Find a smaller bitmap in a larger bitmap using ScanLine?
lazarus / 39-Graphics

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gwiesenekker · 2014-06-20 10:58

Hi,

I am trying to find a smaller bitmap in a larger bitmap. The code
below that compares the bitmaps pixel by pixel works but is very slow.
I tried to use ScanLine to speed things up, but the code below that
uses it never finds ScanLine[0]. I guess the problem is with these
statements:

      P := TheP + 3 * X;
      if CompareByte(P, TheSmallP, 3 * TheSmallPicture.Bitmap.Width) =
0 then

The bitmaps are 24-bit. As a final test I commented out these lines:

  //ThePicture.Bitmap.Width := TheBitmap.Width;
  //ThePicture.Bitmap.Height := TheBitmap.Height;
  //ThePicture.Bitmap.Canvas.Draw(0, 0, TheBitmap);

and replaced them with:

  ThePicture.LoadFromFile(BitmapFileName);

So now it tries to compare the same bitmaps using ScanLine but that
also fails to find ScanLine[0]. Any ideas?

Thanks,
Gijsbert

procedure TMyApplication.FindBitmap(BitmapFileName: string);
var
  hWnd: THandle;
  hdcSrc: THandle;
  DX, DY: Integer;
  X, Y: Integer;
  TheBitmap: TBitmap;
  ThePicture: TPicture;
  TheRect: TRect;
  TheSmallPicture: TPicture;
  Found: Boolean;
  X1, Y1: Integer;
  TheP, TheSmallP : pByteArray;
  P: pByteArray;
begin
  hWnd := GetForeGroundWindow;
  if hWnd = 0 then Exit;
  hdcSrc := GetDCEx(hWnd, 0, 0);

  GetWindowRect(hWnd, TheRect);
  DX := TheRect.Right - TheRect.Left;
  DY := TheRect.Bottom - TheRect.Top;
  //writeln('width and height of foreground window: ', DX, ' ', DY);

  //grab the bitmap of the foreground window
  TheBitmap := TBitmap.Create;
  TheBitmap.Width := DX;
  TheBitmap.Height := DY;
  BitBlt(TheBitmap.Canvas.Handle, 0, 0, TheBitmap.Width,
TheBitmap.Height, hdcSrc, 0, 0, SRCCOPY);

  //copy the bitmap in case it is black
  ThePicture := TPicture.Create;
  //ThePicture.Bitmap.Width := TheBitmap.Width;
  //ThePicture.Bitmap.Height := TheBitmap.Height;
  //ThePicture.Bitmap.Canvas.Draw(0, 0, TheBitmap);
  ThePicture.LoadFromFile(BitmapFileName);

  TheSmallPicture := TPicture.Create;
  TheSmallPicture.LoadFromFile(BitmapFileName);

  if ThePicture.Bitmap.PixelFormat = pf24Bit then writeln('24 bit');
  if TheSmallPicture.Bitmap.PixelFormat = pf24Bit then writeln('24
bit');

  //find bitmap using scanline
  ExitCode := -1;
  for Y := 0 to ThePicture.Bitmap.Height -
TheSmallPicture.Bitmap.Height do
  begin
    TheP := ThePicture.Bitmap.ScanLine[Y];
    TheSmallP := TheSmallPicture.Bitmap.ScanLine[0];
    for X := 0 to ThePicture.Bitmap.Width -
TheSmallPicture.Bitmap.Width do
    begin
      writeln('Scanline ', Y, ' ', X);
      P := TheP + 3 * X;
      if CompareByte(P, TheSmallP, 3 * TheSmallPicture.Bitmap.Width) =
0 then
      begin
        writeln('Found ScanLine 0');
        Found := True;
        //now compare the remaining scanlines
        for Y1 := 1 to TheSmallPicture.Bitmap.Height - 1 do
        begin
          TheP := ThePicture.Bitmap.ScanLine[Y + Y1];
          TheSmallP := TheSmallPicture.Bitmap.ScanLine[Y1];
          P := TheP + 3 * X;
          if CompareByte(P, TheSmallP, 3 *
TheSmallPicture.Bitmap.Width) <> 0 then
          begin
            Found := False;
            break;
          end;
        end;
        if Found then
        begin
          writeln('Scanline found at ', X, ' ', Y);
          writeln('1');
          ExitCode := 1;
          Exit;
        end;
      end;
    end;
  end;
  writeln('0');
  ExitCode := 0;

  ExitCode := -1;
  for X := 0 to ThePicture.Bitmap.Width - TheSmallPicture.Bitmap.Width
do
  begin
    for Y := 0 to ThePicture.Bitmap.Height -
TheSmallPicture.Bitmap.Height do
    begin
      if ThePicture.Bitmap.Canvas.Pixels[X, Y] =
TheSmallPicture.Bitmap.Canvas.Pixels[0, 0] then
      begin
        Found := True;
        for X1 := 0 to TheSmallPicture.Bitmap.Width - 1 do
        begin
          for Y1 := 0 to TheSmallPicture.Bitmap.Height - 1 do
          begin
            if not(ThePicture.Bitmap.Canvas.Pixels[X + X1, Y + Y1] =
TheSmallPicture.Bitmap.Canvas.Pixels[X1, Y1]) then
            begin
              Found := False;
              break;
            end;
          end;
          if not Found then break;
        end;
        if Found then
        begin
          writeln('Pixel found at ', X, ' ', Y);
          writeln('1');
          ExitCode := 1;
          Exit;
        end;
      end;
    end;
  end;
  writeln('0');
  ExitCode := 0;
end;

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gwiesenekker · 2014-06-20 12:08

I added these debug statements:

writeln('Width ', TheSmallPicture.Bitmap.Width);
writeln('Delta ', Abs(Integer(TheSmallPicture.Bitmap.Scanline[1]) -
Integer(TheSmallPicture.Bitmap.Scanline[0])));

The width of the bitmap is 147 pixels, but the delta between two
scanlines is 444 bytes, which would mean 148 pixels if the bitmap is
24 bit. Why?

Gijsbert

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gwiesenekker · 2014-06-20 12:57

I found the problem! The calls to CompareByte should be:

CompareByte(TheP^[3 * X], TheSmallP^, 3 *
TheSmallPicture.Bitmap.Width)

Gijsbert