How create thumbnails?


I have a bunch of scanned photographs, about 600 x 430 pixels in size. In my application I want to create thumbnails for all these pictures and save them to disk in BMP format. How do I transfer/resize the big pictures into thumbnails? I tried using CopyRect from the big image to a small one, but the colors were all mixed up and looked really weird in the small picture, almost like graffiti.
NOTE: I want this done in the background, so reading the form's canvas or anything like that is no good.


//Daniel, Sweden
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ZifNabConnect With a Mentor Commented:
Hello Dippen and Inter,

Yep Igor is right (like always), but there are lots of other ways getting such thumbnails. A few months ago I found a really good looking site. It 's the site of Anders Melander. He has some great components and some great links to graphical stuff (Algorithemes etc.). And one of his freeware components is a bitmap resampler (lots of algorithemes included). So I think this is the easiest way of implementing your thumbnails....

(Igor : It's really worth looking at it!)

Regards, ZiF.
For fancy zoom out you should do averaging. i.e. you should produce the thumb nail by hand. Let the following be the 2x2 picture
1 2
3 4   -- is transformed to  a single pixel by  (1+2+3+4) / 4
The resulting image is better than just picking one pixel from that 4 pixels. So after deciding the zoom ration you take the average of some pixel for thumb nail
DippenAuthor Commented:
Thanks for your answer, Zif! I've looked at Anders Melanders components, but it seems as if they're a little to much for my needs right now, if you know what I mean. Delphi obviously has some sort of routine for rescaling pictures, since Delphi produces a high-quality thumbnail if I set the Stretch property of an TImage to True and then make the TImage component smaller. Can't I use that capability in some easy way?
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Actually, delphi does nothing special this is the Paint Routine for TImage:

procedure TImage.Paint;
  Dest: TRect;
  if Stretch then
    Dest := ClientRect
  else if Center then
    Dest := Bounds((Width - Picture.Width) div 2, (Height - Picture.Height) div 2,
      Picture.Width, Picture.Height)
    Dest := Rect(0, 0, Picture.Width, Picture.Height);
  with inherited Canvas do
    StretchDraw(Dest, Picture.Graphic);

Regards, (so they just use StretchDraw)
Correct, You could save the dest canvas to a file...
DippenAuthor Commented:
Whoomp, there it is!

Thx, guys, I love you both! I feel just a little tiny bit stupid since there was such an easy solution to the problem, but what the h... Cya!
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