Fix square corner issue by andywolff · Pull Request #191096 · flutter/flutter

andywolff · 2026-08-14T03:43:15Z

Fixes #190602, see my comment on that issue for details about the problems I identified.
Also fixes #162172

What changed

When a route contained rounded clips (such as CupertinoSheetRoute), intermediate child ink features, and a backdrop filter (such as a blurred app bar), the clip stack replay logic prematurely popped parent clips and exceeded the depth buffer budget across render pass splits, causing subsequent header draws to render with 90° square corners. This PR addresses both mechanisms:

First, EntityPassClipStack::RecordRestore was popping replay entities unconditionally whenever any clip was restored. In a sheet containing ink decorations (like list item ripple effects), child clip restores at deeper heights would prematurely discard the root parent clip. We now track clip_height in ReplayResult so that restorations only pop entities when clip_height > restore_height, preserving parent clips across sibling restores. When replaying those clips in ClipContents::Render, we also supply the entity's model matrix to properly recompute the MVP matrix against the new pass's orthographic projection instead of reusing the stale matrix from the prior pass.

Second, the pass depth budget was exhausting across backdrop splits. Canvas::kMaxDepth (1 << 24) was mismatched with Entity::kDepthEpsilon (1.0 / 262144.0, or 2^-18), allowing integer depths to exceed the 18-bit precision of the depth buffer. Aligning Canvas::kMaxDepth to 262,143 and capping depth allocations in Save/SaveLayer to min(current_depth_ + total_content_depth, parent_clip_depth) keeps clip replay and subsequent draw operations within valid precision bounds. In Canvas::Restore, we also prevent unconditional depth jumps to kMaxDepth for unclipped saves, preserving the allocated budget for subsequent sibling draws.

Testing

Reproducing this interaction in automated tests required testing the onscreen pass-splitting branch (IsOnscreen()), which normal offscreen unit tests bypass. We added a ScopedOnscreenOverride test hook in Canvas and constructed a multi-layer display list recreating the route clip, child ink items, blurred backdrop bar, and trailing app bar:

clip_stack_unittests.cc: Added EntityPassClipStackTest.RestoringChildClipPreservesParentReplayEntities to isolate parent clip retention across sibling restores.
canvas_unittests.cc: Added AiksTest.ClipDepthMaintainedAcrossBackdropFilterAndLayers to verify depth invariants and precision bounds (<= kMaxDepth), and AiksTest.ParentClipDepthBudgetPreservedAfterBackdropLayerRestore to verify parent depth budget restoration.
dl_golden_unittests.cc (impeller_golden_tests): Added DlGoldenTest.CanRenderClippedBackdropFilterWithSuperellipse for pixel-accurate golden image verification in CI against Skia Gold.

Pre-launch Checklist

andywolff · 2026-09-01T21:22:39Z

/gemini review

gemini-code-assist

Code Review

This pull request refactors clip depth tracking and restoration logic in Impeller's Canvas, including fixes to clip restoration in EntityPassClipStack and the addition of corresponding unit tests. Feedback highlights a critical bug in Canvas::FlipBackdrop where mutating clip_depth to kMaxDepth permanently overwrites parent scope depth budgets. Additionally, the reviewer recommends addressing a style guide violation regarding the naming and encapsulation of the thread-local testing override, replacing std::min<uint64_t> with std::min<uint32_t> to avoid unnecessary type promotion, and removing verbose debug logging statements before merging.

andywolff · 2026-09-01T22:19:44Z

/gemini review

gemini-code-assist

Code Review

This pull request modifies Impeller's Canvas clip depth management to preserve active parent clips across sibling restorations and backdrop filter splits, reducing kMaxDepth and updating EntityPassClipStack. The reviewer feedback identifies a critical issue where tracking and restoring pre-split clip depth can cause debug crashes (FML_DCHECK failures) in split passes. It is recommended to remove the pre_split_clip_depth tracking entirely, simplify FlipBackdrop to use kMaxDepth, and update the unit tests accordingly.

andywolff · 2026-09-02T15:23:55Z

Code Review

This pull request modifies Impeller's Canvas clip depth management to preserve active parent clips across sibling restorations and backdrop filter splits, reducing kMaxDepth and updating EntityPassClipStack. The reviewer feedback identifies a critical issue where tracking and restoring pre-split clip depth can cause debug crashes (FML_DCHECK failures) in split passes. It is recommended to remove the pre_split_clip_depth tracking entirely, simplify FlipBackdrop to use kMaxDepth, and update the unit tests accordingly.

