[Impeller] simplify text shaders by moving UV computation to contents (…

…#39989)

[Impeller] simplify text shaders by moving UV computation to contents

impeller/entity/contents/text_contents.cc

@@ -58,6 +58,12 @@ std::optional<Rect> TextContents::GetCoverage(const Entity& entity) const {
   return bounds->TransformBounds(entity.GetTransformation());
 }
 
+static Vector4 PositionForGlyphPosition(const Matrix& translation,
+                                        Point unit_position,
+                                        Size destination_size) {
+  return translation * (unit_position * destination_size);
+}
+
 template <class TPipeline>
 static bool CommonRender(
     const ContentContext& renderer,
@@ -94,9 +100,6 @@ static bool CommonRender(
 
   typename FS::FragInfo frag_info;
   frag_info.text_color = ToVector(color.Premultiply());
-  frag_info.atlas_size =
-      Point{static_cast<Scalar>(atlas->GetTexture()->GetSize().width),
-            static_cast<Scalar>(atlas->GetTexture()->GetSize().height)};
   FS::BindFragInfo(cmd, pass.GetTransientsBuffer().EmplaceUniform(frag_info));
 
   // Common fragment uniforms for all glyphs.
@@ -136,6 +139,10 @@ static bool CommonRender(
     offset += 4;
   }
 
+  auto atlas_size =
+      Point{static_cast<Scalar>(atlas->GetTexture()->GetSize().width),
+            static_cast<Scalar>(atlas->GetTexture()->GetSize().height)};
+
   for (const auto& run : frame.GetRuns()) {
     auto font = run.GetFont();
 
@@ -147,23 +154,26 @@ static bool CommonRender(
         return false;
       }
 
-      auto atlas_position = atlas_glyph_pos->origin;
-      auto atlas_glyph_size =
-          Point{atlas_glyph_pos->size.width, atlas_glyph_pos->size.height};
       auto offset_glyph_position =
           glyph_position.position + glyph_position.glyph.bounds.origin;
 
+      auto uv_scaler_a = atlas_glyph_pos->size / atlas_size;
+      auto uv_scaler_b = (Point::Round(atlas_glyph_pos->origin) / atlas_size);
+      auto translation = Matrix::MakeTranslation(
+          Vector3(offset_glyph_position.x, offset_glyph_position.y, 0));
+
       for (const auto& point : unit_points) {
         typename VS::PerVertexData vtx;
-        vtx.unit_position = point;
-        vtx.destination_position = offset_glyph_position;
-        vtx.destination_size = Point(glyph_position.glyph.bounds.size);
-        vtx.source_position = atlas_position;
-        vtx.source_glyph_size = atlas_glyph_size;
+        auto position = PositionForGlyphPosition(
+            translation, point, glyph_position.glyph.bounds.size);
+        vtx.uv = point * uv_scaler_a + uv_scaler_b;
+        vtx.position = position;
+
         if constexpr (std::is_same_v<TPipeline, GlyphAtlasPipeline>) {
           vtx.has_color =
               glyph_position.glyph.type == Glyph::Type::kBitmap ? 1.0 : 0.0;
         }
+
         vertex_builder.AppendVertex(std::move(vtx));
       }
     }

impeller/entity/shaders/glyph_atlas.frag

@@ -7,29 +7,19 @@
 uniform sampler2D glyph_atlas_sampler;
 
 uniform FragInfo {
-  vec2 atlas_size;
   vec4 text_color;
 }
 frag_info;
 
-in vec2 v_unit_position;
-in vec2 v_source_position;
-in vec2 v_source_glyph_size;
+in vec2 v_uv;
 in float v_has_color;
 
 out vec4 frag_color;
 
 void main() {
-  vec2 uv_size = v_source_glyph_size / frag_info.atlas_size;
-  vec2 uv_position = v_source_position / frag_info.atlas_size;
-  if (v_has_color == 1.0) {
-    frag_color =
-        texture(glyph_atlas_sampler, v_unit_position * uv_size + uv_position) *
-        frag_info.text_color.a;
-  } else {
-    frag_color =
-        texture(glyph_atlas_sampler, v_unit_position * uv_size + uv_position)
-            .aaaa *
-        frag_info.text_color;
+  vec4 value = texture(glyph_atlas_sampler, v_uv);
+  if (v_has_color != 1.0) {
+    value = value.aaaa;
   }
+  frag_color = value * frag_info.text_color;
 }

impeller/entity/shaders/glyph_atlas.vert

@@ -10,24 +10,15 @@ uniform FrameInfo {
 }
 frame_info;
 
