[Gllug] GIMP

[Chris Bell]

On Wed 17 Mar, Mike Brodbelt wrote:
> 
> On Wed, 2004-03-17 at 20:34, Rich Walker wrote:
> > Mike Brodbelt <mike at coruscant.demon.co.uk> writes:
> 
> > This really is a "don't get us started" area. One of our number runs a
> > photolibrary, and the amount of idiocy in this area is incredible. As
> > you say, the DPI thing - which comes from the fact that Mac software has
> > always had a "DPI" setting for generating jpeg's, I think, so people
> > believe it...

   Colour is a difficult subject at the best of times. The sensitivity of
the eye to different spectral colours is nothing like what most people would
expect, it varies from eye to eye and person to person, and it is very
difficult to match this with any device.
   The highly sensitive but colour-blind rods are mainly used in low light,
and the three types of colour sensitive cones take over when there is more
light available.
   The spectral responses of the cones overlap by a considerable amount,
with the green sensitive cones having a broad response, the blue sensitive
cones having a relatively sharp cut-off, while the red sensitive cones have
considerable residual sensitivity well into the far blue colours.
   The eye sensitivity is also non-linear, approximately logarithmic, and
self adjusting for the intensity and colour of the surroundings. It also
makes allowance for lens flare.
   Add to this the poor colour output from most displays, with a
less-than-smooth colour output which is often unable to simulate a far red,
peculiarities of the display gamma, inability to display a wide brightness
range, and completely different results from different kinds of printers,
and you have a recipe for disaster.
   A considerable amount of research has been done into colour matching, and
most professional video cameras pass the outputs from three colour sensors
through a matrix to re-calculate each of the final colours used. Adjustments
are then applied for flare, gamma correction, excessive brightness range,
and colour noise in darker areas. The different initial colour sensitivities
still give different results for objects which look similar to the eye, and
different fabrics can appear to change colour on camera. Light containing a
very restricted band of colours, especially that from some flowers, is often
rendered incorrectly. Similar errors are seen under mixed, or artificial
lighting such as cheap flourescents.



-- 
Chris Bell

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