obj.c vs obj.p during collision detection

[sven1977]

Hi everyone, I was wondering about an issue I stumbled across during coding my game in quintus. It seemed like a falling sprite in a 2d game with gravity was able to fall into another object from the top and even get rendered in that overlapping position before - in the next frame - the overdue collision would occur.

Here are my thoughts on why this could happen and how to fix it:

• a sprite's update method gets called by the stage to which the sprite has been added. Let's assume the sprite is a falling stone.
• the sprite's step method is called moving the sprite closer to the ground (gravity). The sprite's p.y is increased. However, the sprite's c.y value stays the same for now (we haven't called Q._generateCollisionPoints(sprite) yet).
• the sprite's step method - after moving the sprite by changing p.y - calls the stage's collide() method
• in there, we first call stage.regrid(sprite), which prefers the sprite's c (old y-value) over the sprite's p (new y-value), so the regrid method is already behind the actual current y-position
• after that (still inside the stage's collide method), we deal with _collisionLayer collisions, then we try to find collisions with other sprites on the stage by calling the stage's gridTest method, in which eventually we call Q.overlap before doing an actual Q.collision call
• in the Q.overlap method, we again prefer c (with still the outdated y-position) over p (with the current y-position)
• that sometimes leads to cases where we don't even check for a collision (call to Q.collision) even though we should, because the c y-value is one frame behind the p y-value.
• So in my view, we have to make a call to Q._generateCollisionPoints(sprite) at least before checking Q.overlap(sprite, obj2) or even before calling (stage.regrid(sprite)).

Let me know what you think, thanks, Sven

[cykod]

I think I'm confused , isn't this what's happening in collide (i.e. generateCollisionPoints is called before each regrid and after each collision) - or are you seeing something else? Please re-open I'm misunderstanding

https://github.com/cykod/Quintus/blob/master/lib/quintus_scenes.js#L663

[sven1977]

Oh my, you are absolutely right. I must have deleted that line accidentially from my local version.

Sorry and thanks!