Re: [Openexr-devel] Interpreting Deep

[Larry Gritz]

Very helpful document, Florian, thanks.

I have some questions about the meanings of deep images (mostly from the point 
of view of writing software to manipulate them).

Is the One True Accepted definition of the color and alpha channels that they 
are always the premultiplied, accumulated (that is, pre-composited?) values at 
the depth of each sample?  So we never have to worry about deep images that 
have channels whose values are the "local" contributions (rather than the 
cumulative amounts)?

If so, how can you represent samples "behind" an opaque object?  Or can't you? 
Does the spirit of deep OpenEXR allow samples with a Z that is greater than the 
point where alpha == 1?

For the "flatten" operation, should the flattened Z be the depth at which alpha 
became opaque? That kind of makes sense, in that it would keep the Z paired 
with its alpha at that depth; but on the other hand, that means that a 
flattened deep file would in general not end up with a Z channel that would 
match, say, a traditional render Z output, which usually registers the closest 
hit regardless of opacity.

On p. 2 of Florian's document, it says "Every deep OpenEXR image must contain 
either a single alpha channel, A, or three alpha channels RA, GA, BA."  Are we 
to take this literally, that it is not considered valid to have a deep OpenEXR 
that doesn't contain alpha, or whose channels are not given these precise 
names?  (Example, it will *always* be "RA", and *never* "opacity.R"?)


On Sep 24, 2013, at 3:10 PM, Florian Kainz wrote:

> 
> To the more mathematically inclined OpenEXR users and developers:
> 
> If you can spare the time, I would like you to read the attached
> document and give me feedback on it.
> 
> The IlmImf library defines the file format for deep images and it
> provides convenient methods for writing deep image files.  However,
> the library and the existing documentation (as of September 2013)
> do not explain in detail how deep images are meant to be interpreted.
> 
> The attached document attempts to describe what the deep data in a file
> mean, and how compositing of deep images works.  The document also points
> out numerical issues with the representation of volumetric samples, and
> proposes an alternate volumetric sample representation that would address
> those issues.
> 
> I believe that agreement on the interpretation of deep OpenEXR files
> is desirable because it enables compatibility among different vendors'
> image compositing applications.  In addition, anyone developing algorithms
> such as lossy compression would be able to rely on a standardized
> interpretation of a file's contents.
> 
> Florian
> <Deep Image Data 09-24-13.pdf>_______________________________________________
> Openexr-devel mailing list
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--
Larry Gritz
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