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From: thomsen@spf.trw.com (Mark R. Thomsen)
Newsgroups: comp.sys.next
Subject: Re: ColorStation questions
Message-ID: <281F74CB.1E6C@deneva.sdd.trw.com>
Date: 2 May 91 02:09:15 GMT
References: <1991Apr30.150026.9826@magnus.acs.ohio-state.edu>
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Organization: TRW Inc., Redondo Beach, CA
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There is enough discussion and argument on this that perhaps another attempt to  
explain will help some (and add confusion to others). I'll start slow to try  
minimizing confusion.

NeXTdimension represents a value to display with 24 bits. The 24 bits are the  
concatenation of three 8 bit values, representing intensities of the three  
primaries used (red, green, blue). Since the value does not go into a lookup  
table that the programmer controls (translating the internal value to a display  
value), the system is called 'true color'. To modify an image displayed (e.g.,  
a programmed gamma curve) a program will directly modify each value in the  
image array. If a lookup table was used the table values could be modified and  
the image array would not be altered. The advantage of the LUT is often speed  
relative to a program modifying 100's of thousands of values. NeXTdimension's  
processing of images will usually be done with the i860 so performance is not  
shabby (we worked image processing algorithms on a loaner NeXTdimension in  
February).

(BTW: A gamma correction is a remapping of displayed values so that a linear  
digital data progression - 1, 2, 3, 4 - looks linear on a display or printer. A  
different gamma value is associated with a different device, helping to make  
the displayed image look the same and 'right' on different devices. Recommend  
reading section 13.1.1 of Foley, vanDam, Feiner, and Hughes' Computer Graphics  
text for detailed description).

NeXTstation Color represents a value to display with 12 bits, simply halving  
the concatenated values to 4 bits each for RGB. Everything else remains the  
same, though there is no i860 to pump up performance.

There may be a non-programmable LUT that adjusts displayed colors according to  
display settings. This I do not know, though it would make sense. I think that  
the Sony chips on the ND do this. Perhaps looking at the NSC.

NeXT confused some people when they said "we will bring out 32 bit color". What  
they meant is 24 bit color on NeXTdimension with alpha processing. Most data  
structures a programmer will work with have 32 bits - the extra 8 bits is the  
alpha for transparency/opaqueness. Pixar used the extra bits for other things  
as well. I don't think NeXT has said if they plan to.

Unless you are careful you can easily start letting Display Postscript start  
dithering. It looks suprisingly good, but for people doing image analysis it  
tends to get in the way. It is not hard to be careful. Awareness is sufficient.

A gray value is when R=G=B, whether 8 bits or 4 bits. Thus ND supports 256 gray  
values, or can emulate an 8 bit gray scale display. NSC supports 16 gray  
values. For image processing many images come as gray scale. To display these  
we copy each value into the R, G, and B of an image array - after we have done  
all of the processing on the gray scale value. There are tricks for coaxing  
more visually meaningful gray scale than this - mail me direct if you have a  
question.

In our experience a display architecture that uses an LUT (e.g., a Sun) is  
suited for graphics applications (e.g., CAD). A true color display architecture  
such as NeXT's is suited for image applications (e.g., earth resource satellite  
analysis). You can, of course, do image processing on a Sun (it is hard to do  
it well) and graphics on a NeXT. The bias as you get deeper is this way. Again,  
this is our experiences.

RGB input would have been excellent, but a bit unusual for something in this  
price range. We have a need for RGB and are using other equipment to capture  
'high-res' frames and transfering them to a NeXT. If someone finds a technique  
to directly capture RGB on the NeXT, please post.

Mark R. Thomsen