Hi Everyone! I am currently working on my sphere-line/point-line collision detection. Basically, as how everyone is probably doing it, i do a line segment intersection check using parametrization wherein one line segment is the wall, and and the other line segment is the movement vector of sphere/point as it travels from one game frame to the other. Then after i verified that there is collision, i get the intersection point(which is also the collision point) and set the new coordinates of the movement vector of the sphre/point with the value of collision point. Say: |W1-W2| <- line segment of wall from W1 to W2 {P1-P2| <- movement vector of point/sphere from P1(its location at time 0) to P2(its location at time 1) where time 0 is time the point/sphere moved in the last game frame and time 1 is time the point/sphere moved in the current game frame Checking for line intersection between the 2 vectors and assuming intersection at point H, the new point/sphere movement vector becomes |P1-H| and then i calculate the new velocity. Now here''s my problem. After this frame my sphere/point is now located at H, and H is a point where it collided with wall. Therefore, in the next game frame, if i check the line intersection between wall line segment and sphere/point''s movement vector(which is |P1-H|), then i will still detect collision(at the same point H) wherein i am not supposed to. How do i fix this? I appreciate any help from you guys and i thank you in advance.

Hi Guys,
Thanks everyone for your response on my message. My sincere apologies for not being able to reply so soon.
With regards to my problem, and to explain to you further, i am actually working on a pool/billiards 2d game and this is my first try on writing a pc game. all my collisions ( circle-wall and circle-circle) is working. Also they are integrated and in such a way that all object movements are relative to each other. Here''s the sequence of my game loop:

1. update/move ball positions
2. using movement vectors (ball) and line vectors ( wall ), verify collision and get collision point of each collision detected
3. then calculate earliest collision ( shortest vector from previous position to hit point among all the balls that moved, if collision occured
4. move back the rest of the balls with longer collision vector to points relative to time the first ball collided with something. Also, move back the time elapsed
5. As Jyk mentioned, yes i move back the objects with additional "just short" so the balls will be placed "just barely" hitting another ball or wall.
6. then update new velocities of balls
7. do this until there are no more collisions or time elapsed reach 0

The problem i actually have is in no 5, but i seem to have solved it already as previous i am calculating the new position incorrectly. Now they seem ok.
but now i have noticed a new problem. Ideally after item 6 i would expect that no more balls will still be in collision with something as they have all moved back a certain position or at least "just barely" hitting another ball/wall.
But to be sure, i have placed collision check at the end of item 5, and surprisingly, it still detects ball-ball collision, while no more ball-wall collision.
Therefore it seems that i still have problem with ball-ball collision. may i know if any of you guys experience this as well? what may be causing it? and how to fix/eliminate it?

Thanks in advance!

Hi Jyk,
Yes, you are right. After spending 2 days, i have found that "glitch" in my code that's causing it. I made a mistake in subtracting an EPSILON value to the hit vectors as i subtracted an absolute value of EPSILON in the hit vectors of all the balls so that they will end up "just barely hitting" the wall or another ball. I realized that the value to be subtracted must be relative to the amount i moved back the hit vector of the first ball that collided.
In summary, i am happy to say the problem is fixed.
Thanks for your help




[edited by - harmless on June 13, 2004 2:51:05 AM]