I am trying to program point line segment collision detection and reaction. I am doing this for fun and to learn. The point moves (it has a velocity, and can be controlled by the user), whilst the lines are strait and stationary. The lines are not axis aligned. Everything is in 2D. It is quite straight forward to work out if a collision has occurred. For each frame, the point moves from A to B. AB is a line, and if it crosses the line segment, a collision has occurred (or will occur) and I am able to work out the point of intersection (poi). The problem I am having is with the reaction. Ideally I would like the point to be prevented from moving across the line. In one frame, I can move the point back to the poi (or only alow it to move as far as the poi), and alter the velocity. The problem I am having with this approach (I think) is that, next frame the user may try to cross the line again. Although the point is on the poi, the point may not be exactly on the line. Since it is not axis aligned, I think there is always some subtle rounding issue (A float representation of a point on a line might be rounded to a point that is slightly on one side or the other). Because of this, next frame the path might not intersect the line (because it can start on the other side and move away from it) and the point is effectively allowed to cross the line. Firstly, does the analysis sound correct? Having accepted (maybe) that I cannot always exactly position the point on the line, I tried to move the point away from the line slightly (either along the normal to the line, or along the path vector). I then get a problem at edges. Attempting to fix one collision by moving the point away from the line (even slightly) can cause it to cross another line (one shape I am dealing with is a star, with sharp corners). This can mean that the solution to one collision inadvertently creates another collision, which is ignored. Again, does this sound correct? Anyway, whatever I try, I am having difficulty with edges, and the point is occasionally able to penetrate the polygons and cross lines, which is undesirable. Whilst I can find a lot of information about collision detection on the web (and on this site) I can find precious little information on collision reaction. Does any one know of any good point line collision reaction tutorials? Or is my approach too flawed/over complicated?