[Impeller] GetLocalCoverageLimit unconditionally clamps to root render target, causing incorrect subpass texture sizing and culling

Problem
Canvas::GetLocalCoverageLimit() in engine/src/flutter/impeller/display_list/canvas.cc ends with:
return maybe_coverage_limit->Intersection( Rect::MakeSize(render_target_.GetRenderTargetSize()));
This intersects the coverage limit with the root render target size. For the root pass this is a no-op (the pass texture IS the root RT). For subpasses (saveLayer) it's wrong: a subpass texture can extend beyond the root RT — shifted by a translate, or expanded by an image filter — and the root RT intersection clips off the valid portion outside the screen.

The line above already computes pass_texture ∩ clip_coverage, which is the correct bound.

Symptom
Scaled-down overview of a full-size page (e.g. launcher editing mode at 0.3×): the background fills correctly, but child elements beyond a certain distance from the screen center disappear. Also seen as edge artifacts with ImageFilter.blur on layers near the screen edge.

Suggested fix
Make the root RT intersection conditional:
std::optional<Rect> Canvas::GetLocalCoverageLimit(bool clamp_to_root = true) const { // ... existing pass_texture ∩ clip logic unchanged ... if (!clamp_to_root) return maybe_coverage_limit; return maybe_coverage_limit->Intersection( Rect::MakeSize(render_target_.GetRenderTargetSize())); }
Pass false from SaveLayer (texture sizing) and DrawDisplayList (cull rect). Root pass behavior unchanged.

We have been running with this fix internally and it resolves the reported symptoms.

Environment
Flutter main branch (also in 3.38.3 stable)
Impeller (Vulkan / OpenGLES)
File: impeller/display_list/canvas.cc, function GetLocalCoverageLimit()

Add golden test for DrawDisplayList cull rect not clamped to root RT

Runnable repro with plain dart:ui (no plugins). A SaveLayer with a matrix image filter moves its content +300 px. Inside it, two circles are drawn at x=-200: one with drawCircle, one through drawPicture. Both should appear at x=100.

import 'dart:typed_data';
import 'dart:ui' as ui;
import 'package:flutter_test/flutter_test.dart';

void main() {
  test('drawPicture inside translated saveLayer is not culled', () async {
    const int w = 400, h = 200;

    final pr = ui.PictureRecorder();
    ui.Canvas(pr).drawCircle(const ui.Offset(-200, 150), 30,
        ui.Paint()..color = const ui.Color(0xFF00FF00));
    final inner = pr.endRecording();

    final rec = ui.PictureRecorder();
    final c = ui.Canvas(rec, ui.Rect.fromLTWH(0, 0, w.toDouble(), h.toDouble()));
    c.drawRect(ui.Rect.fromLTWH(0, 0, w.toDouble(), h.toDouble()),
        ui.Paint()..color = const ui.Color(0xFFFFFFFF));
    final m = Float64List.fromList(
        <double>[1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 300, 0, 0, 1]);
    c.saveLayer(null, ui.Paint()..imageFilter = ui.ImageFilter.matrix(m));
    c.drawCircle(const ui.Offset(-200, 50), 30,
        ui.Paint()..color = const ui.Color(0xFFFF0000)); // direct (control)
    c.drawPicture(inner); // via DrawDisplayList
    c.restore();

    final img = await rec.endRecording().toImage(w, h);
    final px = (await img.toByteData(format: ui.ImageByteFormat.rawRgba))!
        .buffer.asUint8List();
    List<int> at(int x, int y) {
      final i = (y * w + x) * 4;
      return [px[i], px[i + 1], px[i + 2]];
    }

    expect(at(100, 50), [255, 0, 0]);   // direct circle
    expect(at(100, 150), [0, 255, 0]);  // drawPicture circle
  });
}

Results on Flutter 3.38.3, Linux host:

Backend	direct circle	drawPicture circle
`flutter test --enable-impeller`	visible	missing (white)
`flutter test` (Skia)	visible	visible

The direct circle shows that the SaveLayer texture is large enough; only the nested DisplayList is culled. Fix: #195860

[Impeller] Do not clamp DrawDisplayList cull rect to the root render …

@caimengnan-ops

@caimengnan-ops Thanks for the detailed report. I reproduced this on Flutter 3.47.9 stable and current master 3.50.0 on macOS 27.2. The test fails with Impeller on both versions, while the same test passes under Skia. Labelling and routing it to the engine team for further investigation.

Related: #195666

A second case from review on
#195860
: the same translated SaveLayer, but the off-screen content is drawn inside a child saveLayer instead of a Picture. The child layer is sized against the root render target, so its off-screen part is dropped.

import 'dart:typed_data';
import 'dart:ui' as ui;
import 'package:flutter_test/flutter_test.dart';

void main() {
  test('child saveLayer inside translated saveLayer is not culled', () async {
    const int w = 400, h = 200;

    final rec = ui.PictureRecorder();
    final c = ui.Canvas(rec, ui.Rect.fromLTWH(0, 0, w.toDouble(), h.toDouble()));
    c.drawRect(ui.Rect.fromLTWH(0, 0, w.toDouble(), h.toDouble()),
        ui.Paint()..color = const ui.Color(0xFFFFFFFF));
    final m = Float64List.fromList(
        <double>[1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 300, 0, 0, 1]);
    c.saveLayer(null, ui.Paint()..imageFilter = ui.ImageFilter.matrix(m));
    c.drawCircle(const ui.Offset(-200, 50), 30,
        ui.Paint()..color = const ui.Color(0xFFFF0000)); // parent layer (control)
    c.saveLayer(null, ui.Paint()); // child layer
    c.drawCircle(const ui.Offset(-200, 150), 30,
        ui.Paint()..color = const ui.Color(0xFF00FF00));
    c.restore();
    c.restore();

    final img = await rec.endRecording().toImage(w, h);
    final px = (await img.toByteData(format: ui.ImageByteFormat.rawRgba))!
        .buffer.asUint8List();
    List<int> at(int x, int y) {
      final i = (y * w + x) * 4;
      return [px[i], px[i + 1], px[i + 2]];
    }

    expect(at(100, 50), [255, 0, 0]);   // circle in parent layer
    expect(at(100, 150), [0, 255, 0]);  // circle in child layer
  });
}

[Impeller] Do not clamp the local coverage limit to the root render t…

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[Impeller] GetLocalCoverageLimit unconditionally clamps to root render target, causing incorrect subpass texture sizing and culling #195859

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[Impeller] GetLocalCoverageLimit unconditionally clamps to root render target, causing incorrect subpass texture sizing and culling #195859

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caimengnan-ops commented on Nov 3, 2026

saurabh-mirajkar commented on Nov 3, 2026

caimengnan-ops commented on Nov 7, 2026

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