[Openexr-devel] Re: exchanging exr's

[Thad Beier]

This proposal, while straightforward, seems fundamentally wrong-minded
to me.

OpenEXR files have linear intensity values in them.  This
is well defined and straightforward.

What a film printer and scanner take as input or create
as output is a function of the scanning or printing process,
but it doesn't seem to me to make sense that it be part of the
image file.

The Cineon file format is perfect for scanning and
printing film, exactly because the information contained in that
file corresponds in a well-defined way to densities on negative
film.  Cineon files are very closely matched to the characteristics
of film, and have very appropriate dynamic range and precision.
The only problem with them for film I/O purposes are that there
is no compression applied.

To me, it would seem that the only extra number needed in the
OpenEXR header would be a scale value, to say how many
footLamberts (or whatever the correct unit is) the value 1.0
corresponds to.  If you want to use OpenEXR files for
input and output, that's fine -- but the responsibility for
conversion between arcane, soon-to-be-archaic units
and linear intensity values should be the responsibility of
the printer/scanner service, and should be transmitted
out-of-band.   OpenEXR files contain linear intensity
values, and that purity of intent is important.

Clearly you would never have a sequence of frames with
different values for these parameters for every frame.   With
the Cineon standard, the description of the relationship between
values in the file and what they physically represent was described
in their Digital Film document, rather than being included in
every frame of every sequence ever exchanged.  I feel that this
is the appropriate mechanism.

Thad Beier
Hammerhead Productions
address@hidden