...redacted defense of the prior approach...

Update: When I took this PR out of draft, the goldens bot notified me that there were untriaged goldens. It turns out, it was breaking some behavior. Resetting current_depth_ = 0 and overriding clip_depth in FlipBackdrop broke depth monotonicity during inline advanced blend emulation (which also calls FlipBackdrop). For this reason, I changed approach, removing pre_split_clip_depth and the depth resets from FlipBackdrop entirely so depth remains monotonic across both backdrop splits and emulated blends. I added a new unit test to confirm this so it can be caught before golden diffing.

gemini-code-assist · 2026-09-02T19:45:04Z

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flutter-dashboard · 2026-09-02T20:31:56Z

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Changes reported for pull request #191096 at sha b04c8a7

andywolff · 2026-09-02T20:37:04Z

Returning to draft because those golden diffs don't look right to me

flutter-dashboard · 2026-09-02T20:38:52Z

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.

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flar · 2026-09-22T21:20:57Z

  auto entry = CanvasStackEntry{};
  entry.transform = transform_stack_.back().transform;
-  entry.clip_depth = current_depth_ + total_content_depth;
+  entry.clip_depth = std::min<uint32_t>(current_depth_ + total_content_depth,


This part of the fix concerns me. The current depth + the total content depth should never be greater than the clip depth. If that happens then something else consumed some depth values that we never accounted for. If the DCHECK is triggering then we need to adjust the clip accounting, not simply choose a lower depth.

andywolff · 2026-09-22T21:50:55Z

reason for revert: seems to have caused a test order dependency #192992

Also, thanks @flar, I'm not sure whether that is the cause of the test order dependency, but agree with that concern and I'll address it when I re-land

flutteractionsbot · 2026-09-22T22:03:34Z

Successfully created revert PR: #193189

@andywolff

Reverts: [Fix square corner issue](flutter#191096) Initiated by: @andywolff Reason for reverting: seems to have caused a test order dependency flutter#192992 Original PR Author: @andywolff Reviewed By: @gaaclarke The original PR description is provided below: Fixes flutter#190602, see my [comment](flutter#190602 (comment)) on that issue for details about the problems I identified. Also fixes flutter#162172 ## What changed When a route contained rounded clips (such as `CupertinoSheetRoute`), intermediate child ink features, and a backdrop filter (such as a blurred app bar), the clip stack replay logic prematurely popped parent clips and exceeded the depth buffer budget across render pass splits, causing subsequent header draws to render with 90° square corners. This PR addresses both mechanisms: First, `EntityPassClipStack::RecordRestore` was popping replay entities unconditionally whenever any clip was restored. In a sheet containing ink decorations (like list item ripple effects), child clip restores at deeper heights would prematurely discard the root parent clip. We now track `clip_height` in `ReplayResult` so that restorations only pop entities when `clip_height > restore_height`, preserving parent clips across sibling restores. When replaying those clips in `ClipContents::Render`, we also supply the entity's model matrix to properly recompute the MVP matrix against the new pass's orthographic projection instead of reusing the stale matrix from the prior pass. Second, the pass depth budget was exhausting across backdrop splits. `Canvas::kMaxDepth` (`1 << 24`) was mismatched with `Entity::kDepthEpsilon` (`1.0 / 262144.0`, or `2^-18`), allowing integer depths to exceed the 18-bit precision of the depth buffer. Aligning `Canvas::kMaxDepth` to `262,143` and capping depth allocations in `Save`/`SaveLayer` to `min(current_depth_ + total_content_depth, parent_clip_depth)` keeps clip replay and subsequent draw operations within valid precision bounds. In `Canvas::Restore`, we also prevent unconditional depth jumps to `kMaxDepth` for unclipped saves, preserving the allocated budget for subsequent sibling draws. ## Testing Reproducing this interaction in automated tests required testing the onscreen pass-splitting branch (`IsOnscreen()`), which normal offscreen unit tests bypass. We added a `ScopedOnscreenOverride` test hook in `Canvas` and constructed a multi-layer display list recreating the route clip, child ink items, blurred backdrop bar, and trailing app bar: * `clip_stack_unittests.cc`: Added `EntityPassClipStackTest.RestoringChildClipPreservesParentReplayEntities` to isolate parent clip retention across sibling restores. * `canvas_unittests.cc`: Added `AiksTest.ClipDepthMaintainedAcrossBackdropFilterAndLayers` to verify depth invariants and precision bounds (`<= kMaxDepth`), and `AiksTest.ParentClipDepthBudgetPreservedAfterBackdropLayerRestore` to verify parent depth budget restoration. * `dl_golden_unittests.cc` (`impeller_golden_tests`): Added `DlGoldenTest.CanRenderClippedBackdropFilterWithSuperellipse` for pixel-accurate golden image verification in CI against Skia Gold. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools. - [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. - [x] All existing and new tests are passing.  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@andywolff

