Moving objects and then checking for collision doesn't work during frame start

[crusher]

Something strange is going on with how collisions are detected between objects during frame start.

The scenario is this, I have 3 different active objects with different starting positions. At frame start I place 2 of the 3 objects on top the 3rd object and then I see if the 2 moved objects are overlapping or colliding. This fails every time if done during frame start. Otherwise it works as it should.

*Start of Frame: 
	-> Move "Obj1" to "Obj2"
	-> Move "Obj3" to "Obj2"

*Start of Frame
+Obj1 is overlapping Obj3
or
*Start of Frame
+Obj3 is overlapping Obj1
	-> End Application

*Start of Frame
+Collision between Obj1 and Obj3
or
*Start of Frame
+Collision between Obj3 and Obj1
	-> End Application

The same problem was discussed in this thread.

Thanks!

[King_Cool]

I can tell you what the problem is.
This is just how MMF2 works, and is also the reason why Moving Platforms for Platform Games are so hard to create.

...
When doing overlap and collision tests, there are Collision Masks for each Object running behind the curtains wich handle this.

When you do a condition like:
- ObjectA is overlapping ObjectB
..or..
- ObjectA collides with ObjectB

Only the Collision Mask for ObjectA is updated to its new position, while ObjectBs Collision Mask is not updated and remains at the possition before ObjectB was moved.
Collision Masks are also automatically updated each time the game screen is redrawn ( end of each Event Editor itteration ).

...
Check out this example if you dont believe me.

[crusher]

Thanks for the explanation King_Cool, I figured the problem was something like that.

So apart from spending an extra frame for the collision masks to update there's nothing one can do about it? That's kind of a letdown...

[King_Cool]

No there is nothing you can do to manually uppdate the collision masks.

However if you do square 'box type' collision/ overlap checking by compairing to the Objects X and Y coordinates, you can overcome this problem.
I have done this for my Platform Engine and collision tests are pixel perfect and flawless.

[netninja]

I made a post about this back in May:

http://community.clickteam.com/showthread.php?t=70742

I actually think it's a recent change, but I could be wrong. I'm not sure if it was always like that.