Thread: trouble understanding collision detection code

That collision detection code won't work with high speed objects.

What you need is to do is compute the time at which the two objects collide. If the interval is empty then there is no collision in the next frame.

Draw the problem out on a piece of paper.


4 cases here:
[*] = moving box -> Box A
[/] = stationary box -> Box B
--- = number line

This sample is taken from 3D Math Primer for Graphics and Game Development, Fletcher Dunn and Ian Parberry

Code:

t=0   (-x)[***]------[/////////]--------(+x)
t=t.e (-x)------[***][/////////]--------(+x)
hit	(-x)------------[/[***]/]--------(+x)
t=t.l  (-x)------------[/////////][***]--(+x)
t=1   (-x)------------[/////////]---[***](+x)

At t=0 box A is completely on the left side of box B.
At t=1 box A is completely on the right side of box B.

t.e(tenter)=point at which box A begins to overlap box B

t.l(tleave)=point at which box A no longer overlaps box B

d is the component of the displacement vector, in this case x.
t is a time on the number line.

For the moving box or box A.
Amin(t)=left side of box A
Amax(t)=right side of box A.

Amin(t)=Amin(0)+td
Amax(t)=Amax(0)+td

For the stationary box or box B
Bmin(t)=Bmin(0)+td
Bmax(t)=Bmax(0)+td

t.e=tenter
t.l=tleave

At point of collision:

Amax(t.e)=Bmin
Amax(0)+(t.e)d=Bmin
(t.e)d=Bmin-Amax(0)
(t.e)=Bmin-Amax(0)/d

Now solve for tleave:

Amin(t.l)=Bmax
Amin(0)+(t.l)d=Bmax
(t.l)d=Bmax-Amin(0)
(t.l)=Bmax-Amin(0)/d

• If denominator is zero then boxes always overlap or never overlap
• If box A begins on right side of box B then tenter will be greater than tleave -> swap their values prior to testing
• If values of tenter and tleave are outside of range 0...1 then: if tenter>1 or tleave<0 there is no overlap and the boxes never collide
• If the interval is empty then the boxes never collide.
• If the interval is outside of the range 0...1 then there is no collision in the period of time being tested against
  

Apply this for your second dimension, namely y, and you will have a way to test for any type of collision in a 2D environment. Notice we are testing time here, not against coordinates which can skip over one another.