Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

{ 
  Here is the routine I use in my thumbnail component and I belive it is quite 
  fast. 
  A tip to gain faster loading of jpegs is to use the TJpegScale.Scale 
  property. You can gain a lot by using this correct. 

  This routine can only downscale images no upscaling is supported and you 
  must correctly set the dest image size. The src.image will be scaled to fit 
  in dest bitmap. 
}


 const
  FThumbSize = 150;

 //Speed up by Renate Schaaf, Armido, Gary Williams... 
procedure MakeThumbNail(src, dest: tBitmap);
 type
   PRGB24 = ^TRGB24;
   TRGB24 = packed record
     B: Byte;
     G: Byte;
     R: Byte;
   end;
 var
   x, y, ix, iy: integer;
   x1, x2, x3: integer;

   xscale, yscale: single;
   iRed, iGrn, iBlu, iRatio: Longword;
   p, c1, c2, c3, c4, c5: tRGB24;
   pt, pt1: pRGB24;
   iSrc, iDst, s1: integer;
   i, j, r, g, b, tmpY: integer;

   RowDest, RowSource, RowSourceStart: integer;
   w, h: integer;
   dxmin, dymin: integer;
   ny1, ny2, ny3: integer;
   dx, dy: integer;
   lutX, lutY: array of integer;

 begin
   if src.PixelFormat <> pf24bit then src.PixelFormat := pf24bit;
   if dest.PixelFormat <> pf24bit then dest.PixelFormat := pf24bit;
   w := Dest.Width;
   h := Dest.Height;

   if (src.Width <= FThumbSize) and (src.Height <= FThumbSize) then
   begin
     dest.Assign(src);
     exit;
   end;

   iDst := (w * 24 + 31) and not 31;
   iDst := iDst div 8; //BytesPerScanline 
  iSrc := (Src.Width * 24 + 31) and not 31;
   iSrc := iSrc div 8;

   xscale := 1 / (w / src.Width);
   yscale := 1 / (h / src.Height);

   // X lookup table 
  SetLength(lutX, w);
   x1 := 0;
   x2 := trunc(xscale);
   for x := 0 to w - 1 do
   begin
     lutX[x] := x2 - x1;
     x1 := x2;
     x2 := trunc((x + 2) * xscale);
   end;

   // Y lookup table 
  SetLength(lutY, h);
   x1 := 0;
   x2 := trunc(yscale);
   for x := 0 to h - 1 do
   begin
     lutY[x] := x2 - x1;
     x1 := x2;
     x2 := trunc((x + 2) * yscale);
   end;

   dec(w);
   dec(h);
   RowDest := integer(Dest.Scanline[0]);
   RowSourceStart := integer(Src.Scanline[0]);
   RowSource := RowSourceStart;
   for y := 0 to h do
   begin
     dy := lutY[y];
     x1 := 0;
     x3 := 0;
     for x := 0 to w do
     begin
       dx:= lutX[x];
       iRed:= 0;
       iGrn:= 0;
       iBlu:= 0;
       RowSource := RowSourceStart;
       for iy := 1 to dy do
       begin
         pt := PRGB24(RowSource + x1);
         for ix := 1 to dx do
         begin
           iRed := iRed + pt.R;
           iGrn := iGrn + pt.G;
           iBlu := iBlu + pt.B;
           inc(pt);
         end;
         RowSource := RowSource - iSrc;
       end;
       iRatio := 65535 div (dx * dy);
       pt1 := PRGB24(RowDest + x3);
       pt1.R := (iRed * iRatio) shr 16;
       pt1.G := (iGrn * iRatio) shr 16;
       pt1.B := (iBlu * iRatio) shr 16;
       x1 := x1 + 3 * dx;
       inc(x3,3);
     end;
     RowDest := RowDest - iDst;
     RowSourceStart := RowSource;
   end;

   if dest.Height < 3 then exit;

   // Sharpening... 
  s1 := integer(dest.ScanLine[0]);
   iDst := integer(dest.ScanLine[1]) - s1;
   ny1 := Integer(s1);
   ny2 := ny1 + iDst;
   ny3 := ny2 + iDst;
   for y := 1 to dest.Height - 2 do
   begin
     for x := 0 to dest.Width - 3 do
     begin
       x1 := x * 3;
       x2 := x1 + 3;
       x3 := x1 + 6;

       c1 := pRGB24(ny1 + x1)^;
       c2 := pRGB24(ny1 + x3)^;
       c3 := pRGB24(ny2 + x2)^;
       c4 := pRGB24(ny3 + x1)^;
       c5 := pRGB24(ny3 + x3)^;

       r := (c1.R + c2.R + (c3.R * -12) + c4.R + c5.R) div -8;
       g := (c1.G + c2.G + (c3.G * -12) + c4.G + c5.G) div -8;
       b := (c1.B + c2.B + (c3.B * -12) + c4.B + c5.B) div -8;

       if r < 0 then r := 0 else if r > 255 then r := 255;
       if g < 0 then g := 0 else if g > 255 then g := 255;
       if b < 0 then b := 0 else if b > 255 then b := 255;

       pt1 := pRGB24(ny2 + x2);
       pt1.R := r;
       pt1.G := g;
       pt1.B := b;
     end;
     inc(ny1, iDst);
     inc(ny2, iDst);
     inc(ny3, iDst);
   end;
 end;