Projectile Collision Detection Is Inferior In 4.2.0

[CritCorsac]

Describe the bug
In version 4.1.3 projectiles had no issue hitting fast moving actors. In 4.2, hit detection frequently fails and the projectile moves through the actor.

To Reproduce
Steps to reproduce the behavior:

1. Create a project in 4.1.3.
2. Create a platformer scene with an actor that moves left and right rapidly in its "on update" tab and set its speed to 4 and collision group to 1.
3. In the "on hit" tab, for the newly created actor, under "group 1", add an event to display a simple message when hit.
4. Attach a script to the B button and have it launch a projectile from the player. Have the projectiles speed set to 6 and make sure it collides with collision group 1.
5. Run the project in 4.1.3 first. The projectile should never miss. Migrate the project to 4.2 and watch as the projectile now frequently misses its target and passes through it. Especially when the actor is moving towards the projectile.

Expected behavior
Collision detection accuracy like that in the previous version, 4.1.3.

Platform (please complete the following information):

• OS: Linux 64-bit
• App version: 4.2

Additional context
I have attached a zip file that contains two projects (one for version 4.1 and one for 4.2) that recreate the steps mentioned above. Run the 4.1 file in version 4.1.3 and press B (X on the keyboard) to shoot. Do not move the player from their start position. The projectile will never miss. Open the 4.2 file in version 4.2 and do the same. The projectile will sometimes miss its target.
Collision Test Projects.zip

[CritCorsac]

I did some further testing on this using the 4.2 project provided in the zip file in my original post...

The issue is not present in 4.2.0-beta1 and it is present in 4.2.0-beta2. So the collision detection issue for projectiles seems to have occurred somewhere between those two beta builds. Looking at the notes for beta2, it does seem like some changes were made to projectiles so I am assuming one of those changes are the cause.

[Mico27]

Yes, for performance reasons, projectiles collision detection is now spread across 4 frames.
So if your projectile is too fast or the target actor is too fast in the incoming direction and if the collision box of the actor or projectile is too small, the projectile may not collide to that actor.

[CritCorsac]

I see. Would it at all be feasible to add an option in the projectile event to choose between performance and accuracy? Where projectiles spawned with performance checked do collision detection over 4 frames while projectiles with accuracy checked do it the old way every frame.

[chrismaltby]

Hi @CritCorsac I'd been thinking about how to deal with this and came to a similar conclusion about having a performance/accuracy setting but probably having this as a per project setting rather than per projectile (adding the check per projectile will probably cause any performance gain to be lost)

I think it's actually only this line in the game engine that needs to change:
https://github.com/chrismaltby/gbvm/blob/0757ea9ffb28969beb1f5502337cbd2560b8a34c/src/core/projectiles.c#L102

If we wrapped the if there with something like this we could have a project wide setting, for switching this feature on/off.

#ifdef PROJECTILES_PERFORMANCE
    if ((tmp_iterator++ & 0x3) == 0) {
#endif

        // projectile collision checks

#ifdef PROJECTILES_PERFORMANCE
    }
#endif

[Mico27]

just want to specify that before that change (in 4.1.3) the projectile collision check was done on even frames, not on all frames.
chrismaltby/gbvm@8d86bab#diff-2e3472c76747055756310d8d0c0296778902fcf137059662cf8cd84774809f71 this was my PR to spread it on the two frames instead of only on the even frame to prevent stutter, but it got bumped to 4 frames here chrismaltby/gbvm@3a23bfd