Rotating a view around a center point

Question

I've got what I thought was a simple question, yet the solution eludes me. The premise of the problem is simple: I have a view (call it a point) with center vx, vy that I want to rotate around a bigger circle cx, cy. I've looked into using sin, cos, radians, atan2, all that jazz, yet for some reason when I rotate my views, it looks as if it's rotating in an oval and not a circle. The math for this eludes me.

What I have so far:

public void update(double matrixAngle) 
        double angleInDegs = Math.toDegrees(mAngle);
        double newAngle = (matrixAngle > 0) ? angleInDegs + matrixAngle : angleInDegs - matrixAngle;
        double angleInRads = Math.toRadians(newAngle);

        float dx = itemCircle.getCenterX() - getCenterX();
        float dy = itemCircle.getCenterY() - getCenterY();
        double radius = Math.sqrt(dx * dx + dy * dy);

        float x = (float) (getCenterX() + radius * Math.cos(angleInRads));
        float y = (float) (getCenterY() - radius * Math.sin(angleInRads));

        itemCircle.setCenterX(x);
        itemCircle.setCenterY(y);

For some clarification:

matrixAngle parameter of the method is the matrix angle used to rotate the entire view. It ranges from 0-180, then from -180 to 0, where 0 is opening from the right-side of the circle.

mAngle is the original angle that the view was drawn at. It ranges from 0 to 360.

itemCircle is the circle representing the view's bounds. It is a circle (an extended FloatingActionButton, to be exact).

getCenterX(), getCenterY() are the coords of the bigger circle that I want to rotate around.

I'd really appreciate some help here. This problem has been bugging me for a day now.

Answer 1

Your object is moving in an ellipse shape because the pixels aren't square. Notice that in class DisplayMetrics , there are actually different values for xdpi and ydpi . You'll need to adjust your computation to account for this.

Answer 2

Got it. Involved looking more closely at cos, sin angles to achieve what I wanted. Working code:

public void update(double matrixAngle) {
        double angleInRads = Math.toRadians(matrixAngle);

        double cosAngle = Math.cos(angleInRads);
        double sinAngle = Math.sin(angleInRads);

        float dx = mItemCircle.getCenterX() - getCenterX();
        float dy = mItemCircle.getCenterY() - getCenterY();

        float rx = (float) (dx * cosAngle - dy * sinAngle);
        float ry = (float) (dx * sinAngle + dy * cosAngle);

        rx += getCenterX();
        ry += getCenterY();

        mItemCircle.setCenterX(rx);
        mItemCircle.setCenterY(ry);
}