change a bitmap's pixel colour - image

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).]

Answer 3

procedure TForm1.Button1Click(Sender: TObject);
var
BitMap1,
BitMap2 : TBitmap;
X, Y, Size : Integer;
P: Cardinal;
begin
BitMap1 := TBitmap.Create;
BitMap1.LoadFromFile('image1.bmp');
BitMap1.Transparent := true;
BitMap1.TransparentColor := clWhite; // old color
BitMap2 := TBitMap.Create;
BitMap2.Height := BitMap1.Height;
BitMap2.Width := BitMap1.Width;
BitMap2.Canvas.Brush.Color := clBlack; // new color
BitMap2.Canvas.FillRect(
Rect(
0,
0,
BitMap2.Width,
BitMap2.Height
)
);
BitMap2.Canvas.Draw(BitMap1);
Image1.Picture.Assign(BitMap2);
BitMap1.Free;
BitMap2.Freel
end;

Answer 4

private void btnLoad2_Click(object sender, System.EventArgs e)
{
Bitmap myBitmap= new Bitmap(openFileDialog1.FileName);
Bitmap myBitmap1 = new Bitmap("C:\\Documents and Settings\\Lalji\\Desktop\\image.png");
for (int x = 0; x < myBitmap.Width; x++)
{
for (int y = 0; y < myBitmap.Height; y++)
{
// Get the color of a pixel within myBitmap.
Color pixelColor = myBitmap.GetPixel(x, y);
string pixelColorStringValue =
pixelColor.R.ToString("D3") + " " +
pixelColor.G.ToString("D3") + " " +
pixelColor.B.ToString("D3") + ", ";
if (pixelColor.R.Equals(0) && pixelColor.G.Equals(0) && pixelColor.B.Equals(0))
{
//MessageBox.Show("black pixel");
}
else if (pixelColor.R.Equals(255) && pixelColor.G.Equals(255) && pixelColor.B.Equals(255))
{
//MessageBox.Show("white pixel");
myBitmap1.SetPixel(x, y, Color.White);
}
//switch (pixelColorStringValue)
//{
// case "255 255 255":
// {
// // white pixel
// MessageBox.Show("white pixel");
// break;
// }
// case "000 000 000,":
// {
// // black pixel
// MessageBox.Show("black pixel");
// break;
// }
//}
}
}
myBitmap1.Save("C:\\Documents and Settings\\Lalji\\Desktop\\image1.png");
MessageBox.Show("Process done");
}