Bob Friesenhahn wrote:
Kai-Uwe's experiments are with image data which is log encoded to
represent a density range of up to (typically) 2.048. This is a common
representation for motion picture film images.
Up until now, it seems that the film industry has been soundly ignoring
ICC CMS and has instead implemented many proprietary methods for
performing color adjustments on film images. This is in spite of the
fact that color adjustments (a.k.a "grading") are done for every film
and it plays a significant role in the production of each movie. The
necessary color adjustments may be different for a different reel of
film, different scene., etc.
Accurately preserving/presenting "above white" information is quite
important to the film people since film itself is able to support it and
people can see the difference.
My impression is that the ICC profile format has been optimized for
rendered (e.g. output reference) spaces. I suspect that trying to create
a general profile (e.g. using a Lut based representation) of a high
dynamic range space, with sufficient detail to be useful,
would be very challenging. On the other hand, a strictly additive HDR
space should be reasonably well be handled by a matrix and a set of curves.
I also get the impression that HDR mapping (or rendering, which would
obviously include artistic and practical input such as white point
matching ("color grading") etc.) hasn't been as much studied in the context
of device independent color spaces, as it has from a device dependent
(ie. RGB) spaces. This isn't much of a surprise since the main impetus
seems to be from the computer graphics side, rather than the color science side.
So a practical HDR profile may well fit within the current ICC profile format,
while a standardized HDR mapping mechanism (or framework) to work with it,
doesn't seem to be readily available at the moment.
An enhanced "rendered" PCS would be interesting (e.g. a PCS allowing
a compressed highlight representation), as would matching device
colorspaces, both allowing higher fidelity image transition for
purposes such as theatre projection, or perhaps a 2 stage HDR rendering scheme.
(the first stage being the major, artistic driven one, the second,
minor one being for highlights to be rendered to a particular output
medium.)
An alternative (which may be cleaner, in separating the profile from the
image representation), would be a device independent space annotation scheme.
If raw HDR image data is stored in a device independent colorspace,
then annotation including white and black points, gamut extent, rendering
curve (+ possibly other effects) would allow a downstream rendering
into a specific device space to be guided as to what artistic result
to aim for.
Graeme Gill.
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