Dave Martindale wrote:
[re Photoshop's 16 bit representation]

The Pixar Image Computer used tricks like this many years ago. The
memory was 12 bits per component, with 2048 representing a value of
1.0. Values up to 3071 were brighter than white, up to 1.5. The range
3072-4095 represented negative values in [-0.5, 0]. When a 12-bit
value was loaded into the processor, it was automatically extended to
16 bits in a way that preserved the positive or negative meaning,
then the arithmetic was all 16/32 bit.

Dave

It did indeed, and I'm astonished that you remember these details so long
after I, who worked on this stuff for years, have forgotten them!

You touch on another important aspect of having displayable values occupy
only a subset of the total 16 bits. This is the ability to represent
negative
intermediate values.

Negative numbers are an important component of most
graphic calculations, and the ability to represent negative values in place
saves,
storage, and the time required to convert between storage formats, while
retaining the ability to make calculations that require per pixel negative
values. An example would be the subtraction of two channels, or the Pirl
function, which uses negative terms in calculating the theoretically perfect
resampling filter.

OTOH, Adobe's weird 16 bit format makes it more difficult to interface to
other graphics libraries, requiring additional passes to convert to and from
16 bit mode.
--

Mike Russell
www.curvemeister.com
www.geigy.2y.net