Still, I can now throw enemies on the screen and the player can shoot at them. Granted, the bullets don't do anything, which brings me to my next point...

I'm skipping Phase 4 (enable the enemies to fire) and heading straight to Phase 5 (collision detection). The more I've been thinking about it, the more I realized how tough this could be. Short version of the story: If a bullet is travelling at 10 pixels per frame and the player's hitbox is 5 pixels, it is possible that bullet could jump right over the player in one frame! (I made a post about this on gamedev.net: http://www.gamedev.net/community/forums/topic.asp?topic_id=570333 )

In any case, between the suggestions made on gamedev.net and some collision detection articles I read, I've decided to go with something like this:

- Assume every collision object is circular. Not ideal, but will work for now.
- For each frame, calculate the relative velocity of two items (say, a ship and a bullet) by subtracting one motion vector from the other.
- Now, all I'll have to do is see if this "relative velocity" vector ever get's too close to the ship (or bullet, depending on how I subtract the vectors). This will be done by...
- Taking the inverse slope of the "relative velocity" vector
- Use this slope and the position of the ship to determine the equation of the line
*perpendicular*to the vector that passes through the point. - Find the intersection of this line with the relative velocity vector. This is the
*closest*that the vector comes to the ship. Calculate this distance. - If this distance is less than the radius of the ship and the bullet, there's a collision

Disclaimer: I may be horribly misrepresenting the notion of a vector when I refer to the "relative velocity" vector. But hopefully the idea is clear. The idea is that I'll run through these steps for each pair of objects that need a collision check. Eventually, I'll add some code to minimize the number of checks that need to be made (but I'll save that for another post).

Time to post these ideas to gamedev and make sure they make sense!

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