[CP][Impeller] Switch to nearest sampling for the text atlas (#39180)

* [Impeller] Switch to nearest sampling for the text atlas (#39104)

* [Impeller] Linear sample atlas glyphs when the CTM isn't translation/scale only (#39112)

---------

Co-authored-by: Brandon DeRosier <bdero@google.com>

impeller/entity/contents/text_contents.cc

@@ -78,8 +78,18 @@ static bool CommonRender(
   VS::BindFrameInfo(cmd, pass.GetTransientsBuffer().EmplaceUniform(frame_info));
 
   SamplerDescriptor sampler_desc;
-  sampler_desc.min_filter = MinMagFilter::kLinear;
-  sampler_desc.mag_filter = MinMagFilter::kLinear;
+  if (entity.GetTransformation().IsTranslationScaleOnly()) {
+    sampler_desc.min_filter = MinMagFilter::kNearest;
+    sampler_desc.mag_filter = MinMagFilter::kNearest;
+  } else {
+    // Currently, we only propagate the scale of the transform to the atlas
+    // renderer, so if the transform has more than just a translation, we turn
+    // on linear sampling to prevent crunchiness caused by the pixel grid not
+    // being perfectly aligned.
+    // The downside is that this slightly over-blurs rotated/skewed text.
+    sampler_desc.min_filter = MinMagFilter::kLinear;
+    sampler_desc.mag_filter = MinMagFilter::kLinear;
+  }
   sampler_desc.mip_filter = MipFilter::kNone;
 
   typename FS::FragInfo frag_info;
@@ -146,10 +156,10 @@ static bool CommonRender(
       for (const auto& point : unit_points) {
         typename VS::PerVertexData vtx;
         vtx.unit_position = point;
-        vtx.destination_position = offset_glyph_position + Point(0.5, 0.5);
+        vtx.destination_position = offset_glyph_position;
         vtx.destination_size = Point(glyph_position.glyph.bounds.size);
-        vtx.source_position = atlas_position + Point(0.5, 0.5);
-        vtx.source_glyph_size = atlas_glyph_size - Point(1.0, 1.0);
+        vtx.source_position = atlas_position;
+        vtx.source_glyph_size = atlas_glyph_size;
         if constexpr (std::is_same_v<TPipeline, GlyphAtlasPipeline>) {
           vtx.has_color =
               glyph_position.glyph.type == Glyph::Type::kBitmap ? 1.0 : 0.0;

impeller/entity/shaders/glyph_atlas.frag

@@ -21,13 +21,14 @@ out vec4 frag_color;
 
 void main() {
   vec2 uv_size = v_source_glyph_size / frag_info.atlas_size;
-  vec2 offset = v_source_position / 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 + offset);
+        texture(glyph_atlas_sampler, v_unit_position * uv_size + uv_position);
   } else {
     frag_color =
-        texture(glyph_atlas_sampler, v_unit_position * uv_size + offset).aaaa *
+        texture(glyph_atlas_sampler, v_unit_position * uv_size + uv_position)
+            .aaaa *
         frag_info.text_color;
   }
 }

impeller/entity/shaders/glyph_atlas.vert

@@ -26,7 +26,8 @@ void main() {
   gl_Position = IPPositionForGlyphPosition(
       frame_info.mvp, unit_position, destination_position, destination_size);
   v_unit_position = unit_position;
-  v_source_position = source_position;
+  // Pixel snap the source (sampling) start position.
+  v_source_position = round(source_position);
   v_source_glyph_size = source_glyph_size;
   v_has_color = has_color;
 }

impeller/geometry/geometry_unittests.cc

@@ -424,6 +424,32 @@ TEST(GeometryTest, MatrixIsAligned) {
   }
 }
 
+TEST(GeometryTest, MatrixTranslationScaleOnly) {
+  {
+    auto m = Matrix();
+    bool result = m.IsTranslationScaleOnly();
+    ASSERT_TRUE(result);
+  }
+
+  {
+    auto m = Matrix::MakeScale(Vector3(2, 3, 4));
+    bool result = m.IsTranslationScaleOnly();
+    ASSERT_TRUE(result);
+  }
+
+  {
+    auto m = Matrix::MakeTranslation(Vector3(2, 3, 4));
+    bool result = m.IsTranslationScaleOnly();
+    ASSERT_TRUE(result);
+  }
+
+  {
+    auto m = Matrix::MakeRotationZ(Degrees(10));
+    bool result = m.IsTranslationScaleOnly();
+    ASSERT_FALSE(result);
+  }
+}
+
 TEST(GeometryTest, MatrixLookAt) {
   {
     auto m = Matrix::MakeLookAt(Vector3(0, 0, -1), Vector3(0, 0, 1),

impeller/geometry/matrix.h

@@ -322,6 +322,19 @@ struct Matrix {
     );
   }
 
+  /// @brief  Returns true if the matrix has a scale-only basis and is
+  ///         non-projective. Note that an identity matrix meets this criteria.
+  constexpr bool IsTranslationScaleOnly() const {
+    return (
+        // clang-format off
+        m[0] != 0.0 && m[1]  == 0.0 && m[2]  == 0.0 && m[3]  == 0.0 &&
+        m[4] == 0.0 && m[5]  != 0.0 && m[6]  == 0.0 && m[7]  == 0.0 &&
+        m[8] == 0.0 && m[9]  == 0.0 && m[10] != 0.0 && m[11] == 0.0 &&
+                                                       m[15] == 1.0
+        // clang-format on
+    );
+  }
+
   std::optional<MatrixDecomposition> Decompose() const;
 
   constexpr bool operator==(const Matrix& m) const {