@andywolff

Fixes flutter#190602, see my [comment](flutter#190602 (comment)) on that issue for details about the problems I identified. Also fixes flutter#162172 ## What changed When a route contained rounded clips (such as `CupertinoSheetRoute`), intermediate child ink features, and a backdrop filter (such as a blurred app bar), the clip stack replay logic prematurely popped parent clips and exceeded the depth buffer budget across render pass splits, causing subsequent header draws to render with 90° square corners. This PR addresses both mechanisms: First, `EntityPassClipStack::RecordRestore` was popping replay entities unconditionally whenever any clip was restored. In a sheet containing ink decorations (like list item ripple effects), child clip restores at deeper heights would prematurely discard the root parent clip. We now track `clip_height` in `ReplayResult` so that restorations only pop entities when `clip_height > restore_height`, preserving parent clips across sibling restores. When replaying those clips in `ClipContents::Render`, we also supply the entity's model matrix to properly recompute the MVP matrix against the new pass's orthographic projection instead of reusing the stale matrix from the prior pass. Second, the pass depth budget was exhausting across backdrop splits. `Canvas::kMaxDepth` (`1 << 24`) was mismatched with `Entity::kDepthEpsilon` (`1.0 / 262144.0`, or `2^-18`), allowing integer depths to exceed the 18-bit precision of the depth buffer. Aligning `Canvas::kMaxDepth` to `262,143` and capping depth allocations in `Save`/`SaveLayer` to `min(current_depth_ + total_content_depth, parent_clip_depth)` keeps clip replay and subsequent draw operations within valid precision bounds. In `Canvas::Restore`, we also prevent unconditional depth jumps to `kMaxDepth` for unclipped saves, preserving the allocated budget for subsequent sibling draws. ## Testing Reproducing this interaction in automated tests required testing the onscreen pass-splitting branch (`IsOnscreen()`), which normal offscreen unit tests bypass. We added a `ScopedOnscreenOverride` test hook in `Canvas` and constructed a multi-layer display list recreating the route clip, child ink items, blurred backdrop bar, and trailing app bar: * `clip_stack_unittests.cc`: Added `EntityPassClipStackTest.RestoringChildClipPreservesParentReplayEntities` to isolate parent clip retention across sibling restores. * `canvas_unittests.cc`: Added `AiksTest.ClipDepthMaintainedAcrossBackdropFilterAndLayers` to verify depth invariants and precision bounds (`<= kMaxDepth`), and `AiksTest.ParentClipDepthBudgetPreservedAfterBackdropLayerRestore` to verify parent depth budget restoration. * `dl_golden_unittests.cc` (`impeller_golden_tests`): Added `DlGoldenTest.CanRenderClippedBackdropFilterWithSuperellipse` for pixel-accurate golden image verification in CI against Skia Gold. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools. - [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. - [x] All existing and new tests are passing.  