How to move point along circle?

Question

I want to move a sprite in a circular motion by using a circle radius and movingangle.

I know for instance that the sprite is moving in a circle with the radius 10 and It's current position is (387,38) and angle is 28 degrees. Now I wann't move it say, 100px along the circle perimeter.

p1 = x&y coordinate, known (387,38) in example with movingangle 28 degrees
r = radius, known (10 in example)
l = distance to move along bend, known (100px in example)
p2 = new point, what is this x and y value?

I have a solution that works but I dont quite like it. It kind of don't make sense to me and it requires more calculations than I think is required. It works by first calculating the center point using p1 and then doing the same thing backwards so to speak to get p2 (using cosinus and sinus). There should be a quicker way I believe but I cant find anyone doing exactly this.

The reason i tagged cocos2d is because thats what I'm working with and sometimes game frameworks provide functions to help with the trigonometry.

Answer 1

Try this:

a1 = 28*(π/180)
cen.x = p1.x - r*SIN(a1)
cen.y = p2.y - r*COS(a1)
a2 = a1 + l/r
p2.x = cen.x + r*SIN(a2)
p2.y = cen.y + r*COS(a2)

It does not get any simpler than this. Move from p1 to the circle center, and then to p2.

Answer 2

You can do it with magic of anchor point in a really simple way =) All the transformations with any node are done relatieve to it's anchor point. Rotation is a kind of transformations too. So, you can do something like this

CGFloat anchorY = neededRadius / spriteHeight;
[yourSprite setAnchorPoint:ccp(0.5f, anchorY)];
[yourSprite setPosition:neededPosition]; //remember that position is also for anchor point
[self addChild: yourSprite];
[yourSprite runAction:anyRotateAction]; // i mean, CCRotateBy or CCRotateTo