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From: rjc@geech.gnu.ai.mit.edu (Ray Cromwell)
Subject: Re: Color palette correction (NeXT)
Message-ID: <1991Apr20.224627.26851@mintaka.lcs.mit.edu>
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Organization: The Internet
References: <1991Apr19.003352.6042@cc.helsinki.fi> <1991Apr19.014549.15293@mintaka.lcs.mit.edu> <1991Apr20.211035.6064@cc.helsinki.fi>
Date: Sat, 20 Apr 91 22:46:27 GMT
Lines: 69

In article <1991Apr20.211035.6064@cc.helsinki.fi> jalkio@cc.helsinki.fi writes:
>In article <1991Apr19.014549.15293@mintaka.lcs.mit.edu>, rjc@churchy.gnu.ai.mit.edu (Ray Cromwell) writes:
>> In article <1991Apr19.003352.6042@cc.helsinki.fi> jalkio@cc.helsinki.fi writes:
>>>
>>>Perhaps saying that the Color station has a palette of 16M is a bit too
>>>much and saying that it only has a palette of 4096 is a bit too
>>>low... The thruth lies somewhere in between.
>>>
>>>:-)
>>>
>>>			Jouni
>> 
>>   I don't agree with this. Most Amiga graphic programs store images
>> internally as 24bits and most load/save IFF24 bit images, however
>> the standard Amiga can only quantize these images to 4096 colors.
>> The NeXT hardware palette is 12(16?)bits, period. It doesn't matter if the
>> OS stores pictures internally as 100 bits, the hardware is only
>> capable of 4096 different colors.
>> 
>
>I don't think NeXT is just like this. I understood that there really
>_ARE_ 16M different shades, of which the operating system automatically
>chooses the best suited for each situation (determined by the "basic"
>color, of course). You only can't directly access all those shades.
>Thus, those 12 (or 16) physical bits just refer to a table where each
>entry can contain a 24-bit color value. 
>
>I might have understood wrong, of course.
>
>			Jouni

  Ahh, so the NeXT has a 12bit display. My definition of 'bit display'
is the number of bits per pixel. For instance, most people
incorrectly called some Mac and VGA display cards '24bit'. Actually
these cards are 8bit (can only display 256 colors simulataneously) while
their palletes are 24bit (16M colors).

 For instance, the Amiga HAM mode is only 6 bits per pixel, so it's
a 6bit display. HAM mode groups pixels together (allowing a previous
pixel to be combined with the current color modification). This gives
you an effective 12bits, but you can only partially change a pixel, or
look up a new one.  DCTV allows you to have 6 or 8 bit pixels
grouped together into an NTSC encoding which allows brightness resolution
to change, but chroma(color) will be less accurate (color bleeding).
HAM-E gives the Amiga an 8bit display mode(256 colors at once, something
that should have been done in the custom chips a long time ago). HAM-E
also uses HAM's trick of pixel combining modifications. This time, instead
of 4bit modification you get 6 bit modification which gives you a pseudo-18bit
display.

  My own definition of a 24bit display is this:

 Atleast 24bits of data displayed for each pixel. This means there is
no color lookup table. The display is 'palette mapped'. Each pixel
specifies it's color directly. This means a 640x480 24bit frame
is 921,600 bytes large.

 HAM and DYUV (DCTV and CD-I) are good methods for animation since
they don't require huge amounts of data, and the eye is relatively
poor at perceiving high resolution in fast moving images.




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