How to rotate an object 90 degrees around its center?

Question

I am trying to set up rotation for my falling object (a composite shape) so that it rotates 90 degrees every time a key is pressed around a center point. I am wanting to use the java.awt.Point library to designate one of the pieces of my composite shape as the "center of rotation" and then use the .Point location to for the points prior position, and then set the new x and y locations...and then somehow set the new coordinates of new position.

I am still very new to programming. Many times I will understand the theory behind what I need to do, but get really confused with how to actually go about implementing it.

Can anyone help with a general example at how to go about rotating a comp. shape around a center?

Lior Ohana Lior Ohana · Accepted Answer · 2011-11-16T06:51:16

Without getting into the java Point implementation, the general concept is very simple.

First you need to transform all your shape points so that the center of your shape is (0,0). For example if the center (Cx, Cy) and one of you points is (Px, Py) than the new point should be NewP = (Px - Cx, Py - Cy). Now after all points were transformed, you should do the following for each one of the points. Assuming you want to rotate 90 degress clock wise (check the link below to see how to do it for other rotations): RotatedP = (-NewPy, NewPx)

Once rotated, you should transform you points back to their original location: FinalP = (RotatedPx - NewPx, RotatedPy - NewPy).

Some pseudo code:

Point p;
Center center;

Point relativePoint = p - center;

// For 90deg CW: -y, x
// For 90deg CC: y, -x
Point rotatedPoint = new Point(-1 * relativePoint.Y, relativePoint.X); // 90deg CW
Point finalPoint = rotatedPoint + center;

Just read a little bit about transformations and rotation and it should be very clear for you: Rotations - Read only the first part (Rotations in two dimensions)

How to rotate an object 90 degrees around its center?

1 Answers