Question

I have an application for drawing and editing vector graphics in WinForms

I have images, rectangles, ellipses, regions etc. and I know how to resize them by mouse move. But I don’t know how to rotate them by mouse move.

I draw objects into Graphics.

I’ve tried this, but it didn’t work.

g.TranslateTransform((float)(this.Rectangle.X + this.Rectangle.Width / 2), (float)(this.Rectangle.Y + this.Rectangle.Height / 2));
g.RotateTransform(this.Rotation);
g.TranslateTransform(-(float)(this.Rectangle.X + this.Rectangle.Width / 2), -(float)(this.Rectangle.Y + this.Rectangle.Height / 2));
//g.TranslateTransform(-(float)(rect.X + rect.Width / 2), -(float)(rect.Y + rect.Height / 2));

g.DrawImage(img, rect);

g.ResetTransform();

This didn’t work, because I don’t know how to find corners of objects in new (rotated) position, so I’m not able to resize that…

Answer 1

You need to apply high school trigonometry. There are lots of articles if you google “graphics.drawimage rotation”

But to start with, you should NOT be transforming the Graphics object itself. You are just looking to get the new bounding box of your image. To do this:

1. Take the bounding box of the image centered on the origin. Remember this is defined as three points for the benefit of DrawImage(Image, Point[])

   Point[] boundingBox = { new Point(-width /2, -height/2),
                           new Point(width/2, -height/2), 
                           new Point(-width/2, height/2) };
   

2. Use trig to rotate it. Feed each point through the following function:

   Point rotatePointAroundOrigin(Point point, float angleInDegrees) {
       float angle = angleInDegrees * Math.PI/180; // get angle in radians
       return new Point( point.X * Math.Cos(angle) - point.Y * Math.Sin(angle),
                         point.X * Math.Sin(angle) + point.Y * Math.Cos(angle));
   }
   

3. Translate the boundind box to where it has to go. Add the width/2 and height/2 to each of its points, plus whatever extra amount you want.

4. Call DrawImage(image, boundingBox)