.rgb, opacity);
-  }
-  else {
-    /* mix with 1.0 -> is like opacity when using multiply blend mode */
-    fragColor = vec4(mix(vec3(1.0), color.rgb, opacity), 1.0);
-  }
+  fragColor = vec4(color.rgb, opacity);
 }
diff --git a/source/blender/draw/engines/overlay/shaders/paint_weight_vert.glsl b/source/blender/draw/engines/overlay/shaders/paint_weight_vert.glsl
index b3baa8c7b07..31b6dc42cf4 100644
--- a/source/blender/draw/engines/overlay/shaders/paint_weight_vert.glsl
+++ b/source/blender/draw/engines/overlay/shaders/paint_weight_vert.glsl
@@ -1,8 +1,13 @@
+#ifdef FAKE_SHADING
+uniform vec3 light_dir;
+#endif
 
 in float weight;
 in vec3 pos;
+in vec3 nor;
 
 out vec2 weight_interp; /* (weight, alert) */
+out float color_fac;
 
 void main()
 {
@@ -14,6 +19,16 @@ void main()
   /* Separate actual weight and alerts for independent interpolation */
   weight_interp = max(vec2(weight, -weight), 0.0);
 
+  /* Saturate the weight to give a hint of the geometry behind the weights. */
+#ifdef FAKE_SHADING
+  vec3 view_normal = normalize(normal_object_to_view(nor));
+  color_fac = abs(dot(view_normal, light_dir));
+  color_fac = color_fac * 0.9 + 0.1;
+
+#else
+  color_fac = 1.0;
+#endif
+
 #ifdef USE_WORLD_CLIP_PLANES
   world_clip_planes_calc_clip_distance(world_pos);
 #endif
diff --git a/source/blender/draw/tests/shaders_test.cc b/source/blender/draw/tests/shaders_test.cc
index 0c7cbd4dac8..a9810b4cc77 100644
--- a/source/blender/draw/tests/shaders_test.cc
+++ b/source/blender/draw/tests/shaders_test.cc
@@ -267,7 +267,8 @@ static void test_overlay_glsl_shaders()
     EXPECT_NE(OVERLAY_shader_paint_point(), nullptr);
     EXPECT_NE(OVERLAY_shader_paint_texture(), nullptr);
     EXPECT_NE(OVERLAY_shader_paint_vertcol(), nullptr);
-    EXPECT_NE(OVERLAY_shader_paint_weight(), nullptr);
+    EXPECT_NE(OVERLAY_shader_paint_weight(false), nullptr);
+    EXPECT_NE(OVERLAY_shader_paint_weight(true), nullptr);
     EXPECT_NE(OVERLAY_shader_paint_wire(), nullptr);
     EXPECT_NE(OVERLAY_shader_particle_dot(), nullptr);
     EXPECT_NE(OVERLAY_shader_particle_shape(), nullptr);