change a bitmap's pixel colour

Question

I am trying to change a bit-map's pixel color if it's white. I wrote following code. But it's awfully slow!. i want to check if a pixel's color is white or not, and if it is white, change the color to black.

Can anybody suggest a better approach?

procedure TForm1.Button1Click(Sender: TObject);
var
  BitMap1 : TBitmap;
  X, Y, Size : Integer;

  P: Cardinal;
begin
  BitMap1 := TBitmap.Create;
  bitMap1.LoadFromFile('image1.bmp');

  for Y := 0 to Bitmap1.Height - 1 do
  begin
    for X := 0 to Bitmap1.width  * size - 1 do
    begin
    p := BitMap1.Canvas.Pixels[X,Y];
    if p = 255 then
      BitMap1.Canvas.Pixels[X,Y] := 0;

    end;
  end;

  Image1.Picture.Assign(BitMap1);
end;

Answer 1

For sure use the ScanLine property to access bitmap pixels since you're working with a large array of pixels where the Pixels access is slow. For replacing colors of your choice with support for 24-bit and 32-bit bitmaps, I would use something like this:

procedure ReplaceColor(ABitmap: TBitmap; ASource, ATarget: TColor);
type
  TRGBBytes = array[0..2] of Byte;
var
  I: Integer;
  X: Integer;
  Y: Integer;
  Size: Integer;
  Pixels: PByteArray;
  SourceColor: TRGBBytes;
  TargetColor: TRGBBytes;
const
  TripleSize = SizeOf(TRGBBytes);
begin
  case ABitmap.PixelFormat of
    pf24bit: Size := TripleSize;
    pf32bit: Size := SizeOf(TRGBQuad);
  else
    raise Exception.Create('Bitmap must be 24-bit or 32-bit format!');
  end;

  for I := 0 to TripleSize - 1 do
  begin
    // fill the array of bytes with color channel values in BGR order,
    // the same would do for the SourceColor from ASource parameter:
    // SourceColor[0] := GetBValue(ASource);
    // SourceColor[1] := GetGValue(ASource);
    // SourceColor[2] := GetRValue(ASource);
    // but this is (just badly readable) one liner
    SourceColor[I] := Byte(ASource shr (16 - (I * 8)));
    // the same do for the TargetColor array from the ATarget parameter
    TargetColor[I] := Byte(ATarget shr (16 - (I * 8)));
  end;

  for Y := 0 to ABitmap.Height - 1 do
  begin
    // get a pointer to the currently iterated row pixel byte array
    Pixels := ABitmap.ScanLine[Y];
    // iterate the row horizontally pixel by pixel
    for X := 0 to ABitmap.Width - 1 do
    begin
      // now imagine, that you have an array of bytes in which the groups of
      // bytes represent a single pixel - e.g. the used Pixels array for the
      // first 2 pixels might look like this for 24-bit and 32-bit bitmaps:

      // Pixels   [0][1][2]     [3][4][5]
      // 24-bit    B  G  R       B  G  R
      // Pixels   [0][1][2][3]  [4][5][6][7]
      // 32-bit    B  G  R  A    B  G  R  A

      // from the above you can see that you'll need to multiply the current
      // pixel iterator by the count of color channels to point to the first
      // (blue) color channel in that array; and that's what that (X * Size)
      // is for here; X is a pixel iterator, Size is size of a single pixel:          

      // X * 3    (0 * 3)       (1 * 3)
      //           ⇓             ⇓
      // Pixels   [0][1][2]     [3][4][5]
      // 24-bit    B  G  R       B  G  R

      // X * 4    (0 * 4)       (1 * 4)
      //           ⇓             ⇓
      // Pixels   [0][1][2][3]  [4][5][6][7]
      // 32-bit    B  G  R  A    B  G  R  A

      // so let's compare a BGR value starting at the (X * Size) position of
      // the Pixels array with the SourceColor array and if it matches we've
      // found the same colored pixel, if so then...
      if CompareMem(@Pixels[(X * Size)], @SourceColor, TripleSize) then
        // copy the TargetColor color byte array values to that BGR position
        // (in other words, replace the color channel bytes there)
        Move(TargetColor, Pixels[(X * Size)], TripleSize);
    end;
  end;
end;

And the usage:

procedure TForm1.Button1Click(Sender: TObject);
var
  Bitmap: TBitmap;
begin
  Bitmap := TBitmap.Create;
  try
    Bitmap.LoadFromFile('d:\Image.bmp');
    ReplaceColor(Bitmap, clWhite, clBlack);
    Image1.Picture.Assign(Bitmap);
  finally
    Bitmap.Free;
  end;
end;

---

For pure GDI and bitmaps having at most 256 colors you might use the CreateMappedBmp function.

