Stephen Williams wrote:
> firstname.lastname@example.org said:
> > (Yes, it's 3 - the luminance sensors (the 'rods') have the same
> > sensitivity as the 'green' channel 'cones')
> Not quite, they respond to a wider spectrum then the green sensors.
> Though they are highly sensitive to green, they respond to most all
> of the visible color range. I think.
> (Actually, after saying that I'm not really sure how wide the spectral
> response is for luminance, but I do know that luminance calculations
> certainly all include some red and blue, and I presume there is a
> psychological/physiological reason for that.)
Its not just about spectrum, but about resolution and sensitivity.
The luminence sensors are much denser than the chroma sensors. They are
most sensitive at green, drop off badly below red, and drop off much worse
in the blue.
The green/red sensing is at a much lower density than the luminence, but
has fairly good sensitivity. NTSC and PAL colour TV take advantage of this,
and use only about 1/4 of the bandwidth for the chroma info, as they use
for the luminence info. The density and sensitivity of the blue sensors are
very very poor. A pure red or green TV image looks bright and clear. A pure
blue TV image looks dark and noisy. That is mostly due to the limitations
of the eye, although the blue emitter in a TV tube is also poorer than the
red or green..
The bottom line is we cannot see colour in detail. Try grouping a lot of
tiny varously coloured images together. You can still the fine detail of
their shapes, but they all appear white.
When you try to scan a colour photo or magazine print you are using three
sensors which don't accurately match the spectral response of the eye, but
have peak responses in roughly the same places. You are scanning images
made of three dyes which strange spectral responses that only *very*
roughly approxiamate the response of the eye's sensors. You will display
the result on a CRT, LCD, or printer, with their own set of very rough
approxiamations to the right spectral response. In addition, some of these
steps are working in cyan, magenta, and yellow, rather than red, green and
blue. There are further limitations in colour accuracy caused by this
swapping of primary/secondary colour mode.
If the eye's colour sensing worked in an absolute way the result would
probably always look awful. However, the eye senses colour only in a
relative way. Basically, an averaging process takes place across the entire
field of vision, and the eye assumes this average to be white. All colours
are rendered within the brain's signal processing according to this
averaging. Now, if the sun changes to flourescent light as night comes we
don't see the world turn the awful blue which most flourescent tubes
produce. We see just a minor tinting of the colouration of the scene. More
puzzling, and not fully explained, is why we appear to see a somewhat
colourful image under low pressure sodium lamps. These produce extremely
narrow band monochromatic light. We see zero colour variation in the
relected light from any part of the scene, and yet we don't see the world
in pure orange. The eye is clearly cooking up some fake, but realistic,
colourfulness. How that realism works is a mystery, as we seem to detect
the approxiamately correct colour of objects we have never seen in white
Unless you understand these things, imaging will always be disappointing.
Without a feel for the limitations of the chain of factors resulting in a
final image, the nature of that result tends to be an unpleasant suprise.
Often, the only way to prove to people just how bad something will look is
to produce a sample! A sad, but necessary, waste of effort.
-- Source code, list archive, and docs: http://www.mostang.com/sane/ To unsubscribe: echo unsubscribe sane-devel | mail email@example.com
This archive was generated by hypermail 2b29 : Tue Dec 12 2000 - 18:52:31 PST