We help IT Professionals succeed at work.

We've partnered with Certified Experts, Carl Webster and Richard Faulkner, to bring you two Citrix podcasts. Learn about 2020 trends and get answers to your biggest Citrix questions!Listen Now


Bitmap Rotation

elessar asked
Medium Priority
Last Modified: 2010-04-06
how do I rotate the bitmap in a standard TBitmap say 90 degrees left or right
Watch Question

I had no clue whatsoever howto rotate a bitmap fast...
...so I had to find out.
This code is fast(!/?). And it can rotate a TImage at ANY
Set the arbangle real to what you want and the rest will be done
for you:

procedure TMainForm.Image1DblClick(Sender: TObject);
  points      : array[0..3] of TPoint;
  bmp         : TBitmap; { temporary storage }
  index       : Integer;
  minx, miny  : Integer;
  maxx, maxy  : Integer;
  arbangle, r : Real;

  function min(a, b: Integer): Integer;
    if a<b then result := a else result := b;

  function max(a, b: Integer): Integer;
    if a>b then result := a else result := b;

  arbangle := 5; { arbitrary angle, can be any angle }
  if sender is TImage then with (sender as TImage) do { Is it a TImage? }
  if not picture.bitmap.empty then begin              { Is it a bitmap? }
    bmp := TBitmap.Create;
    r := Math.Degtorad(arbangle);
    { Some lame maths }
    points[0].x := round(width-(cos(r)*width-sin(r)*height));
    points[0].y := round(height-(sin(r)*width+cos(r)*height));
    points[1].x := round(width+sin(r)*height);
    points[1].y := round(height-cos(r)*height);
    points[2].x := round(width-cos(r)*width);
    points[2].y := round(height-sin(r)*width);
    points[3].x := Width;
    points[3].y := Height;
    { Remove negative coordinates }
    minx := min(points[0].x, min(points[1].x, min(points[2].x, points[3].x)));
    miny := min(points[0].y, min(points[1].y, min(points[2].y, points[3].y)));
    for index := 0 to 3 do begin
      dec(points[index].x, minx);
      dec(points[index].y, miny);
    maxx := max(points[0].x, max(points[1].x, max(points[2].x, points[3].x)));
    maxy := max(points[0].y, max(points[1].y, max(points[2].y, points[3].y)));
    { Set size }
    bmp.width  := maxx;
    bmp.Height := maxy;
    { Here's the magic }
    if not PlgBlt(
      bmp.canvas.handle, // handle of destination device context
      points,               // vertices of destination parallelogram
      canvas.handle,       // handle of source device context
      0,                // x-coord. of upper-left corner of source rect.
      0,                // y-coord. of upper-left corner of source rect.
      Width,             // width of source rectangle
      Height,             // height of source rectangle
      0,                 // handle of bitmask
      0,                 // x-coord. of upper-left corner of bitmask rect.
      0                   // y-coord. of upper-left corner of bitmask rect.
     ) then { errorhandling stuff };
     { Could possibly have been done in a better way }
     picture.bitmap.Width  := bmp.Width;
     picture.bitmap.Height := bmp.Height;
     canvas.draw(0, 0, bmp);


Hope you can use it!

/// John

Not the solution you were looking for? Getting a personalized solution is easy.

Ask the Experts


exactly what I was looking for ta! - since I have also posted this question on newsgroups and some people have requested that if I get any replies could I forward them... any objections if I pass this code around?

No objections whatsoever!
Just make sure to mention that you got it via
the Experts Exchange.

/// John
Access more of Experts Exchange with a free account
Thanks for using Experts Exchange.

Create a free account to continue.

Limited access with a free account allows you to:

  • View three pieces of content (articles, solutions, posts, and videos)
  • Ask the experts questions (counted toward content limit)
  • Customize your dashboard and profile

*This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.


Please enter a first name

Please enter a last name

8+ characters (letters, numbers, and a symbol)

By clicking, you agree to the Terms of Use and Privacy Policy.