[Tig] LUT file format

Jim Houston jim.houston
Thu May 11 02:24:47 BST 2006


--
Thanks to oktobor for supporting the TIG
--

On May 10, 2006, at 8:52 AM, Rajesh Ramachandran wrote:
>  I really need to understand this file so that I can
> incorporate support for it in our Digital Cinema mastering application.

The file gives you a rough idea of what the images looked like on
their screen (although you need to know what device it was and
in particular what primaries and white point they were using.)   The 
file has both
a desaturation matrix at the top and a 'print curve' that the filmed
images would produce on a print.  The desaturation matrix was likely
figured out by eye to approximate the look of a film clip.  The
print curve was likely measured from an actual print sample that
was using a specific recorder calibration for output to a specific lab.
  Being 3 1D Luts, this is not very satisfactory for producing a film 
looking image, but it
may be the best you can get from them.   You should
be able to create a Digital master that looks the same as what
they saw on their monitor/projector using this data.  However,
it won't look that close to a filmout in my opinion.  State of the
art, today,  is to use a 3Dlut for this part of the imaging chain,
and then to bake the 3Dlut into the data when making the
digital cinema master.

To figure this out correctly you need to work backwards from
what they were seeing and then forwards to what you want
on your own display device.  This can be fairly complicated
and describing it fully is unfortunately beyond what a
concise e-mail can handle.

In a simple fashion, Lustre should be able to bake this data
into the output of their files.  So instead of just getting printing
density, you could get the device output encoding for their
output device.  This makes it an easier problem.

You then just have to put the encoding into a roughly linear
space, apply a color correction matrix from the original device
to the destination device,  apply a white balance correction
matrix to account for white point differences between the original
device and the destination device,  then re-apply the inverse of
the linearizing operation to get back into an output-referred
space.   This output file should then look pretty close on your
destination device (cross your fingers).

I hope this at least provides a trail of bread crumbs out of the
woods.  Good luck.

Jim Houston
Starwatcher Digital
consulting on all things in the digital workflow...





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