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Fixes flutter#190602, see my [comment](flutter#190602 (comment)) on that issue for details about the problems I identified. Also fixes flutter#162172 When a route contained rounded clips (such as `CupertinoSheetRoute`), intermediate child ink features, and a backdrop filter (such as a blurred app bar), the clip stack replay logic prematurely popped parent clips and exceeded the depth buffer budget across render pass splits, causing subsequent header draws to render with 90° square corners. This PR addresses both mechanisms: First, `EntityPassClipStack::RecordRestore` was popping replay entities unconditionally whenever any clip was restored. In a sheet containing ink decorations (like list item ripple effects), child clip restores at deeper heights would prematurely discard the root parent clip. We now track `clip_height` in `ReplayResult` so that restorations only pop entities when `clip_height > restore_height`, preserving parent clips across sibling restores. When replaying those clips in `ClipContents::Render`, we also supply the entity's model matrix to properly recompute the MVP matrix against the new pass's orthographic projection instead of reusing the stale matrix from the prior pass. Second, the pass depth budget was exhausting across backdrop splits. `Canvas::kMaxDepth` (`1 << 24`) was mismatched with `Entity::kDepthEpsilon` (`1.0 / 262144.0`, or `2^-18`), allowing integer depths to exceed the 18-bit precision of the depth buffer. Aligning `Canvas::kMaxDepth` to `262,143` and capping depth allocations in `Save`/`SaveLayer` to `min(current_depth_ + total_content_depth, parent_clip_depth)` keeps clip replay and subsequent draw operations within valid precision bounds. In `Canvas::Restore`, we also prevent unconditional depth jumps to `kMaxDepth` for unclipped saves, preserving the allocated budget for subsequent sibling draws. Reproducing this interaction in automated tests required testing the onscreen pass-splitting branch (`IsOnscreen()`), which normal offscreen unit tests bypass. We added a `ScopedOnscreenOverride` test hook in `Canvas` and constructed a multi-layer display list recreating the route clip, child ink items, blurred backdrop bar, and trailing app bar: * `clip_stack_unittests.cc`: Added `EntityPassClipStackTest.RestoringChildClipPreservesParentReplayEntities` to isolate parent clip retention across sibling restores. * `canvas_unittests.cc`: Added `AiksTest.ClipDepthMaintainedAcrossBackdropFilterAndLayers` to verify depth invariants and precision bounds (`<= kMaxDepth`), and `AiksTest.ParentClipDepthBudgetPreservedAfterBackdropLayerRestore` to verify parent depth budget restoration. * `dl_golden_unittests.cc` (`impeller_golden_tests`): Added `DlGoldenTest.CanRenderClippedBackdropFilterWithSuperellipse` for pixel-accurate golden image verification in CI against Skia Gold. - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools. - [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. - [x] All existing and new tests are passing.  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@flar

