[Impeller] applied the lerp hack to blur (roughly 2x speedup?) (flutter/engine#50790)

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/impeller/entity/contents/filters/gaussian_blur_filter_contents.cc

@@ -174,7 +174,8 @@ fml::StatusOr<RenderTarget> MakeBlurSubpass(
         GaussianBlurVertexShader::BindFrameInfo(
             pass, host_buffer.EmplaceUniform(frame_info));
         GaussianBlurFragmentShader::BindKernelSamples(
-            pass, host_buffer.EmplaceUniform(GenerateBlurInfo(blur_info)));
+            pass, host_buffer.EmplaceUniform(
+                      LerpHackKernelSamples(GenerateBlurInfo(blur_info))));
         return pass.Draw().ok();
       };
   if (destination_target.has_value()) {
@@ -507,4 +508,32 @@ KernelPipeline::FragmentShader::KernelSamples GenerateBlurInfo(
   return result;
 }
 
+// This works by shrinking the kernel size by 2 and relying on lerp to read
+// between the samples.
+KernelPipeline::FragmentShader::KernelSamples LerpHackKernelSamples(
+    KernelPipeline::FragmentShader::KernelSamples parameters) {
+  KernelPipeline::FragmentShader::KernelSamples result;
+  result.sample_count = ((parameters.sample_count - 1) / 2) + 1;
+  int32_t middle = result.sample_count / 2;
+  int32_t j = 0;
+  for (int i = 0; i < result.sample_count; i++) {
+    if (i == middle) {
+      result.samples[i] = parameters.samples[j++];
+    } else {
+      KernelPipeline::FragmentShader::KernelSample left = parameters.samples[j];
+      KernelPipeline::FragmentShader::KernelSample right =
+          parameters.samples[j + 1];
+      result.samples[i] = KernelPipeline::FragmentShader::KernelSample{
+          .uv_offset = (left.uv_offset * left.coefficient +
+                        right.uv_offset * right.coefficient) /
+                       (left.coefficient + right.coefficient),
+          .coefficient = left.coefficient + right.coefficient,
+      };
+      j += 2;
+    }
+  }
+
+  return result;
+}
+
 }  // namespace impeller

engine/src/flutter/impeller/entity/contents/filters/gaussian_blur_filter_contents.h

@@ -21,6 +21,11 @@ struct BlurParameters {
 KernelPipeline::FragmentShader::KernelSamples GenerateBlurInfo(
     BlurParameters parameters);
 
+/// This will shrink the size of a kernel by roughly half by sampling between
+/// samples and relying on linear interpolation between the samples.
+KernelPipeline::FragmentShader::KernelSamples LerpHackKernelSamples(
+    KernelPipeline::FragmentShader::KernelSamples samples);
+
 /// Performs a bidirectional Gaussian blur.
 ///
 /// This is accomplished by rendering multiple passes in multiple directions.

engine/src/flutter/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc

@@ -11,6 +11,12 @@
 #include "impeller/geometry/geometry_asserts.h"
 #include "impeller/renderer/testing/mocks.h"
 
+#if FML_OS_MACOSX
+#define IMPELLER_RAND arc4random
+#else
+#define IMPELLER_RAND rand
+#endif
+
 namespace impeller {
 namespace testing {
 
@@ -487,5 +493,138 @@ TEST(GaussianBlurFilterContentsTest, Coefficients) {
   }
 }
 
