Negative pixel values in source footage

general questions about Neat Video
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c_bechmann
Posts: 1
Joined: Tue May 28, 2019 2:07 pm

Negative pixel values in source footage

Post by c_bechmann »

Hi.

I have been handed a few sequences with negative values (in this case EXR converted from Alexa, but problem have been seen before). The values goes down to -0.0005 in one or more channels.

The negative values seems to create some artifacts in the denoise process. Can't show any footage (NDA), but the error looks similar to the spots you get when setting the Noise Levels->Cr/CB values to low.

Is there a "best practice" to get around this? I havent been able to find negative values or plate preperation mentioned in the documentation.

My own suggestions would be one of the following:
1: Clamp negative values to 0 or 0.000001. This is a destructive process, so not happy about that.
2: Offset the footage slightly to all positive values, denoise, then offset footage back again. This might (?) effect the grains as they are being calculated on a plate with other than original values.

Also, is Neat Video supposed to be used before or after any LUT-convertion nodes (e.g. log-C -> lin), or is it just controlled with the "linear input data"-checkbox?

I am working in Nuke using Neat Video v3 and v4 (working at two different companies).

Thanks in advance
NVTeam
Posts: 2525
Joined: Thu Sep 01, 2005 4:12 pm
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Re: Negative pixel values in source footage

Post by NVTeam »

Regarding negative values in general, that should work correctly and not cause any artifacts in the OFX plug-in.
Please submit a bug report with a reproducible test case.
We will check what is going on. Perhaps it is a matter of updating the software. Possibly a bug in the source footage (that is possible too).

Regarding order of effects, Neat Video is generally supposed to be applied to linear data in Nuke (the default in Nuke itself).
You can tell Neat Video to treat the input video data as linear or gamma-corrected using the control in Neat Video window.
In your situation I believe you should use the linear option and ensure that Nuke sends linear data to Neat Video.

Thank you,
Vlad
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