Digital Film

Digital Film
If a monitor’s maximum brightness is considered to be 1.0,
the brightest value fi lm can represent is offi cially considered
by Kodak to be 13.53 (although using the more effi -
cient ICC color conversion, outlined later in the chapter,
reveals brightness values above 70). Note this only applies
to a fi lm negative that is exposed by light in the world as
opposed to a fi lm positive, which is limited by the brightness
of a projector bulb, and is therefore not really considered
high dynamic range. A Telecine captures the entire
range of each frame and stores the frames as a sequence of
10-bit Cineon fi les. Those extra two bits mean that Cineon
pixel values can range from 0 to 1023 instead of the 0 to
255 in 8-bit fi les.

Having four times as many values to work with in a Cineon
fi le helps, but considering you have 13.53 times the range
to record, care must be taken in encoding those values.
The most obvious way to store all that light would simply
be to evenly squeeze 0.0 to 13.53 into the 0 to 1023 range.
The problem with this solution is that it would only leave
75 code values for the all-important 0.0 to 1.0 range, the
same as allocated to the range 10.0 to 11.0, which you are
far less interested in representing with much accuracy.
Your eye can barely tell the difference between two highlights
that bright—it certainly doesn’t need 75 brightness
variations between them.
A proper way to encode light on fi lm would quickly fi ll up
the usable values with the most important 0.0 to 1.0 light
and then leave space left over for the rest of the negative’s
range. Fortunately, the fi lm negative itself with its logarithmic
response behaves just this way.
Cineon fi les are often said to be stored in log color space.
Actually it is the negative that uses a log response curve
and the fi le is simply storing the negative’s density at each
pixel. In any case,  describes how
light exposes a negative and is encoded into Cineon color
values according to Kodak, creators of the format.

One strange feature in this graph is that black is mapped
to code value 95 instead of 0. Not only does the Cineon
fi le store whiter-than-white (overbright) values, it also has
some blacker-than-black information. This is mirrored in
the fi lm lab when a negative is printed brighter than usual
and the blacker-than-black information can reveal itself.
Likewise, negatives can be printed darker and take advantage
of overbright detail. The standard value mapped to
monitor white is 685, and everything above is considered
overbright.
Although the Kodak formulas are commonly used to
transform log images for compositing, other methods have
emerged. The idea of having light values below 0.0 is dubious
at best, and many take issue with the idea that a single
curve can describe all fi lm stocks, cameras, and shooting
environments. As a different approach, some visual effects
facilities take care to photograph well-defi ned photographic
charts and use the resultant fi lm to build custom
curves that differ subtly from the standard Kodak one.
As much as Cineon log is a great way to encode light
captured by fi lm, it should not be used for compositing or
other image transformations. This point is so important
that it just has to be emphasized again:
Encoding color spaces are not compositing color spaces.
To illustrate this point, imagine you had a black pixel with
Cineon value 95 next to an extremely bright pixel with
Cineon’s highest code value, 1023. If these two pixels were
blended together (say, if the image was being blurred), the
result would be 559, which is somewhere around middle
gray (0.37 to be precise). But when you consider that the
extremely bright pixel has a relative brightness of 13.5, that
black pixel should only have been able to bring it down
to 6.75, which is still overbright white! Log space’s extra
emphasis on darker values causes standard image processing
operations to give them extra weight, leading to an
overall unpleasant and inaccurate darkening of the image.
So, fi nal warning: If you’re working with a log source, don’t
do image processing in log space!