Use a lowpass filter to blur images

Problem/Question/Abstract:

Does anyone know of a (preferably online) source of information for blurring algorithms? I'm looking for a simple way of blurring an image.

Answer:

A lowpass filter does the job. Copy the image to memory. Then take a zeroed piece of memory the same size. Pull a 3x3 window over your memory image (for x, for y) within this window, multiply the underlying pixels with the constant window multiplyer :

1 1 1 //
1 1 1
1 1 1

and add them up. This value/9 assign to the new memory image at (x,y).

{ ... }
for x := 0 to image1.width - 1 do
begin
  for y := 0 to image1.height - 1 do
  begin
    s := 0;
    for h := -1 to 1 do
    begin
      for v := -1 to 1 do
      begin
        s := s + memimage[x + h, y + v];
      end;
    end;
    new_memimage[x, y] := s / 9;
  end;
end;
{ ... }

then copy the new memory image to the image. Note that the border of the image has to be treated specially. (array range)