"Warp"  schreef in bericht news:4cf6b3fd@news.povray.org...
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Stephen Klebs <skl### [at] gmailcom> wrote:
> What one would expect to be a smooth, linear gradient, becomes in 3.7 a
> parabolic curve, heavily skewed to white.

  I took your gradient image, rotated it 90 degrees and put alternating
horizontal black and white lines of pixels at both sides. Look at the
image from far enough that you don't distinguish the individual lines
but instead it becomes a gray, and then estimate on both sides which
square in the central gradient corresponds to this half-gray area on
the sides. I think it's pretty illuminating.

  With my monitor the photoshop/pov36 part (at the left) has the same
brightness somewhere between the second-to-last and third-to-last square,
while the pov37 part (at the right) as the same brightness somewhere in
the middle, as it should.
                                                          - Warp
-----------------------------------------------------------------------------------

That's odd, because for me the 3.6 side is correct and the 3.7 side is far 
too bright.
I've a rather new HD LCD TFT monitor (about 6 months) and adjusted it 
conform the windows 7 system with brightness and contrast followed by color 
shade correction.
Further the monitor of my laptop (Acer Aspire, 1 year old, crystal clear 
display) shows exactly the same picture. Both are driven by the NVidia card 
inside the laptop, so they should be the same and for /me/ they are. The 
pictures are displayed by Windows Live Mail.
I'm wondering if the default 3.7 approach is correcting the values for a 2.2 
gamma display in the file, while the display system thinks it gets lineair 
values and correct the values again. That would explain the 3.7 side for me. 
(The jpg and png files are the same for me, so that can't be the problem)
By the way, the challenge from clipka is for me a perfect gradient from 0,0 
to 1,1 in the XY-plane. I suspect that this picture is made with 3.7, so 
that is odd again.

I'm reading the Gamma Stories for months now and all the same I'm confused.
Maybe clipka can answer what is correct:
Povray makes a calculation for a picture and the values of the first three 
pixels are 64,64,64 / 128,128,128 / 192,192,192.
As I understand these are the values of PovRay's internal lineair color 
space.
My questions are:
A. What values are send to the display system at the moment of rendering:
    1. Gamma corrected values for a 2.2. display, so not the lineair values.
    2. Lineair values and PovRay tells the display system to convert them to 
2.2 display values:
B. What values are written in the png file and what is put in the gAMA 
chunk.
    1. Same as A.1. and the gAMA chunk tells the values are gamma corrected 
for 2.2.
    2. Same as A.2. and the gAMA chunk  tells that the correct values should 
be 2.2 corrected values (sounds not correct, but you never can tell).
To complete the story:
C.1. PovRay expects the file for a image-texture on a object to be gamma 
corrected (default 2.2) and counter corrects the values for it's lineair 
color space.
    2.Same, but the correction depends on the gAMA blok. With no gAMA a 2.2 
correction is used.

I think that the answers are A.1 and B.1, but sometimes I begin to doubt 
that, as I read these gamma stories.
As far as I've understood C.1 is 3.6 and C.2 is 3.7 policy.

Jaap Frank

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