Posterize to 1 bits per channel + extra luma bit

Question

ImageMagick's -posterize 2 flag can be used to convert an image to 1 bits (2 levels) per R/G/B channel. But is there a way to convert to a format where I have two sets of 8 colors, with half and with full intensity? In other words, my target format is 4 bits per pixel: 1 bits per R/G/B and 1 fourth bit for luminosity.

EDIT for clarity based on some comments:

With -posterize 2, you get 8 colors total, since you have three channels and two levels per channel (since 2³ = 8):

• 0 0 0
• 0 0 1
• 0 1 0
• 0 1 1
• 1 0 0
• 1 0 1
• 1 1 0
• 1 1 1

Now imagine I have access to seven more colors (because "half-bright black" is still just black):

• 0 0 0
• 0 0 0.5
• 0 0.5 0
• 0 0.5 0.5
• 0.5 0 0
• 0.5 0 0.5
• 0.5 0.5 0
• 0.5 0.5 0.5

So I'd like to quantize (?) my picture to these total of 15 colors.

As for the output format, it shouldn't matter, the same way you can use -posterize 2 to e.g. produce a PNG that is formally 24-bit RGB, it just doesn't contain any non-0 or 255 values in any channel.

Answer 1

Based on @Mark Setchell's comment, using these two answers I was able to do this with a two-step process:

1. Generate an image containing the palette

   $ convert  $(for l in 128 255
     do
       for r in 0 $l
       do
         for g in 0 $l
         do
           for b in 0 $l
           do
             echo "xc:rgb($r,$g,$b)"
           done
         done
       done
     done) +append palette.png
   

   This generates a file with 16 colors, because xc:rgb(0,0,0) is generated twice; but that should be OK AFAICT.

2. Use this palette for color mapping

   $ convert foo.png +dither -map palette.png result.png
   

While this does technically do what I set out to do, it turns out its result is quite poor; but that's for a separate question.