Fix P3-to-sRGB color conversion to operate in linear light by westito · Pull Request #181720 · flutter/flutter

westito · 2026-01-30T10:52:06Z

Problem

The P3-to-sRGB color conversion applies an affine matrix directly to gamma-encoded values in both the C++ engine (dl_color.cc) and Dart (painting.dart). This is mathematically incorrect — color space conversion matrices must operate in linear light. The result is P3 colors that appear nearly identical to sRGB instead of showing the full gamut difference.
For example, P3 #1ECAD3 (30/255, 202/255, 211/255) converts to:

Old (wrong): extendedSRGB (-0.12, 0.86, 0.87)
New (correct): extendedSRGB (-0.38, 0.81, 0.84)
The red channel error of 0.26 is clearly visible — colors appear washed out instead of vivid.

Fix

Replace the single affine matrix multiply with the correct 3-step pipeline:

Linearize — decode gamma via sRGB EOTF
Transform — apply 3x3 P3-to-sRGB matrix in linear space
Encode — re-apply gamma via sRGB OETF
Extended range (negative values from out-of-gamut colors) is handled by mirroring the transfer function.
C++ (dl_color.cc): Replace affine matrix with p3ToExtendedSrgb() using double precision.
Dart (painting.dart): Replace _MatrixColorTransform with _P3ToSrgbTransform and _SrgbToP3Transform classes. Also fixes the sRGB-to-P3 direction.

Performance

Negligible. The C++ conversion runs once per paint setup in ReadColor, not per frame or per pixel. The Dart conversion runs once per Color.withValues() call.

Tests

C++: Added ColorSpaceP3ToExtendedSRGBLinearLight test in dl_color_unittests.cc
Dart: Added 3 tests in colors_test.dart: mid-range P3→extendedSRGB, P3 green→extendedSRGB, sRGB→P3 round-trip
All new tests fail with the old code and pass with the fix. Existing tests continue to pass.

Issue:

#181717

Pre-launch Checklist

I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
I read the [Tree Hygiene] wiki page, which explains my responsibilities.
I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
I signed the [CLA].
I listed at least one issue that this PR fixes in the description above.
I updated/added relevant documentation (doc comments with ///).
I added new tests to check the change I am making, or this PR is [test-exempt].
I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
All existing and new tests are passing. New tests pass, can't run golden tests!

gemini-code-assist

Code Review

This pull request correctly fixes a mathematical error in the P3-to-sRGB color conversion by ensuring the transformation operates in linear light, as it should. The changes replace the direct affine matrix multiplication on gamma-encoded values with a proper 3-step pipeline: gamma decoding, applying a 3x3 matrix in linear space, and then gamma encoding. This fix is consistently applied in both C++ (dl_color.cc) and Dart (painting.dart), and the reverse sRGB-to-P3 conversion is also corrected in the Dart implementation. The addition of new unit tests for both C++ and Dart is excellent, as they verify the new conversion logic, particularly for mid-range colors where the previous error was most significant. My review includes suggestions to replace magic numbers with named constants to improve code readability and maintainability.

engine/src/flutter/display_list/dl_color.cc

engine/src/flutter/lib/ui/painting.dart

jtmcdole · 2026-01-30T15:54:04Z

Re-running flutter-dashboard / Mac_arm64 build_tests_4_4 since it looks like a flake

Thanks for the PR!

gaaclarke

This is awesome, thanks @westito. I double checked all the tests with Apple's color utility and they are good. Apple gives slightly different values but that might be captured in the epsilon of the assertions. The test coverage is great, no need to add tests.

In fact there is nothing else you need to do. I've made a few notes about minor cleanup and deduplication which I'll look into. I'll commit them here or implement in a followup.

gaaclarke · 2026-01-30T17:33:10Z

engine/src/flutter/display_list/dl_color.cc

+// gamma encode.
+DlColor p3ToExtendedSrgb(const DlColor& color) {
+  // Linearize P3 values (P3 uses same transfer function as sRGB).
+  double r_lin = srgbEOTFExtended(static_cast<double>(color.getRedF()));


no action required: do we want to just keep everything in floats?

gaaclarke · 2026-01-30T17:33:50Z

engine/src/flutter/display_list/dl_color.cc

+  double b_lin = srgbEOTFExtended(static_cast<double>(color.getBlueF()));
+
+  // Apply 3x3 P3-to-sRGB matrix in linear space.
+  double r_srgb_lin = kP3ToSrgbLinear[0] * r_lin + kP3ToSrgbLinear[1] * g_lin +


no action required: we have matrix multiplication available in our api

gaaclarke · 2026-01-30T17:34:09Z

engine/src/flutter/display_list/dl_color.cc

-                     matrix[11],                      //
-                 color_space);
+
+// sRGB standard constants for transfer functions.


no action required: comments should be in doxygen format

gaaclarke · 2026-01-30T17:36:45Z

engine/src/flutter/display_list/dl_color_unittests.cc

+  // Red channel diff: 0.2569 — well above isClose tolerance of 1/256.
+  DlColor p3_teal(1.0, 30.0f / 255.0f, 202.0f / 255.0f, 211.0f / 255.0f,
+                  DlColorSpace::kDisplayP3);
+  DlColor expected_teal(1.0, -0.3764f, 0.8065f, 0.8372f,


no action required: apple's color tool gives (-0.3778, 0.8084, 0.8358)

gaaclarke · 2026-01-30T17:37:41Z

engine/src/flutter/lib/ui/painting.dart

+
+// Converts Display P3 (gamma-encoded) to extended sRGB (gamma-encoded).
+// Pipeline: EOTF(decode) -> 3x3 matrix -> OETF(encode).
+class _P3ToSrgbTransform implements _ColorTransform {


no action required: We should call into c++ for this instead of implementing it twice (now that it is a bit more involved)

gaaclarke · 2026-01-30T17:39:29Z

engine/src/flutter/lib/web_ui/lib/painting.dart


-class _MatrixColorTransform implements _ColorTransform {
-  const _MatrixColorTransform(this.values);
+// sRGB standard constants for transfer functions.


