From: xeno@iastate.edu (Gary L Snethen)
Subject: Re: Screen colors (Was Re: Zork/Dungeon)
Organization: Iowa State University, Ames IA
Date: Fri, 16 Oct 1992 23:55:53 GMT

In <1992Oct10.031206.2757@news.columbia.edu> kps@cunixb.cc.columbia.edu (Karthik P Sheka) writes:

>The human eye picks up only three colors, in different receptors (cones?):
>red, green, and blue.  All shading is done by the ratio of the receptors
>with each other.  There is a slightly larger amount of one of these types
>of receptors than the other two. (I can't recall which one, but I think it
>was green.)

Yes, I believe it was green, and then red, and then blue.  This leads to humans
perceiving green and yellow over the others...  Some have theorized this was
an evolutionary development due to our sun's color.

>When a computer creates a color image (usually) it does so by assigning a
>certain number of bits to each of the primary colors. (again, red, green,
>and blue.)  VGA comes in 4 and 8 bit flavors, (2^4 == 16, 2^8 == 256) , so
>the number of colors seen in VGA is either 16 or 256.  Actually, VGA uses 8
>bits, but I believe that they swap between two sets of four bits each, or
>something, due to limitations on the memory throughput. (Add more memory to
>a 16 color VGA card will usually give you a 256 color VGA card.)

>Some of the nicer "super" VGA cards use 16 bits for color, giving 65536
>colors.  Since 2^16 cannot evenly be distributed among the three primary
>colors, the extra bit is usually given to the primary that we can see more
>shades of. (2^6 greens, 2^5 reds, 2^5 blues).

>Most graphics workstations now come with 24 bit color.  This gives 8 bits
>of color for each of the primaries, or about 16 million colors.  Some of
>the more powerful graphics workstations now, and the graphics mainframes
>and such come with 48 bit color, giving two bytes of color information to
>each of the three primaries (giving 281 trillion colors).

>We've already passed the point of diminishing returns, however.  The most
>significant difference is between 16 and 256 colors for most computer
>applications.  For image-processing, 16 bit color is usually enough, with
>24 bit color used for serious tv-like color.

As I've already said, 24 bits in the system you've outlined isn't enough.
This only provides 256 shades of grey, which is insufficient...  If the
2^24 colors were used optimally (according to the color reception curve of
our eyes), then they would be more than enough, I think...  But they aren't
used this way.

>As for how many colors a person can see, its not a constant even for a
>single person in a single one-hour period.  If the colors being viewed are
>very similar, say shades of green, the eye can pick up more colors within
>those shades.  This is because the cones in the eyes that pick up that
>color get "tired", and the cones of the other colors have an easier time on
>picking out the information that they can read.

Very true...

>No matter how many colors a flat computer display can have, however, it
>will never be realistic enough to seem three-d. (I'm not counting stuff you
>can do with polarized glasses or other pheriferals.)  This is because, for
>an object to seem three-d, it has to direct the light at a lot of different
>angles, which is not possible on a flat screen.

Aye?  3d requires no more color information that realistic 2d images.
3d perception comes primarily from three factors:

1) Shading
2) Bi-ocular convergence
3) Focusing depths

Of these three, number 2 is used the most.  Your two eyes converge on the
various points you are looking at and the offset of other points (both behind
and in front of the observation point) are interpreted as depth by your brain.
This is the way the polarized glasses fool your eyes...  They provide two
different images, one tailored for each eye with the offsets built into
the images.

Focusing information is evident in some people far more than others.  Try
playing frisbee with one eye shut.  If you play comfortably, then you probably
have good focal depth perception.

The fact that you can tell the shape of an object in a photograph is due to
the shading and perspective in the pictures.  The shade of reflected light
is dependent on the angle that the normal of the surface makes with the light
source and with your eye.

If the shading of 24 bit graphics is good enough for 'photograph' quality,
then it is easily enough for 3d perception of depth, but this type of depth
perception does not (and can not) make you feel as though you can reach out
and touch the objects you see.  Bi-ocular perception is the best at doing
that.

And on a related note, it's quite possible for a perfectly flat screen to
give you true bi-ocular representations without polarized glasses or any
headgear.  Anyone who has ever seen a hologram (preferably a good one) knows
that flat plates can produce very realistic depth perception.

>This article turned out a lot longer than I initially expected.  I guess
>the moral is that you probably don't need a 24 bit video card, and to
>invest in holographic displays.

>//Karthik

Mine too... ;)

---Xeno