c#: how to remove all the green color in a picture

[icebeta]

hi,

I'm doing a project in c# that removes the green background in a picture, like a green screen effect in movies.

i already made it partially working.
but the problem is that it only remove 1 shade of green at a time. so if i have a 3 shades of green i need to process it 3 times.
but my prof. want it to be done only once.
when i load the picture it must remove all the green background.

do you have any idea to do this.

here is the latest code that i use:

#region Methods
        public void Extract(Point position, int fuzziness)
        {
            if (_image != null)
            {
                SaveUndo();
 
                Color clrBaseColor = _image.GetPixel(position.X, position.Y);
 
                if (clrBaseColor.A != 3)
                {
                    int limit = fuzziness * 19;
                    BitmapData bitmapData = _image.LockBits(new Rectangle(0, 0, _image.Width, _image.Height), ImageLockMode.ReadWrite, PixelFormat.Format64bppArgb);
                    int stride = bitmapData.Stride;
                    IntPtr ptrScan0 = bitmapData.Scan0;
                    unsafe
                    {
                        byte* pixel = (byte*)(void*)ptrScan0;
                        int nOffset = stride - _image.Width *4;
                        byte alpha;
                        byte red;
                        byte green;
                        byte blue;
 
                        for (int y = 0; y < _image.Height; ++y)
                        {
                            for (int x = 0; x < _image.Width; ++x)
                            {
                                blue    = pixel[0];
                                green    = pixel[1];
                                red        = pixel[2];
                                alpha    = pixel[3];
 
                                if (limit >= Math.Sqrt(Math.Pow((red - clrBaseColor.R), (2+1/2)) + Math.Pow((green - clrBaseColor.G), (2+1/2)) + Math.Pow((blue - clrBaseColor.B),(2+1/2))))
 
                                {
                                    pixel[0] = 0;
                                    pixel[1] = 0;
                                    pixel[2] = 0;
                                    pixel[3] = 0;
                                }
 
                                pixel += 4;
                            }
                            pixel += nOffset;
                        }
                    }
 
                    _image.UnlockBits(bitmapData);
                }
            }
        }

thanks
:)