NSAffineTransform question

• Ewan Delanoy NSAffineTransform question Apr 25 2007, 09:19
  Hello all,
  
    I'm using NSAffineTransform to draw a rotated image in
  a view. When I delete the NSAffineTransform part
  in my code, I get the results i expect, but when I add the NSAffineTransform
  instructions I see nothing at all in the view.
  
    The relevant snippet (in the drawRect: method) goes :
  
  /*version 1 : without the NSAffineTransform stuff */
  
  [theImage drawInRect:drawingRect
               fromRect:imageRect
               operation:NSCompositeSourceOver
               fraction:1.0];
  /*version 2 */
                          NSAffineTransform* xform=[NSAffineTransform
  transform];
    [xform rotateByDegrees:90];
    [xform concat];
    [theImage drawInRect:drawingRect
               fromRect:imageRect
               operation:NSCompositeSourceOver
               fraction:1.0];
    [xform invert];
    [xform concat];
  
  Apart from that region, my code is identical
  in the two cases. Did I put the instructions in the wrong order ?
  
                                            Ewan
• Janek Priimann Re: NSAffineTransform question Apr 25 2007, 13:18
  Right now the image is rotated around (0,0). To fix this problem:
  
  float deltaX = imageRect.size.width * 0.5;
  float deltaY = imageRect.size.height * 0.5;
  
  [xform translateXBy:deltaX yBy:deltaY];
  [xform rotateByDegrees:90];
  [xform translateXBy:-deltaX yBy:-deltaY];
  
  On 4/25/07, Ewan Delanoy <delanoy...> wrote:
  

  > Hello all,
  >
  > I'm using NSAffineTransform to draw a rotated image in
  > a view. When I delete the NSAffineTransform part
  > in my code, I get the results i expect, but when I add the NSAffineTransform
  > instructions I see nothing at all in the view.
  >
  > The relevant snippet (in the drawRect: method) goes :
  >
  > /*version 1 : without the NSAffineTransform stuff */
  >
  > [theImage drawInRect:drawingRect
  > fromRect:imageRect
  > operation:NSCompositeSourceOver
  > fraction:1.0];
  > /*version 2 */
  > NSAffineTransform* xform=[NSAffineTransform
  > transform];
  > [xform rotateByDegrees:90];
  > [xform concat];
  > [theImage drawInRect:drawingRect
  > fromRect:imageRect
  > operation:NSCompositeSourceOver
  > fraction:1.0];
  > [xform invert];
  > [xform concat];
  >
  > Apart from that region, my code is identical
  > in the two cases. Did I put the instructions in the wrong order ?
  >
  > Ewan
  >
  
• Gregory Weston Re: NSAffineTransform question Apr 25 2007, 17:28
  Janek Priimann wrote:
  
  

  > Right now the image is rotated around (0,0). To fix this problem:
  >
  > float deltaX = imageRect.size.width * 0.5;
  > float deltaY = imageRect.size.height * 0.5;
  >
  > [xform translateXBy:deltaX yBy:deltaY];
  > [xform rotateByDegrees:90];
  > [xform translateXBy:-deltaX yBy:-deltaY];
  

  
  There's actually a bit more to do here. The above code will work if
  the image is square and the lower-left corner of the result is
  intended to be at the origin. Since we're dealing with a multiple of
  90 degrees, you can just swap the x and y offsets for the final
  translate (which is, for the OP, the first one in the code line). But
  the offsets should also be adjusted by the lower-left corner of the
  intended destination rectangle. And the destination rectangle itself
  should not be modified ahead of time to reflect the rotated dimensions.
  
  Ewan Delanoy wrote:
  
  

  > [xform concat];
  > [theImage drawInRect:drawingRect fromRect:imageRect
  > operation:NSCompositeSourceOver fraction:1.0];
  > [xform invert];
  > [xform concat];
  

  
  Those last two lines aren't a reliable way to remove the transform.
  I've seen what I consider trivial transforms fail to invert. If you
  want to impose a transform just for a little while:
  
  [NSGraphicsContext saveGraphicsState];
  [theTransform concat];
  // draw, draw, draw
  [NSGraphicsContext restoreGraphicsState];