Reduction of JPeg Bitmap

Posted on 2004-04-09
Last Modified: 2013-12-03
How do I propotionally reduce a the width of a jpeg fiele from 2240 pixels to 120 pixels
Question by:Vector7
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LVL 11

Assisted Solution

shaneholmes earned 125 total points
ID: 10792120
Try this:

Procedure ScaleDownJPG(Fname:string;newheight,newWidth:integer) ;
var JPG1,JPG2:TJpegImage;
     With BMP do


Author Comment

ID: 10793629
Thanks Shane.  I've tried this and it works - many thanks.  However, how do you preserve a degree of quality? Stephen
LVL 11

Accepted Solution

shaneholmes earned 125 total points
ID: 10794581
I found this on Google Groups: Give it a try!


I do not really understand what you are getting at, but I assmue the
problem is that you are changing the Size of the Timage, assuming that
will rescale the image?
Well resizing a TImage will make the image look rescaled, but it does
NOT rescale the underlining bitmap that is stored in the TImage.

Also stretchdraw does not give as agood a resized quality (i.e. does no

Try the following SmoothResize code.
It requires 24-bit bitmaps and D3 or higher.  
Call it with something like:


  TRGBArray = ARRAY[0..32767] OF TRGBTriple;
  pRGBArray = ^TRGBArray;

procedure SmoothResize(abmp:TBitmap; NuWidth,NuHeight:integer);
   weight_x, weight_y     : array[0..1] of Single;
   ifrom_y, ifrom_x       : Integer;
   to_y, to_x             : Integer;
   new_red, new_green     : Integer;
   new_blue               : Integer;
   ix, iy                 : Integer;
   xscale, yscale         : Single;
   sfrom_y, sfrom_x       : Single;
   weight                 : Single;
   total_red, total_green : Single;
   total_blue             : Single;
   bTmp                   : TBitmap;
   sli, slo               : pRGBArray;
   // pointers for scanline access
   liPByte, loPByte, p    : PByte;
   // offset increment
   liSize, loSize         : integer;
   abmp.PixelFormat := pf24bit;
   bTmp := TBitmap.Create;
   bTmp.PixelFormat := pf24bit;
   bTmp.Width := NuWidth;
   bTmp.Height := NuHeight;
   xscale := bTmp.Width / (abmp.Width-1);
   yscale := bTmp.Height / (abmp.Height-1);
   liPByte := abmp.Scanline[0];
   liSize := integer(abmp.Scanline[1]) -integer(liPByte);
   loPByte := bTmp.Scanline[0];
   loSize := integer(bTmp.Scanline[1]) -integer(loPByte);
   for to_y := 0 to bTmp.Height-1 do begin
     sfrom_y := to_y / yscale;
     ifrom_y := Trunc(sfrom_y);
     weight_y[1] := sfrom_y - ifrom_y;
     weight_y[0] := 1 - weight_y[1];
     for to_x := 0 to bTmp.Width-1 do begin
       sfrom_x := to_x / xscale;
       ifrom_x := Trunc(sfrom_x);
       weight_x[1] := sfrom_x - ifrom_x;
       weight_x[0] := 1 - weight_x[1];
       total_red   := 0.0;
       total_green := 0.0;
       total_blue  := 0.0;
       for ix := 0 to 1 do begin
         for iy := 0 to 1 do begin
           p := liPByte;
           Inc(p, liSize *(ifrom_y + iy));
           sli := pRGBArray(p);
           new_red := sli[ifrom_x + ix].rgbtRed;
           new_green := sli[ifrom_x + ix].rgbtGreen;
           new_blue := sli[ifrom_x + ix].rgbtBlue;
           weight := weight_x[ix] * weight_y[iy];
           total_red   := total_red   + new_red   * weight;
           total_green := total_green + new_green * weight;
           total_blue  := total_blue  + new_blue  * weight;
       p := loPByte;
       Inc(p, loSize *to_y);
       slo := pRGBArray(p);
       slo[to_x].rgbtRed := Round(total_red);
       slo[to_x].rgbtGreen := Round(total_green);
       slo[to_x].rgbtBlue := Round(total_blue);
   abmp.Width := bTmp.Width;
   abmp.Height := bTmp.Height;

Charles Hacker
Lecturer in Electronics and Computing
School of Engineering
Griffith University - Gold Coast

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