Answer 2

You should use scanlines for this. Example:

procedure ChangeWhiteToBlack(var Bitmap: TBitmap);
var
  scanline: PRGBTriple;
  y: Integer;
  x: Integer;
begin
  Assert(Bitmap.PixelFormat = pf24bit);
  for y := 0 to Bitmap.Height - 1 do
  begin
    scanline := Bitmap.ScanLine[y];
    for x := 0 to Bitmap.Width - 1 do
    begin
      with scanline^ do
      begin
        if (rgbtBlue = 255) and (rgbtGreen = 255) and (rgbtRed = 255) then
          FillChar(scanline^, sizeof(TRGBTriple), 0);
      end;
      inc(scanline);
    end;
  end;
end;

To try this:

procedure TForm5.FormCreate(Sender: TObject);
var
  bm: TBitmap;
begin
  bm := TBitmap.Create;
  try
    bm.LoadFromFile('C:\Users\Andreas Rejbrand\Desktop\test.bmp');
    ChangeWhiteToBlack(bm);
    bm.SaveToFile('C:\Users\Andreas Rejbrand\Desktop\test2.bmp');
  finally
    bm.Free;
  end;
end;

Update: You need only a very minor modification of the code to make it work on 32-bit bitmaps instead:

procedure ChangeWhiteToBlack32(var Bitmap: TBitmap);
var
  scanline: PRGBQuad;
  y: Integer;
  x: Integer;
begin
  Assert(Bitmap.PixelFormat = pf32bit);
  for y := 0 to Bitmap.Height - 1 do
  begin
    scanline := Bitmap.ScanLine[y];
    for x := 0 to Bitmap.Width - 1 do
    begin
      with scanline^ do
      begin
        if (rgbBlue = 255) and (rgbGreen = 255) and (rgbRed = 255) then
          FillChar(scanline^, sizeof(TRGBQuad), 0);
      end;
      inc(scanline);
    end;
  end;
end;

In fact, you could do

procedure ChangeWhiteToBlack24(var Bitmap: TBitmap);
var
  scanline: PRGBTriple;
  y: Integer;
  x: Integer;
begin
  Assert(Bitmap.PixelFormat = pf24bit);
  for y := 0 to Bitmap.Height - 1 do
  begin
    scanline := Bitmap.ScanLine[y];
    for x := 0 to Bitmap.Width - 1 do
    begin
      with scanline^ do
      begin
        if (rgbtBlue = 255) and (rgbtGreen = 255) and (rgbtRed = 255) then
          FillChar(scanline^, sizeof(TRGBTriple), 0);
      end;
      inc(scanline);
    end;
  end;
end;

procedure ChangeWhiteToBlack32(var Bitmap: TBitmap);
var
  scanline: PRGBQuad;
  y: Integer;
  x: Integer;
begin
  Assert(Bitmap.PixelFormat = pf32bit);
  for y := 0 to Bitmap.Height - 1 do
  begin
    scanline := Bitmap.ScanLine[y];
    for x := 0 to Bitmap.Width - 1 do
    begin
      with scanline^ do
      begin
        if (rgbBlue = 255) and (rgbGreen = 255) and (rgbRed = 255) then
          FillChar(scanline^, sizeof(TRGBQuad), 0);
      end;
      inc(scanline);
    end;
  end;
end;

procedure ChangeWhiteToBlack(var Bitmap: TBitmap);
begin
  case Bitmap.PixelFormat of
    pf24bit: ChangeWhiteToBlack24(Bitmap);
    pf32bit: ChangeWhiteToBlack32(Bitmap);
  else
    raise Exception.Create('Pixel format must be pf24bit or pf32bit.');
  end;
end;

if you don't want to make a single procedure that works with both 24-bit and 32-bit bitmaps, as TLama did. [One benefit of having two separate procedures is that these short procedures are easier to read (and maintain).]