Standardize around blur sigma<->radius conversion factor (#123)

impeller/entity/contents/filters/filter_contents.cc

@@ -61,26 +61,28 @@ std::shared_ptr<FilterContents> FilterContents::MakeBlend(
 
 std::shared_ptr<FilterContents> FilterContents::MakeDirectionalGaussianBlur(
     FilterInput::Ref input,
-    Vector2 blur_vector,
+    Sigma sigma,
+    Vector2 direction,
     BlurStyle blur_style,
     FilterInput::Ref source_override) {
   auto blur = std::make_shared<DirectionalGaussianBlurFilterContents>();
   blur->SetInputs({input});
-  blur->SetBlurVector(blur_vector);
+  blur->SetSigma(sigma);
+  blur->SetDirection(direction);
   blur->SetBlurStyle(blur_style);
   blur->SetSourceOverride(source_override);
   return blur;
 }
 
 std::shared_ptr<FilterContents> FilterContents::MakeGaussianBlur(
     FilterInput::Ref input,
-    Scalar sigma_x,
-    Scalar sigma_y,
+    Sigma sigma_x,
+    Sigma sigma_y,
     BlurStyle blur_style) {
-  auto x_blur =
-      MakeDirectionalGaussianBlur(input, Point(sigma_x, 0), BlurStyle::kNormal);
-  auto y_blur = MakeDirectionalGaussianBlur(
-      FilterInput::Make(x_blur), Point(0, sigma_y), blur_style, input);
+  auto x_blur = MakeDirectionalGaussianBlur(input, sigma_x, Point(1, 0),
+                                            BlurStyle::kNormal);
+  auto y_blur = MakeDirectionalGaussianBlur(FilterInput::Make(x_blur), sigma_y,
+                                            Point(0, 1), blur_style, input);
   return y_blur;
 }
 

impeller/entity/contents/filters/filter_contents.h

@@ -29,19 +29,71 @@ class FilterContents : public Contents {
     kInner,
   };
 
+  /// For filters that use a Gaussian distribution, this is the `Radius` size to
+  /// use per `Sigma` (standard deviation).
+  ///
+  /// This cutoff (sqrt(3)) is taken from Flutter and Skia (where the
+  /// multiplicative inverse of this constant is used (1 / sqrt(3)):
+  /// https://api.flutter.dev/flutter/dart-ui/Shadow/convertRadiusToSigma.html
+  ///
+  /// In practice, this value is somewhat arbitrary, and can be changed to a
+  /// higher number to integrate more of the Gaussian function and render higher
+  /// quality blurs (with exponentially diminishing returns for the same sigma
+  /// input). Making this value any lower results in a noticable loss of
+  /// quality in the blur.
+  constexpr static float kKernelRadiusPerSigma = 1.73205080757;
+
+  struct Radius;
+
+  /// @brief  In filters that use Gaussian distributions, "sigma" is a size of
+  ///         one standard deviation in terms of the local space pixel grid of
+  ///         the filter input. In other words, this determines how wide the
+  ///         distribution stretches.
+  struct Sigma {
+    Scalar sigma = 0.0;
+
+    constexpr Sigma() = default;
+
+    explicit constexpr Sigma(Scalar p_sigma) : sigma(p_sigma) {}
+
+    constexpr operator Radius() const {
+      return Radius{sigma > 0.5f ? (sigma - 0.5f) * kKernelRadiusPerSigma
+                                 : 0.0f};
+    };
+  };
+
+  /// @brief  For convolution filters, the "radius" is the size of the
+  ///         convolution kernel to use on the local space pixel grid of the
+  ///         filter input.
+  ///         For Gaussian blur kernels, this unit has a linear
+  ///         relationship with `Sigma`. See `kKernelRadiusPerSigma` for
+  ///         details on how this relationship works.
+  struct Radius {
+    Scalar radius = 0.0;
+
+    constexpr Radius() = default;
+
+    explicit constexpr Radius(Scalar p_radius) : radius(p_radius) {}
+
+    constexpr operator Sigma() const {
+      return Sigma{radius > 0 ? radius / kKernelRadiusPerSigma + 0.5f : 0.0f};
+    };
+  };
+
   static std::shared_ptr<FilterContents> MakeBlend(Entity::BlendMode blend_mode,
                                                    FilterInput::Vector inputs);
 
   static std::shared_ptr<FilterContents> MakeDirectionalGaussianBlur(
       FilterInput::Ref input,
-      Vector2 blur_vector,
+      Sigma sigma,
+      Vector2 direction,
       BlurStyle blur_style = BlurStyle::kNormal,
       FilterInput::Ref alpha_mask = nullptr);
 
   static std::shared_ptr<FilterContents> MakeGaussianBlur(
       FilterInput::Ref input,
-      Scalar sigma_x,
-      Scalar sigma_y,
+      Sigma sigma_x,
+      Sigma sigma_y,
       BlurStyle blur_style = BlurStyle::kNormal);
 
