Re: Linear and non-linear brightness scaling

Gernot,

I don't remember Poynton saying anything about an eternal law and
particularly I don't remember him saying that perceived brightness followed
a gamma 2.2 function. The whole gamma discussion was simply saying that
mapping luminance to a 1/2.2 gamma gave a near linear representation of that
luminance when viewed on a CRT screen. The display system took a linear
signal and mapped it to a nominal gamma 2.2 signal, hence the input signal
had to have the inverse gamma applied to the data for proper display. The
reasons the relative brightnesses are maintained (as long as the white
points are not too different), is that luminance signal is linear on the
display.

Now getting on the original topic of this thread:

Within the biological field there is strong interest in the basis for
certain physical human responses. In English we use the phrase: "Nurture vs.
Nature". Work in this area tries to answer the question "Is this learned or
is this genetic?" Within the general area that I work in, there is a body
of work (much of which Danny mentioned in his last post) which deals with
the issue of appearance as "Nature". We understand that the average level
of illumination affects our perception. We understand that the average
color of the illumination plays a role in our perception of color. There is
a growing body of work that seeks to examine the spatial nature of our
visual process. This means that the appearance of a scene is affected by
the objects in the scene and their proximity to one another. All of this
work basically assumes a Genetic model of vision exists and that first and
second order statistics can help with that model. What was hinted at in the
paper being discussed in this thread was a biological imperative. The
authors state the following:
"Here we examine the possibility that brightness scaling is actually the
signature of a biological strategy for dealing with inevitably ambiguous
visual stimuli, in which percepts of lightness/brightness are determined by
the probabilistic relationship between luminances in the image plane and
their possible real-world sources. "

What the authors seek to find is the NURTURE side of brightness. They imply
that if we had two different scenes that had identical luminances and
luminance distributions (first and second order statistics), our sensation
of lightness/brightness is affected by the real-world sources of the scene
as the result of a biological strategy that developed over time. For
instance, when we test visual judgments of scene contrast as a function
illumination level, we note that the scene contrast appears to get lower
with lower levels of illumination. Now, does the same effect hold for a
hungry hunter peering across a field at sunset, or does the hunter's visual
process adapt differently to maintain scene contrast? I firmly believe
that there is something to be said for contextual adaptation. I'm not sure
how one would demonstrate it and the authors certainly did not do it this
paper.
--
Tom Lianza
Director of Display and Capture Technologies
GretagMacbeth LLC
3 Industrial Drive
Unit 7&8
Windham, NH 03087
603.681.0315 x232 Tel
603.681.0316 Fax


"Gernot Hoffmann" <hoffmann@xxxxxxxxxxxx> wrote in message
news:1146593483.918453.124440@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Danny,

some conclusions, based on your profound knowledge,
would be helpful for the layman.

Perhaps you don't know those harsh discussions about
'gamma' some years ago, mainly influenced by Poynton's
propaganda (not Boynton).
At this time, anybody was blamed who didn't believe in
the eternal law, that perceived brightness is a power
function of physical luminance, and the exponent should
be near to 2.2 (or 1/2.2).
I'm remembering very well the advice of a member of the
Adobe staff:
Read the books - that's well established by science...
(not quoted literally).

IMO, the function 'brightness versus luminance' doesn't
exist, or it exists cum grano salis for some assumptions
about the viewing conditions.

Best regards --Gernot Hoffmann



.

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