[Image-SIG] 3-bit color palette

[jcupitt at gmail.com]

On 30/03/2008, Gokce Taskan <gokcetaskan at gmail.com> wrote:
> I've been searching all over the internet for a method to convert an
>  image to a 8 color (3-bit) image. I've used these:
>  I am trying find a method to output a resonable image which would look
>  like the one in
>  http://en.wikipedia.org/wiki/List_of_monochrome_and_RGB_palettes#3-bit_RGB
>
>  Do you have any advices?

To convert a 24-bit RGB image to a 3-bit one, just take the top bit
from each channel and OR them together. In C:

  bit3 = ((r & 128) >> 5) | ((g & 128) >> 6) | ((b & 128) >> 7)

You can make it look much better with a dither. A 2D error diffusion
dither is fast and works well.

To convert back to 24-bit RGB (again, in C):

  unsigned char lut[2] = {0, 255}
  r = lut[(bit3 & 4) >> 2]
  g = lut[(bit3 & 2) >> 1]
  b = lut[bit3 & 1]

John