Question

Hello all you math whizzes out there!

I am struggling with a math problem I am hoping you can help me with. I have calculated an angle of direction using radians. Within OpenGL ES I move my guy by changing my point value as such:



spriteLocation.x -= playerSpeed * cosf(playerRadAngle);
spriteLocation.y -= playerSpeed * sinf(playerRadAngle);

// playerRadAgnle is my angle of direction using radians

This works very well to move my sprite in the correct direction. However, I have decided to keep my sprite "locked" in the middle of the screen and move the background instead. This requires me to Reverse my calculated angle. If my sprite s direction in radians is equivalent to 90 degrees, I want to convert it to 270 degrees. Again, keeping everything in radians.

I will admit that my knowledge of Trig is poor at best. Is there a way to figure out the opposite angle using radians? I know I could convert my radians into degrees, then add/subtract 180 degrees, then convert back to radians, but I m looking for something more efficient.

Thanks in advance....

-Scott

Answer 1

Add/subtract pi instead.

Answer 2

You need to add Pi and then use the remainder after division by 2 Pi (to make it restricted within [0; 2 Pi] range).

JavaScript:

function invertAngle(angle) {
  return (angle + Math.PI) % (2 * Math.PI);
}

Answer 3

object_sprite.rotation = warp_direction - 3.14;

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