Do not derive a decode size of 0 for very thin images by YeungKC · Pull Request #194669 · flutter/flutter

YeungKC · 2026-10-18T06:45:36Z

Fixes #194668

A very thin image does not decode when the decode size is derived from one side.
Example: a 4000x1 PNG with cacheWidth: 100. The derived height is
100 ~/ 4000 = 0, and the decode fails.

Observed on stable 3.47.7 (the same code is on master):

Call (4000x1 or 1x4000 PNG)	Result
`Image.memory(..., cacheWidth: 100)` / `ResizeImage(width: 100)`	`Exception: Codec failed to produce an image` (log: `Could not resize to empty dimensions.`)
`ResizeImage(height: 100)` on 1x4000	same exception
`ResizeImage(width: 100, policy: ResizeImagePolicy.fit)` (with or without `allowUpscaling`, with or without `height`)	debug: the assertion `height == null \|\| height > 0` (or `width == null \|\| width > 0`) in `TargetImageSize` fails. By reading the code, in release mode the 0 becomes null, `instantiateCodec` derives 0 again, and the codec fails.
`ImageDescriptor.instantiateCodec(targetWidth: 100)` / `ui.instantiateImageCodec(targetWidth: 100)`	same exception

This change clamps each derived side to at least 1 pixel:

ImageDescriptor.instantiateCodec in dart:ui (lib/ui/painting.dart).
This is the path for cacheWidth or cacheHeight with one side, and for
instantiateImageCodec with one target side.
scaledImageSize in the web engine, which has the same derivation.
ResizeImage with ResizeImagePolicy.fit. The clamp is on each side that
a branch derives from the aspect ratio.

A side that the caller gives is not changed, so an explicit 0 still fails at
decode time, as before, and is not changed to 1. useLogicalSize uses ceil, so it cannot give 0.

Tests

packages/flutter/test/painting/image_provider_resize_image_test.dart:
4000x1 and 1x4000 images with policy: fit decode to 100x1 and 1x100.
This test fails without the change (assertion in TargetImageSize) and
passes with it. The painting, image and fade tests pass locally.
packages/flutter/test/painting/image_provider_resize_image_test.dart:
ResizeImage with width or height of 0 still fails to decode with
policy: exact and policy: fit.
engine/src/flutter/testing/dart/codec_test.dart:
instantiateImageCodec(targetWidth: 100) on 4000x1 gives 100x1, and
targetHeight: 100 on 1x4000 gives 1x100.
engine/src/flutter/lib/web_ui/test/ui/image_test.dart: scaledImageSize
returns 100x1 and 1x100 for the same inputs.

I did not build the engine locally, so the codec_test.dart and web tests run
only in CI.

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 ///). No doc change is necessary.
I added new tests to check the change I am making, or this PR is test-exempt.
All existing and new tests are passing. The framework tests pass locally. The engine and web tests run only in CI.

Fixes flutter#194668

gemini-code-assist

Code Review

This pull request prevents image scaling calculations from resulting in a width or height of 0 for very thin images by enforcing a minimum dimension of 1 in painting.dart, image_decoder.dart, and image_provider.dart. It also adds corresponding unit tests to verify this behavior. Feedback on the changes points out a potential native memory leak in codec_test.dart because the ui.Image returned by frameInfo.image is not disposed.

harryterkelsen · 2026-10-19T16:35:45Z

              }

-              return ui.TargetImageSize(width: targetWidth, height: targetHeight);
+              // A very thin image can derive a side of 0, which cannot be decoded.


This change mostly looks good to me, but I think we are changing behavior when a user explicitly passes a size of 0. Currently if you do ResizeImage(width: 0, ResizeImagePolicy.fit) it will throw an error, as it should. With this code, I think it will silently just create a 1x1 image. Let's change the ResizeImage constructor to assert that the width and height are > 0 and add a test for this case.

Thanks, you are correct. I checked it: before this PR, ResizeImage(width: 0, policy: ResizeImagePolicy.fit) failed the TargetImageSize assertion. With the first version of this PR, it decoded a 1x1 image.

