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.
--
Lorenz Christophe - R&D
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