Convertir un Bitmap en nuances de gris

function Gray(Bitmap: TBitmap): TBitmap;
var i, j: Integer;
Grayshade, Red, Green, Blue: Byte;
PixelColor: Longint;
begin
with Bitmap do
for i := 0 to Width - 1 do
for j := 0 to Height - 1 do
begin
PixelColor := ColorToRGB(Canvas.Pixels[i,j]);
Red := PixelColor;
Green := PixelColor shr 8;
Blue := PixelColor shr 16;

Grayshade := (Red + Green + Blue) div 3;
Canvas.Pixels[i,j] := RGB(Grayshade,Grayshade,Grayshade);
end;
Result := Bitmap;
end;