{
This function resizes a bitmap calculating the average color of a rectangular
area of pixels from source bitmap to a pixel or a rectangular area to target
bitmap.
It produces a soft-color and undistorsioned result image unlike the StretchDraw
method
I think that this method have a tenichal name, but I am not sure.
As you can see, this function could be very optimized :p
}
 procedure TFormConvertir.ResizeBitmap(imgo, imgd: TBitmap; nw, nh: Integer);
 var
  xini, xfi, yini, yfi, saltx, salty: single;
  x, y, px, py, tpix: integer;
  PixelColor: TColor;
  r, g, b: longint;
  function MyRound(const X: Double): Integer;
  begin
  Result := Trunc(x);
  if Frac(x) >= 0.5 then
  if x >= 0 then Result := Result + 1
  else
  Result := Result - 1;
  // Result := Trunc(X + (-2 * Ord(X < 0) + 1) * 0.5);
 end;
 begin
  // Set target size
 imgd.Width := nw;
  imgd.Height := nh;
  // Calcs width & height of every area of pixels of the source bitmap
 saltx := imgo.Width / nw;
  salty := imgo.Height / nh;

  yfi := 0;
  for y := 0 to nh - 1 do
  begin
  // Set the initial and final Y coordinate of a pixel area
  yini := yfi;
  yfi := yini + salty;
  if yfi >= imgo.Height then yfi := imgo.Height - 1;
  xfi := 0;
  for x := 0 to nw - 1 do
  begin
  // Set the inital and final X coordinate of a pixel area
  xini := xfi;
  xfi := xini + saltx;
  if xfi >= imgo.Width then xfi := imgo.Width - 1;

  // This loop calcs del average result color of a pixel area
  // of the imaginary grid
  r := 0;
  g := 0;
  b := 0;
  tpix := 0;
  for py := MyRound(yini) to MyRound(yfi) do
  begin
  for px := MyRound(xini) to MyRound(xfi) do
  begin
  Inc(tpix);
  PixelColor := ColorToRGB(imgo.Canvas.Pixels[px, py]);
  r := r + GetRValue(PixelColor);
  g := g + GetGValue(PixelColor);
  b := b + GetBValue(PixelColor);
  end;
  end;
  // Draws the result pixel
  imgd.Canvas.Pixels[x, y] :=
  rgb(MyRound(r / tpix),
  MyRound(g / tpix),
  MyRound(b / tpix)
  );
  end;
  end;
 end;