Help with game development. Render loop?

Question

I'm working on a simple game, this is my first game project.

Most of the samples I find have a Render Loop where all the game logic is made too and I just don't like this. Let's say I have a ball with X=0, and a wall in X=10 and in a slow machine, the first loop places the ball in X=7 and in a second loop, it places the ball in X=14. It would just crash the game!

Is this "render loop" the right way to make games? Should I write code to check for things like this in every frame? Example, new frame X=14, last frame have X=7, so I should check if there's anything from X=7 to X=14??

I was thinking that I should have a separated thread for the game logic and in the render loop, I should just "take a snapshot" of the current game logic and display that, no?

How do you guys, experienced game developers work around this?

thanks!

Answer 1

As another answer stated, the problem you're seeing is called "tunneling" It's the "bullet through paper" problem, the bullet is moving fast, the paper is thin, how do you know that a collision happened?

It's easy if your world boundaries are simple. E.g. in Tetris, the blocks are only allowed to move left and right until they hit the sides, and it's easy to test if the bottom-most coordinate is hitting the "ground." These tests are simple because you can do one axis at a time, and collisions against the sides means something different than collisions against the bottom of the pit. If you have a rectangular room, just "stop" the moving object if its movement has put it outside the room by clamping its coordinates. I.e. if the room width is from -3 to +3, and your object has an X of 5, just change it to 3 and you're done.

If you want to handle more complicated worlds, it's a bit trickier. You'll want to read up on "swept" geometry collision. Basically, if you have a circle, you need to do collision tests with a capsule instead, the shape that would be made by "sweeping" the circle from its start point to its end point. It'll be like a rectangle with semicircles on either end. The math is surprisingly straight forward (IMHO), but it can be tricky to get it right and to truly understand what's going on. It's worth it though!

Edit: On the thread issue- no need to complicate things. One thread is fine. Skipping update frames can get messy too, and is pretty advanced since you actually need to figure out "the future" and then do interpolation of all interesting values up to that point. I don't call it the "render" loop, myself, as the render loop is just one part of the process.

def GameLoop():
   while True:
      ReadInputs()
      FigureOutWhatStuffDoes()
      DrawItAll()

Edit 2: This seems like an interesting discussion: http://www.gamedev.net/community/forums/topic.asp?topic_id=482397