fixture.computeDistance gives incorrect results for CircleShape

[drrnbrns]

Using version 0.14.0 of forge2d, CircleShape.computeDistanceToOut isn't returning the right values. Note: I've included a fix that works well below.

This code logs distance and normals for both CircleShape and PolygonShape (the latter works fine and is just for comparison). If you draw or think about the locations, you'll see that the circleDistance reported is way off. Flip CircleShape to CircleShapeWithFix and you'll see correct values.

Expand for test case

Test case

import 'dart:math';
import 'package:forge2d/forge2d.dart';

void testFixtures() {
  final world = World();
  final body = world.createBody(BodyDef(
    angle: 0.1,
    position: Vector2(1, 2),
  ));

  final normal = Vector2.zero();
  final testPoint = Vector2(10, 7);

  // Flip CircleShape to CircleShapeWithFix to see the fix.
  final circleShape = CircleShape()
    ..radius = 2
    ..position.x = 3
    ..position.y = 4;
  final circleFixture = body.createFixtureFromShape(circleShape);
  final circleDistance = circleFixture.computeDistance(testPoint, 0, normal);
  print("circleDistance $circleDistance normal $normal");

  final polygonShape = PolygonShape()..setAsBoxXY(2, 3);
  final polygonFixture = body.createFixtureFromShape(polygonShape);
  final polygonDistance = polygonFixture.computeDistance(testPoint, 0, normal);
  print("polygonDistance $polygonDistance normal $normal");
}

Expand for provided working code

Working code for CircleShape.computeDistanceToOut

Provided as a class, but the intent is to simply use the code in computeDistanceToOut. In my app, I have a large number of different cases that all work well with this fixed code.

// CircleShapeWithFix only exists to demonstrate the fixed code in computeDistanceToOut.
class CircleShapeWithFix extends CircleShape {
  @override
  double computeDistanceToOut(
    Transform xf,
    Vector2 p,
    int childIndex,
    Vector2 normalOut,
  ) {
    final q = xf.q;
    final tp = xf.p;
    final dx = -(q.cos * position.x - q.sin * position.y + tp.x - p.x);
    final dy = -(q.sin * position.x + q.cos * position.y + tp.y - p.y);
    final d1 = sqrt(dx * dx + dy * dy);
    normalOut.x = dx * 1 / d1;
    normalOut.y = dy * 1 / d1;
    return d1 - radius;
  }

  @override
  Shape clone() => CircleShapeWithFix()
    ..radius = radius
    ..position.setFrom(position);
}

[spydon]

Thanks for the fix, could you provide it as a PR and add some test cases?

[drrnbrns]

I'll give it a shot, but I'm not too familiar with the internals of this project. It will actually be my first PR on github. Do you have any guidance about where tests would go, and is there a document I should read first?

[spydon]

You can read this one if you want:
https://github.com/flame-engine/flame/blob/main/CONTRIBUTING.md
The gist is basically that you should install melos and run melos bootstrap first.

The test can go in here (there are sadly not that many tests yet, but we try to at least make them for bug fixes so that we don't get regressions):
https://github.com/flame-engine/forge2d/tree/main/packages/forge2d/test

And feel free to ask questions if you get stuck. :)

[drrnbrns]

Thanks, I'll read through that contributing doc. You have the same link twice though.

[spydon]

Thanks, I'll read through that contributing doc. You have the same link twice though.

Fixed!

[drrnbrns]

I created a PR (#100) but I'm not sure what happens next. I can't assign anyone.

The github docs suggest that I can't assign it since I don't have write access, so I'm not clear on what normally happens to get it assigned.