Physics collision are in wrong position when objects have a non-default center points and non-zero angle

[tristanbob]

Describe the bug

Objects with non-default center points and non-zero angles have incorrect physics collisions.

https://user-images.githubusercontent.com/8879811/209406105-9d668ba5-ed67-4c4b-8500-9d3bd2b94001.mp4

To Reproduce

Steps to reproduce the behavior:

1. Create an object and place it in the scene
2. Edit the center point to be on the left edge
3. Rotate the object in the scene editor
4. Start a preview and notice how the physics collision doesn't match the visual object.

Note: The location of the ghost collision matches the incorrect bounding box drawn in the scene editor #4732.

Other details

Windows 11 GDevelop 5.1.153

[tristanbob]

Could the problem be found in this function?

Extensions\Physics2Behavior\physics2runtimebehavior.ts

    updateBodyFromObject() {

      <TRIMMED>
      
      // The object object transform has changed, update body transform:
      if (
        this._objectOldX !== this.owner.getX() ||
        this._objectOldY !== this.owner.getY() ||
        this._objectOldAngle !== this.owner.getAngle()
      ) {
        const pos = this.b2Vec2(
          (this.owner.getDrawableX() + this.owner.getWidth() / 2) *
            this._sharedData.invScaleX,
          (this.owner.getDrawableY() + this.owner.getHeight() / 2) *
            this._sharedData.invScaleY
        );
        body.SetTransform(pos, gdjs.toRad(this.owner.getAngle()));
        body.SetAwake(true);
      }
    }

I tried changing the position vector to this, but it broke a lot of things.

        const pos = this.b2Vec2(
          this.owner.getAABBCenterX() * this._sharedData.invScaleX,
          this.owner.getAABBCenterY() * this._sharedData.invScaleY
        );

Am I on the right track?

[AlexandreSi]

Hi @tristanbob! Great job investigating that! I think you're on the right track, this is where the issue is happening. This issue is similar to the other one you opened that should be solved with https://github.com/4ian/GDevelop/pull/4814. We were not using the object custom center to operate the rotation transformation. So this function should:

• Test if the object is a Sprite
• If that's the case, check if the currently displayed sprite (in the animation) has a custom center
• If that's the case, we should use the position of this center (taking the object scale into account) to do the rotation.

Do you want to submit a PR for this? I can take over if needed!

[tristanbob]

@AlexandreSi thanks for your great work on this. I'd prefer you to complete this and I'll try again on the next puzzle! :)

[AlexandreSi]

Good call cause that's not easy! 😅 I'll try to submit a PR for this soon

[tristanbob]

Any updates on this? It breaks things like pinball paddles that pivot from one side.

https://user-images.githubusercontent.com/8879811/233738153-6c01a73b-51e7-4061-a02f-3c3649992ef4.mp4

[tristanbob]

This bug also breaks games where you want to build a structure with diagonals that are made by rotating objects with non-default centers.

https://twitter.com/VictrisGames/status/1653580321101860864?s=20

https://user-images.githubusercontent.com/8879811/235821117-f0af93d0-9ff0-4c9a-be44-6354e90e0682.mp4

[AlexandreSi]

I'm wondering if that's even possible with Box2D to have a center of rotation that is not the center of the body.

[tristanbob]

I'm wondering if that's even possible with Box2D to have a center of rotation that is not the center of the body.

It might be possible when you use the "SetAsBox" function:

https://gamedev.stackexchange.com/a/3760

https://box2d.org/documentation/classb2_polygon_shape.html#af80eb52027ffe85dd4d0a3110eae9d1b

Do you think this will accomplish what we want? (Collision shape to rotate around a non-center point)

[tristanbob]

Here is another use-case where this bug stops users from using Physics to swing an Axe from the handle:

https://twitter.com/VictrisGames/status/1686222172833091584

[tristanbob]

This bug is preventing pole-vaulting game mechanics that use physics.

https://twitter.com/VictrisGames/status/1788796595435344017