…195716) Fixes flutter#162172 Fixes flutter#190602 Fixes flutter#192992 Fixes flutter#195423 Revised reland of the square corners fix in flutter#191096. Previously, `EntityPassClipStack::RecordRestore` unconditionally popped a single clip replay entry even when the restored scope had skipped a redundant clip or recorded multiple clips, which caused parent rounded clips to get dropped across `BackdropFilter` flips (see my [comment on flutter#190602](flutter#190602 (comment))). `EntityPassClipStack` now tracks `clip_height` on `ReplayResult` so `RecordRestore` only pops entries above `restore_height`. This is the only production code change needed to fix the issue. I structured this PR as two commits to make the diff against the original landing easy to see. Commit 1 is flutter#191096 unmodified, and Commit 2 reverts all of the `Canvas` and `ClipContents` changes back to `master`. In flutter#191096, I gated `should_use_onscreen` on `is_onscreen_` in `Canvas::SaveLayer` to make unit testing easier, which routed `DisplayListToTexture` snapshots through `Canvas::BlitToOnscreen` with an uncleared 4x MSAA depth attachment and caused the `action_sheet_test.dart` alpha flake (see my [comment on flutter#195423](flutter#195423 (comment))). Reverting the `Canvas` changes fixes that flake and removes the clip-depth clamping that @flar flagged in review. Instead of modifying `Canvas` for testability, I replaced the `CreateTestCanvas` tests from Commit 1 with `DisplayListToTexture` pixel-readback tests in `canvas_unittests.cc`, along with unit tests in `clip_stack_unittests.cc` and `aiks_dl_clip_unittests.cc`. I rewrote those tests significantly so they are smaller for the same coverage, and added a test which covers the problem which resulted in the alpha flake behavior. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools. - [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 in-code documentation (doc comments with `///`). - [x] If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in [flutter/website] (or verified none is needed). - [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. - [x] All existing and new tests are passing.  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [AI contribution guidelines]: https://github.com/flutter/flutter/blob/main/CONTRIBUTING.ai.md [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/ [flutter/tests]: https://github.com/flutter/tests [flutter/website]: https://github.com/flutter/website [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

github-actions Bot added engine flutter/engine related. See also e: labels. e: impeller Impeller rendering backend issues and features requests labels Aug 14, 2026

andywolff force-pushed the corners branch from e45bdb1 to 2991ea4 Compare August 18, 2026 00:31

andywolff added CICD Run CI/CD and removed CICD Run CI/CD labels Aug 18, 2026

andywolff force-pushed the corners branch from 2991ea4 to f876042 Compare August 21, 2026 17:34

andywolff force-pushed the corners branch from f876042 to 68ef4fb Compare September 1, 2026 20:52

andywolff added the CICD Run CI/CD label Sep 1, 2026

andywolff force-pushed the corners branch from 68ef4fb to a9fc7e0 Compare September 1, 2026 21:19

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andywolff force-pushed the corners branch from 1c9f013 to b04c8a7 Compare September 2, 2026 15:36

andywolff marked this pull request as ready for review September 2, 2026 19:45

andywolff requested review from b-luk and gaaclarke September 2, 2026 19:45

flutter-dashboard Bot added the will affect goldens Changes to golden files label Sep 2, 2026

andywolff removed request for b-luk and gaaclarke September 2, 2026 20:36

andywolff marked this pull request as draft September 2, 2026 20:37

andywolff force-pushed the corners branch 3 times, most recently from 418ee31 to 7d48f5d Compare September 2, 2026 23:16

gaaclarke mentioned this pull request Sep 21, 2026

DCHECK failing on debug engines ( clip_depth assert ) #193124

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justinmc mentioned this pull request Sep 21, 2026

Test randomization for framework_tests_shard failing on skia gold #192992

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flar reviewed Sep 22, 2026

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andywolff added the revert Autorevert PR (with "Reason for revert:" comment) label Sep 22, 2026

flutteractionsbot mentioned this pull request Sep 22, 2026

Revert: Fix square corner issue #193189

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flutteractionsbot removed the revert Autorevert PR (with "Reason for revert:" comment) label Sep 22, 2026

andywolff mentioned this pull request Sep 23, 2026

[Impeller] RenderTargetCache reuses offscreen render targets without GPU completion synchronization #193229

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justinmc mentioned this pull request Sep 25, 2026

Flutter Notable Commits - 2026 #181433

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This was referenced Oct 13, 2026

Reland square corners fix #194314

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Remove clamping and restore clip depth assertion in Canvas::SaveLayer #194412

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gaaclarke mentioned this pull request Oct 27, 2026

Mac framework_tests_impeller is flakey #195423

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flutteractionsbot mentioned this pull request Oct 27, 2026

Revert: Reland square corners fix #195436

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andywolff mentioned this pull request Oct 30, 2026

[Impeller] Reland clipped BackdropFilter square corners fix #195716

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andywolff commented Aug 14, 2026 •

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