Digital BW, The Print

> Austin Franklin wrote:
>  >>All other facts and misinformation aside, here is a bit of information
>  >>which should give you pause:  8 bits per channel of RGB data gives you
>  >>(only) 9 bits of lightness (black & white) information.
>  >>
>  >>-Jon Dubovsky ( entropy@... )
>  >
>  >
>  > Jon,
>  >
>  > Please explain the basis for this claim.
>
> In the best of worlds, three equally-weighted 8 bit channels give you,
> at most, about 9.6 bits of lightness information.  ( ln(256*3) / ln(2) )
>    It doesn't actually work out to that much because your eye does not
> equally weight the three channels.

I think you missed my question.  Where does your equation come from?  Also,
the channels aren't typically equally weighted.

Mind you, I'm not disagreeing with you, but just want to know the origin of
your claim.

> (On a lesser note, if you're actually trying to extract the lightness
> info, the most common RGB to lightness conversions compute the L channel
> by taking (max3(r,g,b) + min3(r,g,b)) / 2, which gives you one more bit
> of resolution than the source per-channel space (in this case, it gives
> you 9 bits).)
>
> Mr. Wesley, I only bring this up because I often hear people say that 24
> bit RGB space has a great deal more contrast capacity than 8 bit
> black-and-white space.

That I agree with, that it doesn't.  There is a difference between luminance
and chrominance, and what's significant to the human eye.

Austin