Rotating Rectangle and calculating corner

Question

I'm rotating a rectangle and now want to calculate the new position of the upper left corner.

my current calculation is:

Point upLeft = new Point(
        // x-coordinate
                (int) Math.round((oldx * Math.cos(objectAngleRad))
                        - (oldy * Math.sin(objectAngleRad))),
                // y-coordinate
                (int) Math.round((oldx * Math.sin(objectAngleRad))
                        + (oldy * Math.cos(objectAngleRad))));

The calculation doesn't work. Can somebody see the error?

Answer 1

You need to subtract the midpoint of the rectangle before you rotate it and then add it back afterwards, otherwise you're rotating the corner about the origin (0,0)

Point upLeft = new Point(
    // x-coordinate
        (int) Math.round(midx + ((oldx-midx) * Math.cos(objectAngleRad))
                              - ((oldy-midy) * Math.sin(objectAngleRad))),
    // y-coordinate
        (int) Math.round(midy + ((oldx-midx) * Math.sin(objectAngleRad))
                              + ((oldy-midy) * Math.cos(objectAngleRad))));