   FilterContents();

impeller/entity/contents/filters/gaussian_blur_filter_contents.cc

@@ -22,12 +22,23 @@ DirectionalGaussianBlurFilterContents::DirectionalGaussianBlurFilterContents() =
 DirectionalGaussianBlurFilterContents::
     ~DirectionalGaussianBlurFilterContents() = default;
 
-void DirectionalGaussianBlurFilterContents::SetBlurVector(Vector2 blur_vector) {
-  if (blur_vector.GetLengthSquared() < kEhCloseEnough) {
-    blur_vector_ = Vector2(0, kEhCloseEnough);
+void DirectionalGaussianBlurFilterContents::SetSigma(Sigma sigma) {
+  if (sigma.sigma < kEhCloseEnough) {
+    // This cutoff is an implementation detail of the blur that's tied to the
+    // fragment shader. When the blur is set to 0, having a value slightly above
+    // zero makes the shader do 1 finite sample to pass the image through with
+    // no blur (while retaining correct alpha mask behavior).
+    blur_sigma_ = Sigma{kEhCloseEnough};
     return;
   }
-  blur_vector_ = blur_vector;
+  blur_sigma_ = sigma;
+}
+
+void DirectionalGaussianBlurFilterContents::SetDirection(Vector2 direction) {
+  blur_direction_ = direction.Normalize();
+  if (blur_direction_.IsZero()) {
+    blur_direction_ = Vector2(0, 1);
+  }
 }
 
 void DirectionalGaussianBlurFilterContents::SetBlurStyle(BlurStyle blur_style) {
@@ -93,8 +104,8 @@ bool DirectionalGaussianBlurFilterContents::RenderFilter(
     return false;
   }
 
-  auto transformed_blur =
-      entity.GetTransformation().TransformDirection(blur_vector_);
+  auto transformed_blur = entity.GetTransformation().TransformDirection(
+      blur_direction_ * blur_sigma_.sigma);
 
   // LTRB
   Scalar uv[4] = {
@@ -142,7 +153,8 @@ bool DirectionalGaussianBlurFilterContents::RenderFilter(
 
   VS::FrameInfo frame_info;
   frame_info.texture_size = Point(input_bounds->size);
-  frame_info.blur_radius = transformed_blur.GetLength();
+  frame_info.blur_sigma = transformed_blur.GetLength();
+  frame_info.blur_radius = Radius{Sigma{frame_info.blur_sigma}}.radius;
   frame_info.blur_direction = transformed_blur.Normalize();
   frame_info.src_factor = src_color_factor_;
   frame_info.inner_blur_factor = inner_blur_factor_;
@@ -174,10 +186,14 @@ std::optional<Rect> DirectionalGaussianBlurFilterContents::GetCoverage(
     return std::nullopt;
   }
 
-  auto transformed_blur =
-      entity.GetTransformation().TransformDirection(blur_vector_).Abs();
-  auto extent = bounds->size + transformed_blur * 2;
-  return Rect(bounds->origin - transformed_blur, Size(extent.x, extent.y));
+  auto transformed_blur_vector =
+      entity.GetTransformation()
+          .TransformDirection(blur_direction_ *
+                              ceil(Radius{blur_sigma_}.radius))
+          .Abs();
+  auto extent = bounds->size + transformed_blur_vector * 2;
+  return Rect(bounds->origin - transformed_blur_vector,
+              Size(extent.x, extent.y));
 }
 
 }  // namespace impeller

impeller/entity/contents/filters/gaussian_blur_filter_contents.h

@@ -18,7 +18,9 @@ class DirectionalGaussianBlurFilterContents final : public FilterContents {
 
   ~DirectionalGaussianBlurFilterContents() override;
 
-  void SetBlurVector(Vector2 blur_vector);
+  void SetSigma(Sigma sigma);
+
+  void SetDirection(Vector2 direction);
 
   void SetBlurStyle(BlurStyle blur_style);
 
