conversion/cineon.html

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<h1>Cineon files</h1>

In film production negative is usually scanned to the Cineon file
format designed by Kodak.

<p>Kodak documents the Cineon file quite carefully. Each number in the
file is supposed to represent .002 times the base-10 logarithim of the
density of the film. They also document that the negative's density is
proportional to the amount of light that struck the film raised to a
power called the "gamma" of the film, which is about .6. There is also
a "base" density that is added to this value (unexposed film does not
have zero density) but normally the Cineon scanner or software is
adjusted to subtract this during scanning. There is also very
non-proportional roll-off sections of the transfer function at the top
and bottom, but I will ignore them here, it turns out that you can't
do much better with them anyway.

<p>Therefore you should be able to recreate the original light levels
by converting each number x in the file with
pow(10.0,(x-W)*.002/gamma). In this function W is the "whitepoint"
which is the value from the file that you want to turn into 1.0 on the
screen. Kodak also recommends a value of 685 for this. Since zero
exposure cannot be represented, the lowest value in the file turns
into a non-zero value.

<p>Lots of software tries to convert straight to sRGB by a power
function. This cannot match, as the slope at the low end is zero,
while the slope of the exposure that produced the Cineon file is
always positive:

<p><img src=cineon.png>
<br><i>Green: "correct" Cineon conversion followed by sRGB:</i>
to_sRGB(pow(10,(x-685)*.002/.6))
<br><i>Red: typical conversion:</i> pow(x/685,1/.6)

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Here is a comparison of a well-known sample image converted by
these two methods:

<br><img src=pics/kodaksrgb.jpg name=image>

<br>
<input type=radio onClick="image.src = 'pics/kodaksrgb.jpg'"
checked>"Correct" Cineon conversion
<input type=radio onClick="image.src = 'pics/kodaklinear.jpg'">Typical conversion

<p>Although the typical conversion looks better due to the increased
contrast, the correct conversion is a much more accurate
representation of the original scene luminance. Notice that dark
parts of the background are more visible and the hair detail is clearer.

<p><a href=index.html>Go back</a>

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