Question

I’m trying to create rope physics for a 2D game, so as a starting point I have a small rotating image and I need to add another piece of rope to the end of it. Unfortunately I’m having trouble trying to track the bottom part of the image as the rotation occurs at the top of it. I’ve managed to track the (0,0) coordinate of the image using the following code but I need to be able to track point (32,57). This is what I have so far:

xr = xm + (xPos - xm) * Math.cos(a) - (yPos - ym) * Math.sin(a);
yr = ym + (xPos - xm) * Math.sin(a) + (yPos - ym) * Math.cos(a);

Any help is appreciated!

EDIT:

So hey, I got it working =D Using polar coordinates turned out to be a lot easier then whatever I had going on before.

The top 2 variables are constant and stay the same:

    theta0 = Math.atan2(y, x);
    r = 25;

    theta = theta0 + a;
    xr = (r * Math.cos(theta)) + xm;
    yr = (r * Math.sin(theta)) + ym;

xm and ym are the positions of my image.

Answer 1

Use polar coordinates. Set your origin at the point of rotation of your image, and pick your favorite angular reference (say 0 degrees is directly to the right, and positive rotations go counterclockwise from there).

Compute the polar coordinates of your desired point (32, 57) relative to this coordinate system. Say the answer is (r, theta).

Now, the only thing that’s changing as you spin the image around is the value of theta. Now you can go back to x-y coordinates with your new value of theta.

Hope this helps.