[Openexr-devel] Negative pixels and gamma math issues on half16

[Christophe Lorenz]

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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