+TEST(GaussianBlurFilterContentsTest, LerpHackKernelSamplesSimple) {
+  KernelPipeline::FragmentShader::KernelSamples kernel_samples = {
+      .sample_count = 5,
+      .samples =
+          {
+              {
+                  .uv_offset = Vector2(-2, 0),
+                  .coefficient = 0.1f,
+              },
+              {
+                  .uv_offset = Vector2(-1, 0),
+                  .coefficient = 0.2f,
+              },
+              {
+                  .uv_offset = Vector2(0, 0),
+                  .coefficient = 0.4f,
+              },
+              {
+                  .uv_offset = Vector2(1, 0),
+                  .coefficient = 0.2f,
+              },
+              {
+                  .uv_offset = Vector2(2, 0),
+                  .coefficient = 0.1f,
+              },
+          },
+  };
+
+  KernelPipeline::FragmentShader::KernelSamples fast_kernel_samples =
+      LerpHackKernelSamples(kernel_samples);
+  EXPECT_EQ(fast_kernel_samples.sample_count, 3);
+
+  KernelPipeline::FragmentShader::KernelSample* samples =
+      kernel_samples.samples;
+  KernelPipeline::FragmentShader::KernelSample* fast_samples =
+      fast_kernel_samples.samples;
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Check output kernel.
+
+  EXPECT_FLOAT_EQ(fast_samples[0].uv_offset.x, -1.3333333);
+  EXPECT_FLOAT_EQ(fast_samples[0].uv_offset.y, 0);
+  EXPECT_FLOAT_EQ(fast_samples[0].coefficient, 0.3);
+  EXPECT_FLOAT_EQ(fast_samples[1].uv_offset.x, 0);
+  EXPECT_FLOAT_EQ(fast_samples[1].uv_offset.y, 0);
+  EXPECT_FLOAT_EQ(fast_samples[1].coefficient, 0.4);
+  EXPECT_FLOAT_EQ(fast_samples[2].uv_offset.x, 1.3333333);
+  EXPECT_FLOAT_EQ(fast_samples[2].uv_offset.y, 0);
+  EXPECT_FLOAT_EQ(fast_samples[2].coefficient, 0.3);
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Check output of fast kernel versus original kernel.
+
+  Scalar data[5] = {0.25, 0.5, 0.5, 1.0, 0.2};
+  Scalar original_output =
+      samples[0].coefficient * data[0] + samples[1].coefficient * data[1] +
+      samples[2].coefficient * data[2] + samples[3].coefficient * data[3] +
+      samples[4].coefficient * data[4];
+
+  auto lerp = [](const Point& point, Scalar left, Scalar right) {
+    Scalar int_part;
+    Scalar fract = fabsf(modf(point.x, &int_part));
+    if (point.x < 0) {
+      return left * fract + right * (1.0 - fract);
+    } else {
+      return left * (1.0 - fract) + right * fract;
+    }
+  };
+  Scalar fast_output =
+      /*1st*/ lerp(fast_samples[0].uv_offset, data[0], data[1]) *
+          fast_samples[0].coefficient +
+      /*2nd*/ data[2] * fast_samples[1].coefficient +
+      /*3rd*/ lerp(fast_samples[2].uv_offset, data[3], data[4]) *
+          fast_samples[2].coefficient;
+
+  EXPECT_NEAR(original_output, fast_output, 0.01);
+}
+
+TEST(GaussianBlurFilterContentsTest, LerpHackKernelSamplesComplex) {
+  Scalar sigma = 10.0f;
+  int32_t blur_radius = static_cast<int32_t>(
+      std::ceil(GaussianBlurFilterContents::CalculateBlurRadius(sigma)));
+  BlurParameters parameters = {.blur_uv_offset = Point(1, 0),
+                               .blur_sigma = sigma,
+                               .blur_radius = blur_radius,
+                               .step_size = 1};
+  KernelPipeline::FragmentShader::KernelSamples kernel_samples =
+      GenerateBlurInfo(parameters);
+  EXPECT_EQ(kernel_samples.sample_count, 33);
+  KernelPipeline::FragmentShader::KernelSamples fast_kernel_samples =
+      LerpHackKernelSamples(kernel_samples);
+  EXPECT_EQ(fast_kernel_samples.sample_count, 17);
+  float data[33];
+  srand(0);
+  for (int i = 0; i < 33; i++) {
+    data[i] = 255.0 * static_cast<double>(IMPELLER_RAND()) / RAND_MAX;
+  }
+
+  auto sampler = [data](Point point) -> Scalar {
+    FML_CHECK(point.y == 0.0f);
+    FML_CHECK(point.x >= -16);
+    FML_CHECK(point.x <= 16);
+    Scalar fint_part;
+    Scalar fract = fabsf(modf(point.x, &fint_part));
+    if (fract == 0) {
+      int32_t int_part = static_cast<int32_t>(fint_part) + 16;
+      return data[int_part];
+    } else {
+      int32_t left = static_cast<int32_t>(floor(point.x)) + 16;
+      int32_t right = static_cast<int32_t>(ceil(point.x)) + 16;
+      if (point.x < 0) {
+        return fract * data[left] + (1.0 - fract) * data[right];
+      } else {
+        return (1.0 - fract) * data[left] + fract * data[right];
+      }
+    }
+  };
+
+  Scalar output = 0.0;
+  for (int i = 0; i < kernel_samples.sample_count; ++i) {
+    auto sample = kernel_samples.samples[i];
+    output += sample.coefficient * sampler(sample.uv_offset);
+  }
+
+  Scalar fast_output = 0.0;
+  for (int i = 0; i < fast_kernel_samples.sample_count; ++i) {
+    auto sample = fast_kernel_samples.samples[i];
+    fast_output += sample.coefficient * sampler(sample.uv_offset);
+  }
+
+  EXPECT_NEAR(output, fast_output, 0.1);
+}
+
 }  // namespace testing
 }  // namespace impeller