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EXR's loose their Alpha interpretation settings

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Steve Bentley
EXR's loose their Alpha interpretation settings
on Aug 3, 2018 at 9:11:36 pm

EXR's have been mentioned in this forum a lot of late so I thought I'd ask something that has vexed me ever since EXR's emerged from ILM:
Why can't EXR's keep their interpretation settings in the Project panel?
We often replace footage and always use a Straight alpha. But once we replace an EXR sequence we always have to go back in and remind the EXR's that they should have a Straight alpha even though it was set on first importation. They do seem to keep their "preserve RGB" settings if that has been selected the same goes for their frame rate.
These file have been rendered with a Straight alpha and not premult, so it's not like they are returning to what they were rendered with.
Thoughts?



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Andrew Somers
Re: EXR's loose their Alpha interpretation settings
on Aug 3, 2018 at 9:52:15 pm

Hi Steve, have you modified your Interpretation Rules?

The rules has this line near the bottom:

# OpenEXR is premultiplied by convention
*, *, *, "oEXR", * ~ *, *, *, P, *, *
You can find it here:
~UserName/Library/Preferences/Adobe/After Effects/11.0/Interpretation Rules.txt

Just FYI, the EXR spec defines the RGB channels as premultiplied, it's actually part of best practices/recommendations for use. Here's what they have to say on the subject, starting on page 20:
http://www.openexr.com/TechnicalIntroduction.pdf

Nevertheless, if you want it to always be STRAIGHT, then you can make the line read:

# OpenEXR set default to interpret alpha to straight.
*, *, *, "oEXR", * ~ *, *, *, S, *, *
I set that as a SOFT rule (using the tilde ~ ) so if the file has metadata that specifies premult, then it will come in premult.

In Interpretation rules you can also add in the color profile, frame rate, and linearization to what you most often use. Here I set it as a hard rule (using the equals = ) so this line overides embedded meta data.

# OpenEXR set default to interpret alpha to straight, 2398, sRGB Primaries and Linear.
*, *, 23.976, "oEXR", * = *, *, *, S, "sRGB", 1

Hope that helps.


Andy

Andrew Somers
VFX & Title Supervisor
http://GeneralTitles.com


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Steve Bentley
Re: EXR's loose their Alpha interpretation settings
on Aug 3, 2018 at 10:15:47 pm

Thanks for that Andy. I'd forgotten about that pref file.

I had a heated (in a good way) conversation with Piotr about premult vs straight when the spec was first being born but I'll review it again now that I have more or less gray (hair and brain cells respectively). I've always railed against the "norm" of the premult alpha in the industry given that anytime we were having a matting issue I could solve it by working in a "straight" pipeline. But I've given up yelling into the wind; it seemed for a while I was the only one that cared or knew.

One of the questions I ask my new hires is: What's the diff between straight and premult? - to date I have yet to have anyone answer that question correctly (and I don't even want the math/techie answer); most don't even try.
I think perhaps VFX has gotten to compartmentalized, due probably to sheer size of projects, and that kind knowledge gets lost by all but the TD's. And in some ways that takes some of the fun out of it.



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Andrew Somers
Re: EXR's loose their Alpha interpretation settings
on Aug 3, 2018 at 11:27:52 pm

Yea I agree there are plenty of times when straight is better than premult. In fact I was so used to working in straight that when EXR came along I remember being surprised that they went that way. Nevertheless, I see their rationale as to the value when working in scene linear.

I'll pause briefly to mention that After Effects (and PS) don't handle alpha channels in linear light "quite right" — AE has some definite issues with opacity not functioning as expected when converting to or from a gamma encoded environment for instance.

Nevertheless I use the UNMULT plugin to convert to straight alpha in AE — something so basic and simple I think AE should just "do it".

Andrew Somers
VFX & Title Supervisor
http://GeneralTitles.com


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