[Tig] color vision

[Jim Houston]

On Mar 19, 2007, at 2:29 PM, Rob Lingelbach wrote:
>
> This isn't the explanation I was looking for regarding the need for  
> extra
> contrast after subtracting color from an image...

The vision explanation for this effect is that there are three types of
cones involved in color vision and they combine in different ways
to produce an internal  luminance channel, plus a red-green, and a  
blue-yellow
opponent color channels.   The perception of contrast is the combined
signal from the luminance channel *and* the red-green and blue-yellow  
channels.
In practice, the blue-yellow has so little influence that it is  
ignored. (think about
how either blue or yellow text of equal luminance on a white background
are equally hard to read.)

So contrast mostly comes from the perception of the luminance and
the red-green channel.   If you remove the red-green channel  
information,
then you are only left with a portion (say about 70%) of the contrast  
information the
brain depends on, thus the image looks like it has less contrast.
Boosting the gradient of the luminance of the image by 30% or so  
restores
much of the appearance of the image under the same viewing conditions.

These are human visual system effects that are independent of whether  
it is on film
projected in a dark room, or on a bright screen.

Jim H.
Starwatcher Digital