c++ - Swap two colors using color matrix -

how can swap 2 colors using color matrix? instance swapping red , blue easy. matrix like:

0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 1 

so how can swap 2 colors in general? example, there color1 r1, g1, b1 , color2 r2, g2, b2.

edit: swap mean color1 translate color2 , color2 translate color1. looks need reflection transformation. how calculate it?

gimp reference removed. sorry confusion.

this appears section of color-exchange.c file in gimp source cycles through pixels , if pixel meets chosen criteria(which can range of colors), swaps chosen color:

  (y = y1; y < y2; y++)     {       gimp_pixel_rgn_get_row (&srcpr, src_row, x1, y, width);        (x = 0; x < width; x++)         {           guchar pixel_red, pixel_green, pixel_blue;           guchar new_red, new_green, new_blue;           guint  idx;            /* current pixel-values */           pixel_red   = src_row[x * bpp];           pixel_green = src_row[x * bpp + 1];           pixel_blue  = src_row[x * bpp + 2];            idx = x * bpp;            /* want pixel? */           if (pixel_red >= min_red &&               pixel_red <= max_red &&               pixel_green >= min_green &&               pixel_green <= max_green &&               pixel_blue >= min_blue &&               pixel_blue <= max_blue)             {               guchar red_delta, green_delta, blue_delta;                red_delta   = pixel_red > from_red ?                 pixel_red - from_red : from_red - pixel_red;               green_delta = pixel_green > from_green ?                 pixel_green - from_green : from_green - pixel_green;               blue_delta  = pixel_blue > from_blue ?                 pixel_blue - from_blue : from_blue - pixel_blue;                new_red   = clamp (to_red   + red_delta,   0, 255);               new_green = clamp (to_green + green_delta, 0, 255);               new_blue  = clamp (to_blue  + blue_delta,  0, 255);             }           else             {               new_red   = pixel_red;               new_green = pixel_green;               new_blue  = pixel_blue;             }            /* fill buffer */           dest_row[idx + 0] = new_red;           dest_row[idx + 1] = new_green;           dest_row[idx + 2] = new_blue;            /* copy alpha-channel */           if (has_alpha)             dest_row[idx + 3] = src_row[x * bpp + 3];         }       /* store dest */       gimp_pixel_rgn_set_row (&destpr, dest_row, x1, y, width);        /* , tell user we're doing */       if (!preview && (y % 10) == 0)         gimp_progress_update ((gdouble) y / (gdouble) height);     } 

edit/addition

another way have transformed red blue matrix:

1   0   0   0   0 0   1   0   0   0 0   0   1   0   0 0   0   0   1   0 -1  0   1   0   1 

the values matter bottom ones in matrix.

this same saying subtract 255 red, keep green same, , add 255 blue. cut alpha in half so:

-1  0   1   -0.5    1 

so (just gimp source) need find difference between current color , target color, each channel, , apply difference. instead of channel values 0 255 use values 0 1.

you have changed red green so:

-1  1   0   0   1 

see here info:

http://msdn.microsoft.com/en-us/library/windows/desktop/ms533875%28v=vs.85%29.aspx

good luck.