[Impeller] blur: hold on to 1/8 downsample until the kernel overflows (#50363)
issue: https://github.com/flutter/flutter/issues/142753
After https://github.com/flutter/engine/pull/50262 there were still some
sigmas that could show shimmering around the sigma [50, 100] range which
had a downsample amount of 1/16. This makes those ranges hang on to 1/8
for as long as possible. I'm unable to see any shimmering with
`AiksTest.GaussianBlurAnimatedBackdrop` after this PR. I've also
expanded that test so the clip region could be scaled to make sure that
there aren't sizes which cause it to reappear.
Testing: Expanded on manual testing. Since the error only manifests when
evaluating multiple frames of rendering we don't have infrastructure to
test that.
Here is the graph of `GaussianBlurFilterContents::CalculateScale` after
this change:
<img width="903" alt="Screenshot 2024-02-05 at 2 10 41 PM"
src="https://github.com/flutter/engine/assets/30870216/ac3b735a-95f0-4d7b-af12-58e1ae75278e">
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diff --git a/impeller/aiks/aiks_blur_unittests.cc b/impeller/aiks/aiks_blur_unittests.cc
index e7198aa..7e76260 100644
--- a/impeller/aiks/aiks_blur_unittests.cc
+++ b/impeller/aiks/aiks_blur_unittests.cc
@@ -571,6 +571,10 @@
Point(1024 / 2 - boston->GetSize().width / 2,
(768 / 2 - boston->GetSize().height / 2) + y),
{});
+ auto [handle_a, handle_b] = IMPELLER_PLAYGROUND_LINE(
+ Point(100, 100), Point(900, 700), 20, Color::Red(), Color::Red());
+ canvas.ClipRect(
+ Rect::MakeLTRB(handle_a.x, handle_a.y, handle_b.x, handle_b.y));
canvas.ClipRect(Rect::MakeLTRB(100, 100, 900, 700));
canvas.SaveLayer({.blend_mode = BlendMode::kSource}, std::nullopt,
ImageFilter::MakeBlur(Sigma(sigma), Sigma(sigma),
diff --git a/impeller/entity/contents/filters/gaussian_blur_filter_contents.cc b/impeller/entity/contents/filters/gaussian_blur_filter_contents.cc
index 79d39dd..36bc0c8 100644
--- a/impeller/entity/contents/filters/gaussian_blur_filter_contents.cc
+++ b/impeller/entity/contents/filters/gaussian_blur_filter_contents.cc
@@ -23,6 +23,9 @@
namespace {
+// 48 comes from kernel.glsl.
+const int32_t kMaxKernelSize = 48;
+
SamplerDescriptor MakeSamplerDescriptor(MinMagFilter filter,
SamplerAddressMode address_mode) {
SamplerDescriptor sampler_desc;
@@ -189,6 +192,10 @@
rect.GetSize());
return result.Scale(1.0f / Vector2(reference.GetSize()));
}
+
+int ScaleBlurRadius(Scalar radius, Scalar scalar) {
+ return static_cast<int>(std::round(radius * scalar));
+}
} // namespace
std::string_view GaussianBlurFilterContents::kNoMipsError =
@@ -207,13 +214,21 @@
if (sigma <= 4) {
return 1.0;
}
- Scalar result = 4.0 / sigma;
+ Scalar raw_result = 4.0 / sigma;
// Round to the nearest 1/(2^n) to get the best quality down scaling.
- Scalar exponent = round(log2f(result));
+ Scalar exponent = round(log2f(raw_result));
// Don't scale down below 1/16th to preserve signal.
exponent = std::max(-4.0f, exponent);
Scalar rounded = powf(2.0f, exponent);
- return rounded;
+ Scalar result = rounded;
+ // Only drop below 1/8 if 1/8 would overflow our kernel.
+ if (rounded < 0.125f) {
+ Scalar rounded_plus = powf(2.0f, exponent + 1);
+ Scalar blur_radius = CalculateBlurRadius(sigma);
+ int kernel_size_plus = (ScaleBlurRadius(blur_radius, rounded_plus) * 2) + 1;
+ result = kernel_size_plus < kMaxKernelSize ? rounded_plus : rounded;
+ }
+ return result;
};
std::optional<Rect> GaussianBlurFilterContents::GetFilterSourceCoverage(
@@ -370,8 +385,7 @@
BlurParameters{
.blur_uv_offset = Point(0.0, pass1_pixel_size.y),
.blur_sigma = scaled_sigma.y * effective_scalar.y,
- .blur_radius =
- static_cast<int>(std::round(blur_radius.y * effective_scalar.y)),
+ .blur_radius = ScaleBlurRadius(blur_radius.y, effective_scalar.y),
.step_size = 1,
},
/*destination_target=*/std::nullopt, blur_uvs);
@@ -392,8 +406,7 @@
BlurParameters{
.blur_uv_offset = Point(pass1_pixel_size.x, 0.0),
.blur_sigma = scaled_sigma.x * effective_scalar.x,
- .blur_radius =
- static_cast<int>(std::round(blur_radius.x * effective_scalar.x)),
+ .blur_radius = ScaleBlurRadius(blur_radius.x, effective_scalar.x),
.step_size = 1,
},
pass3_destination, blur_uvs);
@@ -457,8 +470,7 @@
KernelPipeline::FragmentShader::KernelSamples result;
result.sample_count =
((2 * parameters.blur_radius) / parameters.step_size) + 1;
- // 48 comes from kernel.glsl.
- FML_CHECK(result.sample_count < 48);
+ FML_CHECK(result.sample_count < kMaxKernelSize);
// Chop off the last samples if the radius >= 3 where they account for < 1.56%
// of the result.
diff --git a/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc b/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc
index f75b4f3..d84da44 100644
--- a/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc
+++ b/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc
@@ -197,6 +197,8 @@
EXPECT_EQ(GaussianBlurFilterContents::CalculateScale(3.0f), 1);
EXPECT_EQ(GaussianBlurFilterContents::CalculateScale(4.0f), 1);
EXPECT_EQ(GaussianBlurFilterContents::CalculateScale(16.0f), 0.25);
+ // Hang on to 1/8 as long as possible.
+ EXPECT_EQ(GaussianBlurFilterContents::CalculateScale(100.0f), 0.125);
// Downsample clamped to 1/16th.
EXPECT_EQ(GaussianBlurFilterContents::CalculateScale(1024.0f), 0.0625);
}
