Physics behavior not tracking collisions properly.

[HelperWesley]

When collisions happen between frames the physics behavior doesn't count them as collisions, and therefore developers who expect a sound effect or value to change upon collision will be sorely upset.

To Reproduce

Just play the game at 20 min fps and 20 max fps and compare it to 60/60. There will be missing counts.

Plinko.zip

[HelperWesley]

@AlexandreSi

[4ian]

We'll fix that by keeping in memory contacts (that's the term used by the physics engine) for the frame, so we can have:

• a condition "X is currently colliding with Y" (the existing one, renamed)
• a new condition "X is currently or has just collided with Y" (the new one, so you're 100% sure you're not missing a bounce).

[tristanbob]

Tagging these so we can update/close when done:

https://github.com/4ian/GDevelop/discussions/3805

https://github.com/4ian/GDevelop/issues/3853

[tristanbob]

We'll fix that by keeping in memory contacts (that's the term used by the physics engine) for the frame, so we can have:

• a condition "X is currently colliding with Y" (the existing one, renamed)
• a new condition "X is currently or has just collided with Y" (the new one, so you're 100% sure you're not missing a bounce).

We should consider this new feature while we are selecting wording for collision conditions:

https://github.com/4ian/GDevelop/issues/3929

Existing conditions:

ObjectA is colliding with ObjectB = Objects are colliding this frame

New conditions:

ObjectA started colliding with ObjectB = Objects were NOT colliding last frame, but now they are. ObjectA stopped colliding with ObjectB = Objects were colliding last frame, but now they are NOT.

Perhaps we can add a boolean parameter to all of these conditions?

Include collisions that occurred between frames

I think that setting should be enabled by default.

[Silver-Streak]

You probably will need some sort of wording (or detail on the wiki) that explains that any logic associated with it will still only occur during the "current" frame, regardless if the collision occurred mid-frame.

Unless you're all reworking how the engine processes logic entirely 😆

[tristanbob]

@AlexandreSi came up with some use-cases (A, B, C) to analyze:

Let's imagine we are working on 4 frames:
   |Frame0           |Frame1              |Frame2               |Frame3
A.                          < start...................end >
B.                      < start....end >
C.     < start........................................end >
D. .........................end >     < start.........end >

Here is what I am proposing:

ObjectA IS colliding with ObjectB = Objects are colliding this frame

Case A: Frame 2 Case B: Frame 2 Case C: Frame 1, Frame 2 Case D: Frame 0, Frame 1, Frame 2

ObjectA STARTED colliding with ObjectB = Objects were NOT colliding last frame, but now they are.

Case A: Frame 2 Case B: Frame 2 Case C: Frame 1 Case D: None

ObjectA STOPPED colliding with ObjectB = Objects were colliding last frame, but now they are NOT.

Case A: Frame 3 Case B: Frame 2 Case C: Frame 3 Case D: Frame 3

---

EDIT: Removed the boolean parameter "Include collisions between frames" EDIT2: Added case D