{
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.

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;