Rotating sprite 180 deg

Question

I should say first, that I have the rotation down. Its just that I want my square to rotate exactly 180 degrees. Currently, it will rotate but it will rotate but by less each jump. So after several jumps it will be moving on one of its side. There's not much code to show but I'll show you how I'm rotating:

if (jumped) 
        { 
            roation += 0.1f;
            playerVel.Y += gravity;
        } 

I haven't initialized rotation to anything. I've also tried using some Math functions but I just get the same result.

Answer 1

You need to keep track of the total rotation for each rotation. A few other variables will help you out as well. The reason you're likely getting some angle when landing is because your 5.8 degrees per update isn't exactly right, or your timestep isn't a fixed update. You can manage that with something like the following:

float degreesPerSecond = 180f;
float jumpRotation = 0f;

//inside update

if(jumped) {
    float rotationThisFrame = degreesPerSecond * deltaTimeInSeconds;
    jumpRotation += rotationThisFrame;
    if(jumpRotation > 180) { //check if we're done
        rotationThisFrame -= 180 - jumpRotation; //remove the extra rotation
        jumpRotation = 0; //reset total for next time
        jumped = false;
    }
    roation += rotationThisFrame;
}

That should make an entire jump rotation last 1 second. (untested).

Answer 2

Your rounding error is accumulating and overwhelming your calculation, which will always happen when you perform the calculations incrementally like this; that is the nature of floating point.

In order to eliminate rounding error accumulation, you must either:

• use fixed-point arithmetic; or

• recalculate the net value on each cycle instead of incrementing/decrementing an accumulator.

This is done by counting the cycle-number cycle and setting:

if (jumped) {  
   cycle++;  
   rotation = cycle * deltaRotation;  
   playerVel.Y = cycle * gravity;  
}