How EXACTLY does Gamma relate to the Input/Output values in the Levels
dialog box?

If one makes a graduated stepped grayscale, where the input levels are
zero for the darkest black, 255 for the lightest white, and plops the
middle gray at 127, then adjusts the Levels Gamma to 2.0; the black
remains at 0, white at 255, while the middle gray changes to 180.
Overall, the shadows develop more contrast, the highlights, less.

If one then plots the resulting input/output values from each of the
gray panels of the changed grayscale onto the Curves “graph”, one is
left with a shape (and resulting image) that is significantly different
from the curve which would be generated by simply moving the middle gray
input/output value from 127/127 to 127/180.

Regardless of how I fiddle with the input/output numbers, I cannot find
an obvious relationship between those and the value for Gamma.
Compounding the problem is that since gamma appears to be a logarithmic
function, a gamma = 2 should be 10 times the “relationship” of gamma =
1; gamma = 3, 100 times, etc.

How EXACTLY does Photoshop calculate those input/output values? What
internal equation does it use?

Bill Henry

Gamma function, in 8-bit system is (as usual assumes that the black-point is set
up correctly):

output=INT(255*((input/255)^gamma) +0.5)

The middle input box in Photoshop Levels-dialog has an Adobe tweak (Mr. Cox
calls it the "slope limiting"). Due to this tweaking the shadows will be not
properly affected by the "gamma" value in the box. So that control can not be
used for gamma compensation, the error is very large.  Also the number in that
dialog is 1/"gamma".

Adobe really should provide an option so that those who need/want to use the
proper gamma could do so. Because of this problem I have made (using Excel) a
large bunch of *.amp files that you can use in Curves-dialog to apply an
accurate gamma (or inverse gamma) , please download from the bottom of the page
http://www.clinet.fi/~timothy/calibration/linear.htm

The on-line display compensation in Photoshop 5.0 has another "slope limiting".
This is even bigger problem, since the shadows appear differently in Photoshop
than in other application. For that too there should be an option.

Timo Autiokari

Well, Timo's math is almost right, for a change.
Unfortunately he still has no clue about how gamma really works, what gamma
is used/useful for, or
If you want a lot of hand waving and mis-information, go ahead and look at
his web sites -- they're good for a laugh, just don't try to follow any of
his instructions.

Chris

PS. the slope limiting is a good thing, if you understand what's going on,
and it normally has a very small effect.

In article <35f9cbb2.11577...@news.clinet.fi>, timo...@clinet.fi (Timo

Well, most of the people who choose to experiment with linear calibration do not
even think switching back since it gives much better control and results.

So, why is it there in the first place if it has only a "very small effect" ?
And in cases it does have an effect it is only a bad effect, the very same image
shows differently in other sw like a viewer than in PS50 and they print
differently also. Because viewers and printers do not have the Adobe "slope
limiting" tweak.

And why it can not be switched off, there are other photos50.ini settings so
adding one for the "slope limiting" would not be too difficult?  Not all PS
users want to limit their images.

Timo Autiokari

In article <35f380d2.16491...@news.clinet.fi>, timo...@clinet.fi (Timo

No, most of the people who follow your 'calibration' instructions end up
calling Adobe tech support who walk them through how to undo your
instructions and slowly explain all of your mistakes.

It's there because that small effect is quite noticable to the human eye.
Funny, I was just talking to some programmers of other major editing
packages last week - and they all use slope limiting.

It can't be turned off because there is no good reason to turn it off.
So far you're the only one who doesn't want it -- and that's because you
haven't got the faintest clue what it is or why it's there and you have the
worlds most idiotic 'calibration' on your system.

Chris

That is not the case. I get mostly success reports, only a few people have said
they went back to gamma space and the reason has always been because it is
little bit more easy to work with gamma images. But now that the Actions are
working well that small problem is only a question of a simple Action.

Mr. Cox said:

So, the "slope limiting" has normally a very small effect that is quite
noticeable for  human eye.  Sounds interesting. Everyting that is done in PS is
for human eye.

If you e.g compose some dark graphics over your images then the "slope limiting"
will be readily noticeable, not while in PS5 but as soon the image is printed or
viewed in some other application or put to www.

Is there other major packages than Photoshop?  Only two sw packages for PC that
I know applies the slope limiting, all other do on-line viewing accurately.

Yes, I can believe this. It calls some inspecting to find your the reason why
the shadows in some images look bad. Hence, so far most of the people do not
know the root cause of the problem in the first place.

It seems that on the other hand Adobe supports the gamma space due to the
"better" gradation in the shadows. But on the other hand Adobe damages those
same shadows by applying a "slope limiting" there.  Odd reasoning.

The "slope limiting" is not good, simply because it is not universal. Printers
do not follow it nor does browsers or other sw.  Now when you edit images in
Photoshop 5.0 then the slope limiting makes the shadows to appear less than they
do in other apps or devices. So you can over-do the shadows without noticing it.

It is there because the RGB to Lab and RGB to CMYK reduces colors by a factor of
6 to 8. So after the conversion the reduced gradation in the shadows becomes
visible unless it is "slope limited".

So why on earth does the Photoshop allow such an idiotic calibration? In both
Adobe Gamma application and in the RGB setup of PS5?

Timo Autiokari