How to export to Open EXR with alpha chanel without being premultiplied

Participant ,
Nov 14, 2019 Nov 14, 2019

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How to export to (Open EXR with alpha chanel without being premultiplied)

 

just render to Open EXR with alpha packed up within frame and not being premultiplied?

 

 

Any thoughs?

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correct answers 1 Correct answer

Explorer , Feb 12, 2021 Feb 12, 2021
Sorry, this DID work! I created a script file called EXR.jsx and added the code:var section = "Misc Section"; var pref = "Allow writing straight alpha into EXR"; var type = PREFType.PREF_Type_MACHINE_INDEPENDENT; if(app.preferences.havePref(section, pref, type)) { if(app.preferences.getPrefAsBool(section, pref, type)) { alert("EXR straight alpha pref already set to TRUE."); } else { alert("EXR straight alpha pref was set to FALSE. Setting to TRUE."); app.preferences.savePrefAsBool...

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Enthusiast ,
May 03, 2021 May 03, 2021

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The OpenEXR spec says the image should be premultiplied, and that is assumed to mean premutiplied over black. When you premultiply over black and have 16- or 32-bit floating point pixels, you can generally convert back to straight without any noticeable loss, as After Effects does because it works with straight internally. Film compositing programs like Nuke are premultipled internally, so EXRs work smoothly there.

 

The advantage of straight alpha is you can recover RGB values in your transparent areas just by filling in the alpha. Or you can use the alpha for some completely different purpose like the guys above were trying to do.

 

The advantage of premultiplied alpha is if you have RGB values in an area with 0.0 alpha, this composites as a glow or a light ray—a purely additive element that doesn't block the background. If your 3D renderer can create glows, this is how they'll probably turn out, and you can't composite them properly in a program like After Effects without resorting to some tricks.

 

OpenEXR has some cool samples of non-zero RGB over zero Alpha. Try importing them into After Effects and then try setting alpha interpretation to Ignore.

 

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