[Openexr-devel] Technical Introduction update ?

[Richard Hadsell]

There has been discussion on the [nuke-prerelease] e-mail list about DWAA and DWAB compression with OpenExr 2.2.  I checked the OpenExr Technical Introduction, but was disappointed to see that it has not been updated since November 2013.  Is there any chance that someone is working on an update?

It is not difficult to use DWAA and DWAB compressions, which are available in the compression header.  That is not the problem.  The questions that I would like to see answered pertain more to lossy vs. lossless compression.  The [nuke-prerelease] discussion included interesting information like this:

On Thu, Mar 17, 2016 at 1:03 PM, Jonathan Egstad <address@hidden> wrote:
...
> 3. Does the lossyness cause issues with data aov's, motion vectors, uv, position, normals, etc...?

Yup - it will destroy data AOVs like normals, positions, etc.  I believe the codec recognizes typical color-channel names like 'r/R/red/RED', etc and only lossy compresses those.  Channels it doesn't recognize will get lossless compressed with I believe ZIP - alpha for example is lossless compressed.

One gotcha is I think it traps 'Y/y, u/U, v/V' as valid color channels and lossy-compresses those, which is a problem for xyz and uv channels...

This is how our internal codec works so you might want to check that the one in the official OpenEXR2.2+ release has these behaviors.

...
(end quote)

It would be great to see a list of those special channel names.  Are they the only channels that are compressed with DWAA/DWAB?  What compression would be applied to the other channels?  Does this behavior apply to all lossy compressions?

I hope to see answers to these questions in an updated Technical Intro (eventually).

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