delphi - compare screen shots and get screen coordinates of first pixel change

Question

I have two screenshots taken one after another with a few seconds gap. I am comparing both the images and trying to find out if there are any differences between them. If there are any differences, i need to get the screen coordinates of that pixel change.

Can I achieve this using Delphi?

Answer 1

If you have two bitmaps to compare and you need to find a position of the first different pixel, you can iterate the bitmap rows, get the bitmap pixel array for each row of both bitmaps, compare them and if they don't match, find the horizontal position of a different pixel. In code it might look like this:

function CompareBitmaps(ABitmap1, ABitmap2: TBitmap;
  var ADiffPixel: TPoint): Boolean;
var
  X, Y: Integer;
  Size: Integer;
  Pixels1, Pixels2: PByteArray;
begin
  // first check if both bitmaps match in size and pixel format
  Result := (ABitmap1.PixelFormat = ABitmap2.PixelFormat) and
    (ABitmap1.Width = ABitmap2.Width) and (ABitmap1.Height = ABitmap2.Height);
  // if so, then...
  if Result then
  begin
    // assign the size of one pixel
    case ABitmap1.PixelFormat of
      pf24bit: Size := SizeOf(TRGBTriple);
      pf32bit: Size := SizeOf(TRGBQuad);
    else
      raise Exception.Create('Bitmap must be 24-bit or 32-bit format!');
    end;
    // iterate all the bitmap rows    
    for Y := 0 to ABitmap1.Height - 1 do
    begin
      // get pointer to pixel data array of the same row for both bitmaps
      Pixels1 := ABitmap1.ScanLine[Y];
      Pixels2 := ABitmap2.ScanLine[Y];
      // compare those pixel data arrays; if they differ, then...
      if not CompareMem(Pixels1, Pixels2, ABitmap1.Width * Size) then
      begin
        // return negative result
        Result := False;
        // assign vertical pixel position to the output ADiffPixel parameter
        ADiffPixel.Y := Y;
        // iterate the current row to find a different pixel; after it's found
        // the pixel's horizontal position is stored to the output ADiffPixel
        // parameter; the pixel array is treated as a byte array, so there's
        // need to be a division by the pixel size
        for X := 0 to ABitmap1.Width * Size - 1 do
          if (Pixels1[X] <> Pixels2[X]) then
          begin
            ADiffPixel.X := X div Size;
            Exit;
          end;
      end;
    end;
  end
  else
    raise Exception.Create('Bitmaps must match in size and pixel format!');
end;

And the example usage:

procedure TForm1.Button1Click(Sender: TObject);
var
  PixelPos: TPoint;
begin
  if CompareBitmaps(Image1.Picture.Bitmap, Image2.Picture.Bitmap, PixelPos) then
    ShowMessage('Bitmaps are the same!')
  else
    ShowMessage('Bitmaps are different! The first different pixel was found ' +
      'at [' + IntToStr(PixelPos.X) + '; ' + IntToStr(PixelPos.Y) + '].');
end;