-in vec2 unit_position;
-in vec2 destination_position;
-in vec2 destination_size;
-in vec2 source_position;
-in vec2 source_glyph_size;
+in vec4 position;
+in vec2 uv;
 in float has_color;
 
-out vec2 v_unit_position;
-out vec2 v_source_position;
-out vec2 v_source_glyph_size;
+out vec2 v_uv;
 out float v_has_color;
 
 void main() {
-  gl_Position = IPPositionForGlyphPosition(
-      frame_info.mvp, unit_position, destination_position, destination_size);
-  v_unit_position = unit_position;
-  // Pixel snap the source (sampling) start position.
-  v_source_position = round(source_position);
-  v_source_glyph_size = source_glyph_size;
+  gl_Position = frame_info.mvp * position;
+  v_uv = uv;
   v_has_color = has_color;
 }

impeller/entity/shaders/glyph_atlas_sdf.frag

@@ -7,31 +7,23 @@
 uniform sampler2D glyph_atlas_sampler;
 
 uniform FragInfo {
-  vec2 atlas_size;
   vec4 text_color;
 }
 frag_info;
 
-in vec2 v_unit_position;
-in vec2 v_source_position;
-in vec2 v_source_glyph_size;
+in vec2 v_uv;
 
 out vec4 frag_color;
 
 void main() {
-  vec2 scale_perspective = v_source_glyph_size / frag_info.atlas_size;
-  vec2 offset = v_source_position / frag_info.atlas_size;
-
   // Inspired by Metal by Example's SDF text rendering shader:
   // https://github.com/metal-by-example/sample-code/blob/master/objc/12-TextRendering/TextRendering/Shaders.metal
 
   // Outline of glyph is the isocontour with value 50%
   float edge_distance = 0.5;
   // Sample the signed-distance field to find distance from this fragment to the
   // glyph outline
-  float sample_distance =
-      texture(glyph_atlas_sampler, v_unit_position * scale_perspective + offset)
-          .a;
+  float sample_distance = texture(glyph_atlas_sampler, v_uv).a;
   // Use local automatic gradients to find anti-aliased anisotropic edge width,
   // cf. Gustavson 2012
   float edge_width = length(vec2(dFdx(sample_distance), dFdy(sample_distance)));

impeller/entity/shaders/glyph_atlas_sdf.vert

@@ -11,20 +11,12 @@ uniform FrameInfo {
 }
 frame_info;
 
-in vec2 unit_position;
-in vec2 destination_position;
-in vec2 destination_size;
-in vec2 source_position;
-in vec2 source_glyph_size;
+in vec4 position;
+in vec2 uv;
 
-out vec2 v_unit_position;
-out vec2 v_source_position;
-out vec2 v_source_glyph_size;
+out vec2 v_uv;
 
 void main() {
-  gl_Position = IPPositionForGlyphPosition(
-      frame_info.mvp, unit_position, destination_position, destination_size);
-  v_unit_position = unit_position;
-  v_source_position = source_position;
-  v_source_glyph_size = source_glyph_size;
+  gl_Position = frame_info.mvp * position;
+  v_uv = uv;
 }

impeller/geometry/point.h

@@ -38,6 +38,12 @@ struct TPoint {
 
   static constexpr TPoint<Type> MakeXY(Type x, Type y) { return {x, y}; }
 
+  template <class U>
+  static constexpr TPoint Round(const TPoint<U>& other) {
+    return TPoint{static_cast<Type>(std::round(other.x)),
+                  static_cast<Type>(std::round(other.y))};
+  }
+
   constexpr bool operator==(const TPoint& p) const {
     return p.x == x && p.y == y;
   }