Impeller MaskFilter.blur isn't invariant over scale transforms #152778
Description
When drawing a blurred circle whose radius and blur sigma are scaled inversely with the scale applied to the canvas, one would expect a more or less identical result regardless of the scale. This works with Skia, however, with Impeller, there are significant discontinuities with the applied blur.
Sample code
import 'dart:async';
import 'package:flutter/material.dart';
void main() {
runApp(const Test());
}
class Test extends StatefulWidget {
const Test({super.key});
@override
State<Test> createState() => _TestState();
}
class _TestState extends State<Test> {
Timer? _timer;
double _x = 1.0;
@override
void initState() {
super.initState();
_timer = Timer.periodic(const Duration(milliseconds: 200), _update);
}
void _update(Timer timer) {
setState(() {
_x *= 10;
if (_x > 1e20) {
_x = 1e-20;
}
});
}
@override
void dispose() {
_timer?.cancel();
super.dispose();
}
@override
Widget build(BuildContext context) {
return Stack(
textDirection: TextDirection.ltr,
children: [
CustomPaint(
painter: Painter(_x),
),
Positioned(
top: 60.0,
left: 20.0,
child: Text(_x.toStringAsExponential(1), textDirection: TextDirection.ltr),
),
],
);
}
}
class Painter extends CustomPainter {
Painter(this.x);
final double x;
@override
void paint(Canvas canvas, Size size) {
canvas.scale(1.0 / x);
final Paint paint = Paint()
..color = const Color(0xFF66BBFF)
..maskFilter = MaskFilter.blur(BlurStyle.normal, 100.0 * x);
canvas.drawCircle(Offset.zero, 500 * x, paint);
}
@override
bool shouldRepaint(Painter oldDelegate) => x != oldDelegate.x;
}