Calculating the center of rotation after translation

Question

I need to be able to rotate an image around a given point so that what ever part of the image appears in the center of my container is the center of rotation.

To calculate the center points, I am currently just taking the inverse of the translation applied to the image:

Rotate.CenterX = Translate.X * -1;
Rotate.CenterY = Translate.Y * -1;

However, the current calculation i'm using is not sufficient as it does not work if the image has been translated after being rotated.

I'm sure it's a reasonably straight forward trig function, I just can't think what it is!

Cheers

Answer 1

If you are working with GDI+ then use the following:

double ImWidth = (double)Im.Width;
double ImHeight = (double)Im.Height;
double XTrans = -(ImWidth * X);
double YTrans = -(ImHeight * Y);

g.TranslateTransform((float)XTrans, (float)YTrans);    
g.TranslateTransform((float)(ImWidth / 2.0 - XTrans), (float)(ImHeight / 2.0 - YTrans));
g.RotateTransform((float)Angle);
g.TranslateTransform(-((float)(ImWidth / 2.0 - XTrans)), -((float)(ImHeight / 2.0 - YTrans)));

If you are working with WPF graphic objects, use the following transform group:

TransformGroup TC = new TransformGroup();
RotateTransform RT = new RotateTransform(Angle);
RT.CenterX = Im.Width / 2.0;
RT.CenterY = Im.Height / 2.0;
TranslateTransform TT = new TranslateTransform(-X * Im.PixelWidth, -Y * Im.PixelHeight);
TC.Children.Add(TT);
TC.Children.Add(RT);

X & Y are the percent values you want to translate the image in (if the image is 1000 pixels and X is 0.1 then the image will be translated 100 pixels). This is how I needed the function to work but you can easily change it otherwise.