Path: utzoo!utgpu!water!watmath!onfcanim!dave
From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: comp.arch
Subject: Re: 16 & 32 bit vs 32 bit only instructions for RISC.
Message-ID: <15580@onfcanim.UUCP>
Date: 12 Mar 88 20:22:43 GMT
References: <9651@steinmetz.steinmetz.UUCP> <9678@steinmetz.steinmetz.UUCP> <2574@im4u.UUCP> <2116@saturn.ucsc.edu> <1697@winchester.mips.COM>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 35

Another place where 16-bit data is important: computer graphics.

1) Although 8 bits/pixel/colour is most common for "full colour" images,
   an 8-bit linear encoding just isn't good enough for critical work.
   For relatively dark portions of the image, the quantization error
   due to 8 bits causes visible band (Mach bands) or other errors.

   For image storage, you can use better encoding strategies that make
   better use of 8 bits, but then you have to encode and decode the
   pixels every time you touch them (for compositing, etc.).  But just
   using 12 or 16 bits with linear coding is faster.  Pixar uses
   12 bits/colour internally.

   Also, when you interface to the physical world, the ADCs and DACs
   are always linear, so you are forced to work with more than 8 bits
   if you care about quality.  Our digital camera and film recorder
   are both 12 bits per colour per pixel.

2) 16 bits is a good word width for storing screen coordinates in
   display lists.  You always want to store as many of these as will
   fit in the physical memory available.


Any machine that stores a C "short" in 4 bytes is going to make
my image data files twice as big, and allow only half the length of
display lists in memory.  I wouldn't buy it.

Any machine that stores a "short" in 2 bytes but requires multiple
instructions (shift/mask) to access the data is going to have a
difficult time executing the inner loops of some algorithms fast
enough to benchmark comparably to another machine of similar cost
which does support 16-bit data types.

I don't use 16-bit words for many types of data, but I use them in
quantities of millions when I do.