6ff762f changes this:

The ResizeImage constructor now asserts width == null || width > 0 and height == null || height > 0. This is the same check that Image already does for cacheWidth and cacheHeight. An explicit 0 or a negative value now fails at construction, for all policies.

Because the given sides are always greater than 0, the clamp in the fit branch changes only a derived side.

Test: ResizeImage with width or height of 0 or -1, with exact and fit, throws an AssertionError.

The engine change does not change an explicit 0. instantiateCodec already sets a target side that is <= 0 to null before it derives a side, and the clamp is only in the branches that derive a side.

test/painting/, test/widgets/image_test.dart, and flutter analyze pass locally.

harryterkelsen

Thanks! LGTM now with the ResizeImage fix

…onstructor An explicit width or height of 0 now fails at decode time, as before this change.

YeungKC · 2026-10-19T19:17:09Z

Thank you for the approval. After it, the "Google testing" check failed. I cannot see the details of that check.

I suspect that the new constructor asserts cause the failure. They fail as soon as a caller constructs ResizeImage with a width or height of 0, also when the image is never decoded. Some callers can pass 0 without an earlier check. For example, FadeInImage does not assert that imageCacheWidth or placeholderCacheWidth is greater than 0. On master, such a test passes if it does not decode the image.

I prepared an alternative commit. It removes the constructor asserts. It clamps to 1 only the sides that ResizeImagePolicy.fit derives from the aspect ratio, and never a side that the caller gives. Thus, an explicit ResizeImage(width: 0) with fit or exact fails at decode time, as on master. The thin image cases still decode to 100x1 and 1x100. The new tests check both behaviors.

Can you tell me if the internal failure is in fact this assert? If it is, I can push the alternative commit to this PR.

harryterkelsen · 2026-10-19T21:24:13Z

Thank you for the approval. After it, the "Google testing" check failed. I cannot see the details of that check.

I suspect that the new constructor asserts cause the failure. They fail as soon as a caller constructs ResizeImage with a width or height of 0, also when the image is never decoded. Some callers can pass 0 without an earlier check. For example, FadeInImage does not assert that imageCacheWidth or placeholderCacheWidth is greater than 0. On master, such a test passes if it does not decode the image.

I prepared an alternative commit. It removes the constructor asserts. It clamps to 1 only the sides that ResizeImagePolicy.fit derives from the aspect ratio, and never a side that the caller gives. Thus, an explicit ResizeImage(width: 0) with fit or exact fails at decode time, as on master. The thin image cases still decode to 100x1 and 1x100. The new tests check both behaviors.

Can you tell me if the internal failure is in fact this assert? If it is, I can push the alternative commit to this PR.

Yes, I took a look and the Google testing failure is caused by the assert in ResizeImage. I think your fix of only adjusting the "derived" sides sounds correct. Please push it here and I'll take a look

YeungKC · 2026-10-20T01:38:46Z

Thanks. I pushed b84ce82. It removes the new constructor asserts and clamps only the derived sides in the fit branch. A width or height of 0 that the caller gives is not changed, so it still fails at decode time, as before. The assert test is replaced with a test for this case. The painting, image and fade tests pass locally.

