Thanks for your feedback !
There's more in gamma than working with integer systems...
If you consider color corrections, gamma tweaking is a basic tool to
work with.
It is a different story than the one we are used to with linear encoded
images.
Here we don't want to mimic real life light, but we want to alter
reality. And any trick is welcome here ;-)
So we input float and output float... We don't want the grader to clip
all the nice float images to a LDR image. :-o
The negative pixel values still can find some use later on, so we want
to keep them.
You're probably right regarding the half-float precision around zero,
there's no need for a linear area over there.
I'll do some tests and check that.
And it is true that I can simply mirror the gamma curve below zero... I
don't understand why I didn't thought about that ....
The e-sRGB curve is quite nice... Do you have more information on how
that curve is built ? I don't see much on that on the web...
Chris.
gary demos wrote:
I believe that the only time gamma is needed is for interoperation with
gamma-based integer
systems. For such interoperation, 0.0 will mean black which is no
light at all. Negative
values would then be clipped to 0.0 in floating, which will map to 0
(or 16 or 64) in
integer pixel values. There is no meaning defined below the black
value of integer
pixel systems. I believe it is best not to use anything below the
black value of
an integer system.
-Gary Demos
Lars Borg wrote:
Re: [Openexr-devel] Negative pixels and gamma math
issues
I would rather expect you to do the opposite: A linear segment
near 0 and gamma otherwise.
Fractional gamma is not a problem: Just treat values as
positive.
For an example, see how the e-sRGB curves are constructed.
Also, do you really need a flat section when you're using
float
encoding?
The half-float should have enough dynamic range around 0 for
using gamma through the entire curve.
(e-sRGB uses an integer encoding so a flat section is needed
to
save on bit depth.)
Lars Borg
Adobe
At 10:22 AM +0200 7/7/05, Christophe Lorenz wrote:
I'm just curious how you guys get
around
that problem....
We have to apply gamma correction on linear images in half16
format....
The issue is of course that powers of negative numbers with
fractionnal exponents isn't always possible...
The other thing is that even if a power of 2 is always possible, the
curve flattens at zero and goes up again.
This is typically not an issue when displaying an image because the
values are clamped anyhow...
But because we do colorgrading, we want to keep the information
present across the whole range, even after applying a gamma
correction. (otherwise we end up with range clamped between 0-1)
We also want to avoid the flat part around 0, but keep the curve close
to a real power function.
We ended up doing a special gamma function that has 2 linear parts at
0 and 1 and smoothly blend with the real gamma on a 0.1 range.
Then, below 0 and above 1, the linear function is used.
(this is similar to what the broadcast tv guys are doing in the low
lights)
Has anyone faced the same issue ?
Chris.
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