[Impeller] added a fallback that will make sure the blur fragment shader doesn't overflow

impeller/entity/contents/filters/gaussian_blur_filter_contents.cc

@@ -653,6 +653,18 @@ GaussianBlurPipeline::FragmentShader::KernelSamples GenerateBlurInfo(
     x_offset = 1;
   }
 
+  // This is a safe-guard to make sure we don't overflow the fragment shader.
+  // The kernel size is multiplied by 2 since we'll use the lerp hack on the
+  // result. In practice this isn't throwing away much data since the blur radii
+  // are around 53 before the down-sampling and max sigma of 500 kick in.
+  //
+  // TODO(https://github.com/flutter/flutter/issues/150462): Come up with a more
+  // wholistic remedy for this.  A proper downsample size should not make this
+  // required. Or we can increase the kernel size.
+  if (result.sample_count > (2 * (kMaxKernelSize - 1))) {
+    result.sample_count = 2 * (kMaxKernelSize - 1);
+  }
+
   Scalar tally = 0.0f;
   for (int i = 0; i < result.sample_count; ++i) {
     int x = x_offset + (i * parameters.step_size) - parameters.blur_radius;

impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc

@@ -614,5 +614,18 @@ TEST(GaussianBlurFilterContentsTest, LerpHackKernelSamplesComplex) {
   EXPECT_NEAR(output, fast_output, 0.1);
 }
 
+TEST(GaussianBlurFilterContentsTest, ChopHugeBlurs) {
+  Scalar sigma = 30.5f;
+  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};
+  GaussianBlurPipeline::FragmentShader::KernelSamples kernel_samples =
+      GenerateBlurInfo(parameters);
+  EXPECT_EQ(kernel_samples.sample_count, 98);
+}
+
 }  // namespace testing
 }  // namespace impeller