no action required: let's find a way to share this with web.

gaaclarke

I looked briefly into deduplicating the logic and it is a bit involved. If dart:ui calls into c++ then the code can't be shared with web. There is also no existing good way to share logic between the web and standard dart:ui so I don't want to try to do it on top of this PR. Everything else is minor nits and LGTM. Thanks again.

westito · 2026-01-30T18:24:11Z

Thanks! Deduplication may be possible: Color conversion could be done at Dart level when Color is constructed. The problem is, current constructor is constant, any change would cause API break. That's why I chose to implement it at the native level.

jtmcdole

LGTW (looks good to wikipedia specification)

…lutter/flutter#181720)

westito · 2026-01-31T19:18:14Z

gaaclarke: This is awesome, thanks westito. I double checked all the tests with Apple's color utility and they are good. Apple gives slightly different values but that might be captured in the epsilon of the assertions. The test coverage is great, no need to add tests.

In fact there is nothing else you need to do. I've made a few notes about minor cleanup and deduplication which I'll look into. I'll commit them here or implement in a followup.

I found the issue for the slight difference: iOS engine uses MTLPixelFormatBGRA10_XR pixel format that causes precision lost. Should use MTLPixelFormatRGBA16Float for full wide gamut support. It would be acceptable to use 16bit pixel format when developer enables wide gamut? I think it wouldn't have a high performance impact, considering that all native iOS apps use 16bit and supports full-precision wide gamut. If user need performance or experience performance issues, should disable that. I think wide gamut should fully supported. 10bit vs 16bit have no memory usage difference, only affect GPU processing speed. I'm working on macOS wide gamut support where I found this precision lost, then I would include iOS fix also.

…lutter/flutter#181720)

gaaclarke · 2026-02-02T17:12:35Z

I found the issue for the slight difference: iOS engine uses MTLPixelFormatBGRA10_XR pixel format that causes precision lost. Should use MTLPixelFormatRGBA16Float for full wide gamut support. It would be acceptable to use 16bit pixel format when developer enables wide gamut? I think it wouldn't have a high performance impact, considering that all native iOS apps use 16bit and supports full-precision wide gamut. If user need performance or experience performance issues, should disable that. I think wide gamut should fully supported. 10bit vs 16bit have no memory usage difference, only affect GPU processing speed. I'm working on macOS wide gamut support where I found this precision lost, then I would include iOS fix also.

When the feature was originally implemented we looked into this and there was a noticeable difference in performance from what I recall. The team was reticent to give up any performance for wide gamut colors. We can reevaluate that in the future as things change. There isn't a lot of bandwidth to allocate to that now, especially considering wide gamut for android would be a higher priority in my mind.

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…81720)  ### Problem The P3-to-sRGB color conversion applies an affine matrix directly to gamma-encoded values in both the C++ engine (`dl_color.cc`) and Dart (`painting.dart`). This is mathematically incorrect — color space conversion matrices must operate in linear light. The result is P3 colors that appear nearly identical to sRGB instead of showing the full gamut difference. For example, P3 `#1ECAD3` (30/255, 202/255, 211/255) converts to: - **Old (wrong):** extendedSRGB (-0.12, 0.86, 0.87) - **New (correct):** extendedSRGB (-0.38, 0.81, 0.84) The red channel error of 0.26 is clearly visible — colors appear washed out instead of vivid. ### Fix Replace the single affine matrix multiply with the correct 3-step pipeline: 1. **Linearize** — decode gamma via sRGB EOTF 2. **Transform** — apply 3x3 P3-to-sRGB matrix in linear space 3. **Encode** — re-apply gamma via sRGB OETF Extended range (negative values from out-of-gamut colors) is handled by mirroring the transfer function. **C++ (`dl_color.cc`):** Replace affine matrix with `p3ToExtendedSrgb()` using `double` precision. **Dart (`painting.dart`):** Replace `_MatrixColorTransform` with `_P3ToSrgbTransform` and `_SrgbToP3Transform` classes. Also fixes the sRGB-to-P3 direction. ### Performance Negligible. The C++ conversion runs once per paint setup in `ReadColor`, not per frame or per pixel. The Dart conversion runs once per `Color.withValues()` call. ### Tests - **C++:** Added `ColorSpaceP3ToExtendedSRGBLinearLight` test in `dl_color_unittests.cc` - **Dart:** Added 3 tests in `colors_test.dart`: mid-range P3→extendedSRGB, P3 green→extendedSRGB, sRGB→P3 round-trip All new tests fail with the old code and pass with the fix. Existing tests continue to pass. ## Issue: flutter#181717 ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [ ] All existing and new tests are passing. **New tests pass, can't run golden tests!** --------- Co-authored-by: gaaclarke <[email protected]>

westito added 2 commits January 30, 2026 11:20

Fix P3-to-sRGB color conversion to operate in linear light

29b815c

Fix Dart-side P3 color space conversion to operate in linear light

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Fix P3-to-sRGB color conversion on WEB platform

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westito commented Jan 30, 2026

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Fix

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