@@ -34,8 +36,8 @@ class DirectionalGaussianBlurFilterContents final : public FilterContents {
                     const Entity& entity,
                     RenderPass& pass,
                     const Rect& bounds) const override;
-
-  Vector2 blur_vector_;
+  Sigma blur_sigma_;
+  Vector2 blur_direction_;
   BlurStyle blur_style_ = BlurStyle::kNormal;
   bool src_color_factor_ = false;
   bool inner_blur_factor_ = true;

impeller/entity/entity_unittests.cc

@@ -729,7 +729,8 @@ TEST_F(EntityTest, GaussianBlurFilter) {
         Entity::BlendMode::kPlus, FilterInput::Make({boston, bridge, bridge}));
 
     auto blur = FilterContents::MakeGaussianBlur(
-        FilterInput::Make(blend), blur_amount[0], blur_amount[1],
+        FilterInput::Make(blend), FilterContents::Sigma{blur_amount[0]},
+        FilterContents::Sigma{blur_amount[1]},
         blur_styles[selected_blur_style]);
 
     ISize input_size = boston->GetSize();

impeller/entity/shaders/gaussian_blur.frag

@@ -16,19 +16,19 @@ in vec2 v_texture_coords;
 in vec2 v_src_texture_coords;
 in vec2 v_texture_size;
 in vec2 v_blur_direction;
+in float v_blur_sigma;
 in float v_blur_radius;
 in float v_src_factor;
 in float v_inner_blur_factor;
 in float v_outer_blur_factor;
 
 out vec4 frag_color;
 
-const float kTwoPi = 6.283185307179586;
+const float kSqrtTwoPi = 2.50662827463;
 
 float Gaussian(float x) {
-  float stddev = v_blur_radius * 0.5;
-  float xnorm = x / stddev;
-  return exp(-0.5 * xnorm * xnorm) / (kTwoPi * stddev * stddev);
+  float variance = v_blur_sigma * v_blur_sigma;
+  return exp(-0.5 * x * x / variance) / (kSqrtTwoPi * v_blur_sigma);
 }
 
 // Emulate SamplerAddressMode::ClampToBorder.
@@ -38,17 +38,19 @@ vec4 SampleWithBorder(sampler2D tex, vec2 uv) {
 }
 
 void main() {
-  vec4 total = vec4(0);
-  float total_gaussian = 0;
+  vec4 total_color = vec4(0);
+  float gaussian_integral = 0;
   vec2 blur_uv_offset = v_blur_direction / v_texture_size;
+
   for (float i = -v_blur_radius; i <= v_blur_radius; i++) {
     float gaussian = Gaussian(i);
-    total_gaussian += gaussian;
-    total += gaussian * SampleWithBorder(texture_sampler,
-                                         v_texture_coords + blur_uv_offset * i);
+    gaussian_integral += gaussian;
+    total_color +=
+        gaussian * SampleWithBorder(texture_sampler,
+                                    v_texture_coords + blur_uv_offset * i);
   }
 
-  vec4 blur_color = total / total_gaussian;
+  vec4 blur_color = total_color / gaussian_integral;
 
   vec4 src_color = SampleWithBorder(alpha_mask_sampler, v_src_texture_coords);
   float blur_factor = v_inner_blur_factor * float(src_color.a > 0) +

impeller/entity/shaders/gaussian_blur.vert

@@ -7,6 +7,7 @@ uniform FrameInfo {
   vec2 texture_size;
 
   vec2 blur_direction;
+  float blur_sigma;
   float blur_radius;
 
   float src_factor;
@@ -23,6 +24,7 @@ out vec2 v_texture_coords;
 out vec2 v_src_texture_coords;
 out vec2 v_texture_size;
 out vec2 v_blur_direction;
+out float v_blur_sigma;
 out float v_blur_radius;
 out float v_src_factor;
 out float v_inner_blur_factor;
@@ -34,6 +36,7 @@ void main() {
   v_src_texture_coords = src_texture_coords;
   v_texture_size = frame_info.texture_size;
   v_blur_direction = frame_info.blur_direction;
+  v_blur_sigma = frame_info.blur_sigma;
   v_blur_radius = frame_info.blur_radius;
   v_src_factor = frame_info.src_factor;
   v_inner_blur_factor = frame_info.inner_blur_factor;