Who needs 24 bit colors

[Samuel P. Uselton]

In article <95622 at sgi.sgi.com> tarolli at westcoast.esd.sgi.com (Gary Tarolli) writes:
>In article <8009 at eos.arc.nasa.gov>, prevost at eos.arc.nasa.gov (Michael Prevost) writes:
>> What about the case where someone is doing smooth shading? You can burn up a lot
>> of colors that are very close to the same but the eye is very sensitive to 
>> comparing two colors side by side. If you don't want color banding you had 
>> better have many bit planes.
>> 
>
>If you only have 4 or 8 bits of RGB then color banding is noticeable in some
>cases (or many cases, depending on what kind of images you have).  When you
>get up to 12-16 bits of RGB (total not per color channel), then I believe
>the dithering hides most of the color banding.  In fact, in some cases (I
>wont go as far as to say all) dithering can actually hide the color bands.
>
>I haven't done enough comparisons to see if dithered 16 bits looks better
>than 24 bits non-dithered.  I sort of doubt it.  Certainly 8 or 12 bits
>dithered will not look as good as 24 bits, but then again it costs less.
>But for sure, if you hold the number of bits
>constant, dithering greatly improves the image.  The improvement is obviously
>most apparent when there are few bitplanes.  Remarkably, dithering hides
>most of the color bands even when there's only 8 bits.
>
>Interestingly enough,  dithering is still usefull when you have 24 bits.
>If you have a large polygon that is shaded with a very small color slope,
>you will see Mach bands where the color changes.  As you pointed out,
>the eye is very sensitive to comparing two colors side by side.  But if
>you dither the 24 bit image, I believe the banding will disappear.
>
>--------------------
>	Gary Tarolli
>
>

But......
with dithering, you are giving up spatial resolution to get the colors
smoother.  If you need lots of spatial resolution AND smooth colors more bits
is better.

In one of my lives I do ray tracing, and building lighting functions for 
realistic images.  It is entirely possible to have 24 bits per pixel and
still detect the changes at neighboring pixels.

Didn't we have the other end of this discussion not to long ago (when is 
SGI going to start using wider DACs so we can get MORE bits per RGB 
component?) and a person doing vision research said he needed AT LEAST 
12 bits EACH for RGB?


Clearly you don't need more distinct shades than you have pixels on screen,
so I guess a look up table with ~1 Million entries is enough. Thats only 
20 bits per pixel.  But the OUTPUT from the table needs to be BIG!  ( :-) )

So I guess I see why skipping the table step and working in a fixed space
that is easy for interpolation, etc, is good.

Sam Uselton		uselton at nas.nasa.gov	
employed by CSC		working for NASA (Ames)		speaking for myself