Assuming a RGB image with 15 bits per pixel how many shades of grey
are possible? How does an RGB image, which only shows mixes of red,
green, and blue, display grey, anyways?

Hi zoogelsnof,

When the R, G, and B values are all zero, you get a black pixel, and
when they are all at the maximum, they blend together to make a white
pixel.  As the values increase from zero to the maximum, always the
same for R, G, and B, you get all the shades of grey from black to
white.  If there are fifteen bits per pixel and five bits per color
component, that means there are two to the fifth power or 32 shades of
grey possible.

There don't seem to be any standard boundaries separating black or
white from grey.  References I found just considered black and white
as the extreme values of grey.


Additional Links

A page from Kevin J. Walsh on "RGB to Color Name Mapping" shows some
bluish greys with R, G, and B not all equal, as well as 100 greys
ranging from black to white (based on eight bits per component).
http://web.njit.edu/~walsh/rgb.html

Wikipedia's article on grey specifies RBG grey as ranging all the way
from black to white.
http://en.wikipedia.org/wiki/Grey

Wikipedia's article on additive color explains that equal amounts of
R, G, and B form white.
http://en.wikipedia.org/wiki/Additive_color


--efn

The answer is correct, but I want to amplify one thing. White, black,
and gray are relative terms. The period at the end of a sentence read
in sunlight is brighter than the unprinted part of the page read
indoors. It is only by contrast that one distinguishes white from
black (or orange from brown, for that matter).

That's right.  One thing that makes this obvious is an overhead
projector.  Suppose you have a transparency with some black writing on
it.  With the projector lamp off, the screen looks white.  Turn on the
projector, and it shines white light all over the screen, except where
the black writing blocks it.  So the 'black' letters that show up on
the screen are actually exactly the same color white as the whole
screen was with the projector off.  They just look black because the
rest of the screen is now a brighter white than it was before.

Another example is the moon.  I've been told it's the color of a lump
of coal.  But on a dark night, the full moon looks white.

Race Car,

The overhead projector example really brings home the point. The
albedo of the moon is also a good example, but the story is a bit more
complex. See for example:

http://jeff.medkeff.com/astro/lunar/obs_tech/albedo.htm