resizing Pictures

well, less pixels means less data, period.  There are two basic approaches:

1)Firstly, try canvas.stretchdraw.  This is but a wrapper for the stretchBLT api & generally works o.k.

2)Rewrite stretchBlt to do the kind of dithering you want.

You could also convert te bmp to a jpeg & use the TJpegImage to resize & control quality.

GL
Mike

garizpe:

For your 20 points,it is so easy!

var a:Tbitmap;
begin
  a:=TBitmap.Create;
  a.LoadFromFile('c:\a.bmp');  //load a Source bitmap 800x600
  with TBitmap.Create do begin      //Create a new one
    ...  //set other b's property ,for example :pixformat....
    Width:=640;
    Height:=480; //reset the width and height
    Canvas.StretchDraw(b.Canvas.ClipRect,a); //Draw!!
    SaveToFile('c:\b.bmp');  //Save the result.
    free;
  end;
  a.free;
end;

garizpe:

For your 20 points,it is so easy!

var a:Tbitmap;
begin
  a:=TBitmap.Create;
  a.LoadFromFile('c:\a.bmp');  //load a Source bitmap 800x600
  with TBitmap.Create do begin      //Create a new one
    ...  //set other b's property ,for example :pixformat....
    Width:=640;
    Height:=480; //reset the width and height
    Canvas.StretchDraw(b.Canvas.ClipRect,a); //Draw!!
    SaveToFile('c:\b.bmp');  //Save the result.
    free;
  end;
  a.free;
end;

menxin

Hmmmm....

Menxin, don't underestimate a question as the user asked this to be done without loosing definitions in the picture.
The stretchdraw method does not interpolate the image, why the image will sometimes look very weird.

Regards,
Williams