From 86d8689ce29660df461a0e6721a41a73d0a2f7ef Mon Sep 17 00:00:00 2001
From: gaaclarke <30870216+gaaclarke@users.noreply.github.com>
Date: Fri, 27 Oct 2023 16:39:29 -0700
Subject: [PATCH] [Impeller] started scaling the gaussian blur sigma to match
 skia output (flutter/engine#47405)

fixes https://github.com/flutter/flutter/issues/127770

This will help performance as well since our kernels will be smaller.
The justification for scaling the sigma is that we need to account in
the math the scaling down that we are doing of the input.

Here are the side-by-side comparisons with this math and skia, impeller
is always on the right:

## sigma 10

<img width="1395" alt="blur_10"
src="https://github.com/flutter/engine/assets/30870216/d534d71f-b35b-46aa-be79-8337172968a8">

## sigma 75

<img width="1398" alt="blur_75"
src="https://github.com/flutter/engine/assets/30870216/a8a77048-5212-470d-b537-a0aea98efce6">

## sigma 164 (sigma in bug)
<img width="1402" alt="blur_164"
src="https://github.com/flutter/engine/assets/30870216/cc620d02-4460-4cc8-9798-8dd21530eeed">

## sigma 400
<img width="1404" alt="blur_400"
src="https://github.com/flutter/engine/assets/30870216/653c1f1b-8156-4cff-bfcf-bf1ba737b386">

## sigma 1000
<img width="1395" alt="blur_1000"
src="https://github.com/flutter/engine/assets/30870216/c3456275-7876-4733-a27b-6022140c6a24">



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responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
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---
 ...rectional_gaussian_blur_filter_contents.cc | 20 +++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/engine/src/flutter/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents.cc b/engine/src/flutter/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents.cc
index a03abb2eb2..fe5647b1a9 100644
--- a/engine/src/flutter/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents.cc
+++ b/engine/src/flutter/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents.cc
@@ -23,6 +23,23 @@
 
 namespace impeller {
 
+// This function was derived with polynomial regression when comparing the
+// results with Skia.  Changing the curve below should invalidate this.
+//
+// The following data points were used:
+//   0 | 1
+//  75 | 0.8
+// 150 | 0.5
+// 300 | 0.22
+// 400 | 0.2
+// 500 | 0.15
+Sigma ScaleSigma(Sigma sigma) {
+  // Limit the kernel size to 1000x1000 pixels, like Skia does.
+  Scalar clamped = std::min(sigma.sigma, 500.0f);
+  Scalar scalar = 1.02 - 3.89e-3 * clamped + 4.36e-06 * clamped * clamped;
+  return Sigma(clamped * scalar);
+}
+
 DirectionalGaussianBlurFilterContents::DirectionalGaussianBlurFilterContents() =
     default;
 
@@ -76,8 +93,7 @@ std::optional<Entity> DirectionalGaussianBlurFilterContents::RenderFilter(
     return std::nullopt;
   }
 
-  // Limit the kernel size to 1000x1000 pixels, like Skia does.
-  auto radius = std::min(Radius{blur_sigma_}.radius, 500.0f);
+  auto radius = Radius{ScaleSigma(blur_sigma_)}.radius;
 
   auto transform = entity.GetTransformation() * effect_transform.Basis();
   auto transformed_blur_radius =