# Help with 2D Collision Detection

• ### Question

• I have been trying to create a program that models elastic collisions between balls in 2D. It seems to work fine for two balls and ok for three but as I increase the number of balls I seem to get quite a lot of collisions that go undetected and the balls just pass straight through each other.

I think the problem is in the loop used to check the array of balls and the distance between their centres but I'm not sure.

In case anyone was wondering whether the actual collision calculations were derived by me unfortunately not. After making many a hash of doing the velocity calculations I found a great forum post by litdev which explains it quite nicely (http://social.msdn.microsoft.com/Forums/en-US/smallbasic/thread/41543a0d-01cf-4161-b280-f01d6c0511c8)

The exported code is : XFP905

Thanks for any help,

Joe

Wednesday, May 15, 2013 7:30 PM

• The problem is often due to 'tunneling' when a ball from on step to the next passes right though another and is never actually overlapping.

Could remedy by smaller velocities or multiply by a a timestep size (<1) but would appear slower, could reduce delay.

However, I think the issue in this case may also be here:

For j = 2 To Ball_Number

Should be

For j = i+1 To Ball_Number

In this way each ball pair is checked once - also faster!

There is also an issue if the balls are initially overlapping.

Import XFP905-1

Great program by the way.

• Marked as answer by Thursday, May 16, 2013 12:13 PM
Wednesday, May 15, 2013 8:45 PM

### All replies

• The problem is often due to 'tunneling' when a ball from on step to the next passes right though another and is never actually overlapping.

Could remedy by smaller velocities or multiply by a a timestep size (<1) but would appear slower, could reduce delay.

However, I think the issue in this case may also be here:

For j = 2 To Ball_Number

Should be

For j = i+1 To Ball_Number

In this way each ball pair is checked once - also faster!

There is also an issue if the balls are initially overlapping.

Import XFP905-1

Great program by the way.

• Marked as answer by Thursday, May 16, 2013 12:13 PM
Wednesday, May 15, 2013 8:45 PM
• Thanks for your post and posting that thread by LitDev. I studied physics at high school many years ago and that was the end of that until I started programming. When we did motion I don't think we applied the trig to handle the vectors. New territory for me. Thx.

Here's a link to another LitDev thread on calculating and adding acceleration to a ball down a slope.

Cloud quote of the week: "Don’t lose my files bro!" -Andrew Meyer

• Edited by Wednesday, May 15, 2013 10:26 PM
Wednesday, May 15, 2013 10:20 PM
• Thanks for the help both. Sorting out the nested loops seems to have fixed my problems. I'm not quite sure why it didn't work in the first place though because even if I do end up detecting that ball[2] collides with itself the resulting velocity post calculation should be the same. Perhaps it almost rewrites a previous correct calculation with the false positive of a self collision. Any other ideas? Also to Jibba Jabba I'm just starting on that physics challenge atm so I'll wait till I'm finished before I read that post then have a look and compare. Unfortunately I can't install small basic at school due to admin restrictions so it may be slow progress. Thanks again Joe
Thursday, May 16, 2013 12:13 PM
• The way you had it, then it was possible that a ball was checked with itself, found to be overlapping and directions changed.  When another ball was overlapping it then its direction was changed again, but this time from the wrong direction (set when it interacted with itself).
Thursday, May 16, 2013 2:48 PM