Player sprite moving slower on iPhone 4

Question

I just finished getting movement/jump animation for a player sprite in Xcode using Cocos2D. The basic movement algorithm is a timer that updates every 0.01 sec, changing the sprite position to (sprite.position.x + xVel, sprite.position.y + yVel). Each time a movement button is tapped, the appropriate velocity (initialized to 0) is changed to whatever speed I choose, then a stop movement button returns the velocity to 0. It's not an ideal solution but I'm very new at this and stoked to at least have that working with little help from the internet.

So I may not have explained that perfectly, but it is in fact working to my satisfaction in Xcode's iPhone Simulator, however when I build it for my device and run it on my phone, the sprite's movement speed is noticeably slower than in Xcode. At first I thought it must have to do with the resolution of the iPhone 4, making the sprite's movement path twice as long, but I found that if I pull up the multitask bar, then return to the app the speed will sometimes jump back to normal. My second theory was that the code is just inefficient and is bogging the processes down, but I would see this reflected in the frame rate wouldn't I? It stays at 59-60 the whole time, and the spritesheet animation runs at the correct speed.

Has anyone experienced this? Is this a really obvious issue that I'm completely missing? Any help (or tips for optimizing my approach to movement) would be much appreciated!

Answer 1

The reason it is different speeds is because you are not calculating/using a delta time. Basically the frame rate could be any number at any time slowing or speeding up your animations. To counteract this, programmers calculate a delta time:

delta = currentTime - previousFramesTime; // time difference between this frame and the last frame

Then use this in their animation:

position.y = position.y + (velocity * delta);

This way no matter what the framerate is, your animations will remain the same speed.