victorsanni

LGTM for framework side

Fixes flutter#194668 A very thin image does not decode when the decode size is derived from one side. Example: a 4000x1 PNG with `cacheWidth: 100`. The derived height is `100 ~/ 4000 = 0`, and the decode fails. Observed on stable 3.47.7 (the same code is on master): | Call (4000x1 or 1x4000 PNG) | Result | | --- | --- | | `Image.memory(..., cacheWidth: 100)` / `ResizeImage(width: 100)` | `Exception: Codec failed to produce an image` (log: `Could not resize to empty dimensions.`) | | `ResizeImage(height: 100)` on 1x4000 | same exception | | `ResizeImage(width: 100, policy: ResizeImagePolicy.fit)` (with or without `allowUpscaling`, with or without `height`) | debug: the assertion `height == null \|\| height > 0` (or `width == null \|\| width > 0`) in `TargetImageSize` fails. By reading the code, in release mode the 0 becomes null, `instantiateCodec` derives 0 again, and the codec fails. | | `ImageDescriptor.instantiateCodec(targetWidth: 100)` / `ui.instantiateImageCodec(targetWidth: 100)` | same exception | This change clamps each derived side to at least 1 pixel: * `ImageDescriptor.instantiateCodec` in `dart:ui` (`lib/ui/painting.dart`). This is the path for `cacheWidth` or `cacheHeight` with one side, and for `instantiateImageCodec` with one target side. * `scaledImageSize` in the web engine, which has the same derivation. * `ResizeImage` with `ResizeImagePolicy.fit`. The clamp is on each side that a branch derives from the aspect ratio. A side that the caller gives is not changed, so an explicit 0 still fails at decode time, as before, and is not changed to 1. `useLogicalSize` uses `ceil`, so it cannot give 0. ## Tests * `packages/flutter/test/painting/image_provider_resize_image_test.dart`: 4000x1 and 1x4000 images with `policy: fit` decode to 100x1 and 1x100. This test fails without the change (assertion in `TargetImageSize`) and passes with it. The painting, image and fade tests pass locally. * `packages/flutter/test/painting/image_provider_resize_image_test.dart`: `ResizeImage` with `width` or `height` of 0 still fails to decode with `policy: exact` and `policy: fit`. * `engine/src/flutter/testing/dart/codec_test.dart`: `instantiateImageCodec(targetWidth: 100)` on 4000x1 gives 100x1, and `targetHeight: 100` on 1x4000 gives 1x100. * `engine/src/flutter/lib/web_ui/test/ui/image_test.dart`: `scaledImageSize` returns 100x1 and 1x100 for the same inputs. I did not build the engine locally, so the `codec_test.dart` and web tests run only in CI. ## 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. - [ ] I updated/added relevant documentation (doc comments with `///`). No doc change is necessary. - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [ ] All existing and new tests are passing. The framework tests pass locally. The engine and web tests run only in CI. [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/

Do not derive a decode size of 0 for very thin images

511ffb0

Fixes flutter#194668

flutter-dashboard Bot added the CICD Run CI/CD label Oct 18, 2026

github-actions Bot added framework flutter/packages/flutter repository. See also f: labels. engine flutter/engine related. See also e: labels. platform-web Web applications specifically team-web Owned by Web platform team labels Oct 18, 2026

gemini-code-assist Bot reviewed Oct 18, 2026

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Comment thread engine/src/flutter/testing/dart/codec_test.dart Resolved

Dispose the decoded image in the thin image codec test

aeffa2a

YeungKC requested review from harryterkelsen and victorsanni October 19, 2026 13:25

harryterkelsen requested changes Oct 19, 2026

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Assert that ResizeImage width and height are greater than 0

6ff762f

YeungKC requested a review from harryterkelsen October 19, 2026 16:49

harryterkelsen previously approved these changes Oct 19, 2026

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Clamp only derived sizes in ResizeImage instead of asserting in the c…

b84ce82

…onstructor An explicit width or height of 0 now fails at decode time, as before this change.

harryterkelsen self-requested a review October 19, 2026 21:27

YeungKC dismissed harryterkelsen’s stale review via b84ce82 October 20, 2026 01:35

harryterkelsen approved these changes Oct 20, 2026

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harryterkelsen added the autosubmit Merge PR when tree becomes green via auto submit App label Oct 20, 2026

victorsanni approved these changes Oct 20, 2026

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auto-submit Bot added this pull request to the merge queue Oct 20, 2026

Merged via the queue into flutter:master with commit 0af6449 Oct 20, 2026
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flutter-dashboard Bot removed the autosubmit Merge PR when tree becomes green via auto submit App label Oct 20, 2026

justinmc mentioned this pull request Oct 23, 2026

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Do not derive a decode size of 0 for very thin images - #194669

Do not derive a decode size of 0 for very thin images#194669
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