use specialization constant.

impeller/entity/contents/content_context.cc

@@ -404,7 +404,11 @@ ContentContext::ContentContext(
                                                  options_trianglestrip);
   srgb_to_linear_filter_pipelines_.CreateDefault(*context_,
                                                  options_trianglestrip);
-  glyph_atlas_pipelines_.CreateDefault(*context_, options);
+  glyph_atlas_pipelines_.CreateDefault(
+      *context_, options,
+      {static_cast<Scalar>(
+          GetContext()->GetCapabilities()->GetDefaultGlyphAtlasFormat() ==
+          PixelFormat::kA8UNormInt)});
   glyph_atlas_color_pipelines_.CreateDefault(*context_, options);
   geometry_color_pipelines_.CreateDefault(*context_, options);
   yuv_to_rgb_filter_pipelines_.CreateDefault(*context_, options_trianglestrip);

impeller/entity/shaders/glyph_atlas.frag

@@ -8,6 +8,8 @@ precision mediump float;
 
 uniform f16sampler2D glyph_atlas_sampler;
 
+layout(constant_id = 0) const float use_alpha_color_channel = 1.0;
+
 in highp vec2 v_uv;
 
 IMPELLER_MAYBE_FLAT in f16vec4 v_text_color;
@@ -16,5 +18,9 @@ out f16vec4 frag_color;
 
 void main() {
   f16vec4 value = texture(glyph_atlas_sampler, v_uv);
-  frag_color = vec4(max(value.r, value.a)) * v_text_color;
+  if (use_alpha_color_channel == 1.0) {
+    frag_color = value.aaaa * v_text_color;
+  } else {
+    frag_color = value.rrrr * v_text_color;
+  }
 }

impeller/typographer/backends/skia/typographer_context_skia.cc

@@ -470,7 +470,7 @@ std::shared_ptr<GlyphAtlas> TypographerContextSkia::CreateGlyphAtlas(
   PixelFormat format;
   switch (type) {
     case GlyphAtlas::Type::kAlphaBitmap:
-      format = PixelFormat::kR8UNormInt;
+      format = context.GetCapabilities()->GetDefaultGlyphAtlasFormat();
       break;
     case GlyphAtlas::Type::kColorBitmap:
       format = PixelFormat::kR8G8B8A8UNormInt;

impeller/typographer/backends/stb/typographer_context_stb.cc

@@ -516,11 +516,12 @@ std::shared_ptr<GlyphAtlas> TypographerContextSTB::CreateGlyphAtlas(
   PixelFormat format;
   switch (type) {
     case GlyphAtlas::Type::kAlphaBitmap:
-      format = PixelFormat::kR8UNormInt;
+      format = context.GetCapabilities()->GetDefaultGlyphAtlasFormat();
       break;
     case GlyphAtlas::Type::kColorBitmap:
-      format = DISABLE_COLOR_FONT_SUPPORT ? PixelFormat::kR8UNormInt
-                                          : PixelFormat::kR8G8B8A8UNormInt;
+      format = DISABLE_COLOR_FONT_SUPPORT
+                   ? context.GetCapabilities()->GetDefaultGlyphAtlasFormat()
+                   : PixelFormat::kR8G8B8A8UNormInt;
       break;
   }
   auto texture = UploadGlyphTextureAtlas(context.GetResourceAllocator(), bitmap,

impeller/typographer/lazy_glyph_atlas.cc

@@ -68,7 +68,7 @@ const std::shared_ptr<GlyphAtlas>& LazyGlyphAtlas::CreateOrGetGlyphAtlas(
   }
 
   auto& glyph_map = type == GlyphAtlas::Type::kAlphaBitmap ? alpha_glyph_map_
-                                                         : color_glyph_map_;
+                                                           : color_glyph_map_;
   const std::shared_ptr<GlyphAtlasContext>& atlas_context =
       type == GlyphAtlas::Type::kAlphaBitmap ? alpha_context_ : color_context_;
   std::shared_ptr<GlyphAtlas> atlas = typographer_context_->CreateGlyphAtlas(