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Future-proof still image format – follow-up questions
Hi there, excuse me for returning to the subject of archiving for future AE – we’re about to light the blue touchpaper on a major archiving effort with all the attendant costs that entails so I hope to get the best info from the best forums before we commit – my follow-up questions are:
– If saving greenscreen footage to openEXR with Luminance/Chroma checked (to gain smaller storage sizes), will the resultant files be handicapped for keying with Keylight v*.* in AE? We see a slight softening that way but not sure right now if it’s a problem or an improvement.
– Is there a prevailing expectation about backwards/forwards compatibility for openEXR as regards AE? We’ve had zero problems so far but find on the web occasional mentions of compatibility changes as the format moved to version 2. I’ve not been able to get definitive info from ILM. My crystal-ball question is: will material archived in today’s version of openEXR be recoverable five years down the road, especially in AE, despite the format presumably having moved through versions 3, 4, 5 and beyond.
All opinions gratefully received.
Best,
Stephen NorringtonArtproject Independent Production
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4 Replies
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Hi Dave, thanks for the response, we ran a bunch of tests and were unable to determine exactly what changes were made to the material with the Luminance/Chroma save setting checked.
There is some change, some kind of compression (the file size drops by a third) but, oddly, it actually seemed to improve keying of source material that was compressed when originated. This all seems too good to be true but we’ve not been able to glean exactly what the Luminance/Chroma save setting does under-the-hood from ILM’s documentation.
The fear is that saving with the Luminance/Chroma setting enabled may merge parts of the image in a way that will come back to bite us in future iterations of AE and Keylight.
Best,
Stephen NorringtonArtproject Independent Production
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Ah yeah, nice one, we didn’t do that – the visual differences between original and saved are noticeable when A-B’d but fortunately minor – the greater concern is that whatever is being done to the file to drop its size by a third might result in a tech-gotcha somewhere in the future.
For example, I’ve heard dark mutterings that the Luminance/Chroma save setting can’t be read by Nuke which doesn’t matter to us here because we don’t use Nuke. But I can imagine a gotcha like that turning up in future versions of Keylight and AE and our expensive openEXR production archive will then be a headache (we’re engaged in a long-term production, ten years or more – footage shot today won’t be edited for perhaps a decade) – SN
Artproject Independent Production
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openexr is almost, almost there. unfortunately, even though it makes small files, if you do extreme tests and compare the rgb pixels, some data actually gets changed at the single digits. The only 2 I have ever tested that are mathematically perfect are Tiff with LZW turned off set to trillions of color and psd to trillions. but Tiff was created for archive in mind and is cross platform compatible.
if you don’t mind the small change in data, openexr is much more practical for file size and many people use it. and you’re right, there’s different compression settings in openexr.
the nice thing is that it is hardware and operating system independent and open source which is a plus down the road.
you should read the section on Managing un-premultiplied color channels. It recommends premultiplied mattes.
https://www.openexr.com/TechnicalIntroduction.pdf -
Hi Chris, thanks for that, yeah, we see some very slight disturbances, especially with the Luminance/Chroma switch enabled.
We’ve decided to go with 16bit PNG – openEXR is a sledgehammer to crack a nut for our archiving needs and the (slightly) open questions about its future compatibility give us pause – there seems little doubt that PNG will persist unchanged and consistently supported for the foreseeable future and its compressed file sizes are comparable to openEXR’s best efforts, in some (solid-areas) cases squeezing down a 400mb file to 500k while staying completely lossless, at least visually – a neat trick 🙂 – best, SN
Artproject Independent Production
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