Bug 1700434 - Implement faster subpixel text blend mode for SWGL. r=jrmuizel

This adds a swgl_blendSubpixelText() extension that enables us to move some
of the complexity of plumbing dual-source blending out of the shader for
subpixel text. This will enable further speed-ups later by allowing us to use
swgl_commitTexture.

Differential Revision: https://phabricator.services.mozilla.com/D115456

gfx/wr/glsl-to-cxx/src/hir.rs

@@ -3844,6 +3844,13 @@ pub fn ast_to_hir(state: &mut State, tu: &syntax::TranslationUnit) -> Translatio
         Type::new(Void),
         vec![Type::new(Vec4)],
     );
+    declare_function(
+        state,
+        "swgl_blendSubpixelText",
+        None,
+        Type::new(Void),
+        vec![Type::new(Vec4)],
+    );
     declare_function(
         state,
         "swgl_clipMask",

gfx/wr/swgl/README.md

@@ -204,6 +204,8 @@ useful for implementing clip-out modes by inverting the source texture value.
 ```
 // Premultiplied alpha over blend, but with source color set to source alpha modulated with a constant color.
 void swgl_blendDropShadow(vec4 color);
+// Premultiplied alpha over blend, but treats the source as a subpixel mask modulated with a constant color.
+void swgl_blendSubpixelText(vec4 color);
 ```
 
 SWGL allows overriding the blend mode per-primitive by calling `swgl_blend`

gfx/wr/swgl/src/blend.h

@@ -340,6 +340,7 @@ enum SWGLClipFlag {
 static int swgl_ClipFlags = 0;
 static BlendKey swgl_BlendOverride = BLEND_KEY_NONE;
 static WideRGBA8 swgl_BlendColorRGBA8 = {0};
+static WideRGBA8 swgl_BlendAlphaRGBA8 = {0};
 
 // A pointer into the color buffer for the start of the span.
 static void* swgl_SpanBuf = nullptr;
@@ -689,6 +690,12 @@ static PREFER_INLINE WideRGBA8 blend_pixels(uint32_t* buf, PackedRGBA8 pdst,
     return color + dst - muldiv255(dst, alphas(color));
   }
 
+  BLEND_CASE(SWGL_BLEND_SUBPIXEL_TEXT):
+    // Premultiplied alpha over blend, but treats the source as a subpixel mask
+    // modulated with a constant color.
+    return applyColor(src, swgl_BlendColorRGBA8) + dst -
+           muldiv255(dst, applyColor(src, swgl_BlendAlphaRGBA8));
+
   default:
     UNREACHABLE;
     // return src;

gfx/wr/swgl/src/gl.cc

@@ -619,7 +619,8 @@ struct Program {
   macro(GL_HSL_SATURATION_KHR, 0, 0, 0)                                        \
   macro(GL_HSL_COLOR_KHR, 0, 0, 0)                                             \
   macro(GL_HSL_LUMINOSITY_KHR, 0, 0, 0)                                        \
-  macro(SWGL_BLEND_DROP_SHADOW, 0, 0, 0)
+  macro(SWGL_BLEND_DROP_SHADOW, 0, 0, 0)                                       \
+  macro(SWGL_BLEND_SUBPIXEL_TEXT, 0, 0, 0)
 
 #define DEFINE_BLEND_KEY(...) BLEND_KEY(__VA_ARGS__),
 #define DEFINE_MASK_BLEND_KEY(...) MASK_BLEND_KEY(__VA_ARGS__),

gfx/wr/swgl/src/gl_defs.h

@@ -213,3 +213,4 @@ typedef intptr_t GLintptr;
 #define GL_HSL_LUMINOSITY_KHR 0x92B0
 
 #define SWGL_BLEND_DROP_SHADOW 0xB001
+#define SWGL_BLEND_SUBPIXEL_TEXT 0xB002

gfx/wr/swgl/src/swgl_ext.h

@@ -1765,6 +1765,14 @@ static ALWAYS_INLINE int calcAAEdgeMask(bvec4_scalar mask) {
     swgl_BlendColorRGBA8 = packColor<uint32_t>(color);      \
   } while (0)
 
+#define swgl_blendSubpixelText(color)                         \
+  do {                                                        \
+    swgl_ClipFlags |= SWGL_CLIP_FLAG_BLEND_OVERRIDE;          \
+    swgl_BlendOverride = BLEND_KEY(SWGL_BLEND_SUBPIXEL_TEXT); \
+    swgl_BlendColorRGBA8 = packColor<uint32_t>(color);        \
+    swgl_BlendAlphaRGBA8 = alphas(swgl_BlendColorRGBA8);      \
+  } while (0)
+
 // Dispatch helper used by the GLSL translator to swgl_drawSpan functions.
 // The number of pixels committed is tracked by checking for the difference in
 // swgl_SpanLength. Any varying interpolants used will be advanced past the

gfx/wr/webrender/res/ps_text_run.glsl

@@ -245,8 +245,14 @@ void main() {
             v_color = vec4(text.color.a) * text.bg_color;
             break;
         case COLOR_MODE_SUBPX_DUAL_SOURCE:
-            v_mask_swizzle = vec3(text.color.a, 0.0, 0.0);
-            v_color = text.color;
+            #ifdef SWGL_BLEND
+                swgl_blendSubpixelText(text.color);
+                v_mask_swizzle = vec3(1.0, 0.0, 0.0);
+                v_color = vec4(1.0);
+            #else
+                v_mask_swizzle = vec3(text.color.a, 0.0, 0.0);
+                v_color = text.color;
+            #endif
             break;
         default:
             v_mask_swizzle = vec3(0.0, 0.0, 0.0);
@@ -283,7 +289,7 @@ Fragment text_fs(void) {
 
     frag.color = v_color * mask;
 
-    #ifdef WR_FEATURE_DUAL_SOURCE_BLENDING
+    #if defined(WR_FEATURE_DUAL_SOURCE_BLENDING) && !defined(SWGL_BLEND)
         frag.blend = mask * v_mask_swizzle.x + mask.aaaa * v_mask_swizzle.y;
     #endif
 
@@ -299,7 +305,7 @@ void main() {
 
     #if defined(WR_FEATURE_DEBUG_OVERDRAW)
         oFragColor = WR_DEBUG_OVERDRAW_COLOR;
-    #elif defined(WR_FEATURE_DUAL_SOURCE_BLENDING)
+    #elif defined(WR_FEATURE_DUAL_SOURCE_BLENDING) && !defined(SWGL_BLEND)
         oFragColor = frag.color;
         oFragBlend = frag.blend * clip_mask;
     #else