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From: torrie@Neon.Stanford.EDU (Evan James Torrie)
Newsgroups: comp.sys.amiga
Subject: Re: 24 bit color boards
Message-ID: <1990Dec4.214546.7091@Neon.Stanford.EDU>
Date: 4 Dec 90 21:45:46 GMT
References: <1990Dec4.013744.10286@ncsuvx.ncsu.edu> <1990Dec4.061416.16472@uokmax.ecn.uoknor.edu> <1990Dec4.115219.15680@zorch.SF-Bay.ORG> <1990Dec4.155412.23755@cunixf.cc.columbia.edu>
Organization: Computer Science Department, Stanford University
Lines: 51

es1@cunixb.cc.columbia.edu (Ethan Solomita) writes:

>In article <1990Dec4.115219.15680@zorch.SF-Bay.ORG> xanthian@zorch.SF-Bay.ORG (Kent Paul Dolan) writes:
>>Except that I made exactly the opposite point:  just because you can output
>>8 each red, green, and blue bits to the D to A gun intensity controls doesn't
>>mean you have the right to call your system "24 bit color", since that is a
>>term of art that means that you _store_, not _emit_, three bytes of color data
>>per pixel.

>	Kent, take out a calculator and do a little math. The
>resolution of DCTV is somewhere around 600x300, or 180,000
>pixels. Therefore, if you get 18 bits worth of distinct colors
>out of a 24 bit palette, you essentially have a 24 bit
>frame-buffer within the given resolution limit. ie there are only
>enough pixels to display 18 bits worth of color. I believe that

  This is only true if the 18 bits of color are used to index
into a lookup table (which in turn has 24 bits of
information).
  For example, if I have a graduated shade of pure red to
black, then an 18-bit system (with 6 bits for each component)
WITHOUT a lookup table (i.e. direct mapping of 18 bits to the
screen) will only give me 2^6 = 64 different colors.  A true
24-bit system will give me 256 different colors.  A 48-bit
system would give 65536 different colors.
  For a standard sized screen (say 640 across), ONLY the
48-bit system would give more colors than could be used by the
screen (the 18-bit would have bands of 10 pixels across, the
24-bit system would have bands of 2 and a bit pixels).

  If you have a color lookup table though, you can set the
entries in your table to be equal to whatever colors you want
on the screen...  If you have an 18-bit lookup table containing
24-bit entries, and your screen has <2^18 pixels, only then
does your system have exactly the same picture quality as a
24-bit system.

  So do any of these "24-bit" systems have lookup tables?  Or
are they just direct color devices?  [One would suspect the
latter, since an 18-bit lookup table containing 24-bit entries
=> you need 3*2^18 > 700K just for storing the lookup table!].


 


-- 
------------------------------------------------------------------------------
Evan Torrie.  Stanford University, Class of 199?       torrie@cs.stanford.edu   
"I didn't get where I am today without knowing a good deal when I see one,
 Reggie."  "Yes, C.J."