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13.8.8.1 ASC CDL Transforms
The American Society of Cinematographers has specified a set of transforms that
have become a standard in postproduction. They are determined by slope, offset,
power and saturation (applied in that order).
All these parameters are based on simple color manipulations (multiplying with a
factor, adding an offset, raising to an exponent). Noted as "Color Decision List" (ASC
CDL), they offer an exchange format for basic look transformations between color
correction systems and editing tools by different manufacturers.
The ALEXA Mini image processing applies ASC CDL transforms to the Log C
encoded image. This allows manipulations such as exposure correction or bringing
down highlights, before the picture is converted to the display color space with its
steeper contrast curve.
Slope
The linear section of the Log C curve is equivalent to the gamma of a negative film
stock. The Log C curve has a default gamma of approximately 0.51 (adjustable via
slope parameter).
A slope value of 1.2 will have a similar effect as using negative stock with a gamma of
0.6 (= 1.2 * 0.5). A value below 1.0 will lower the gamma accordingly.
Offset
This most intuitive CDL parameter has a similar effect as increasing the exposure
index on the camera. If you are familiar with the motion picture print film process, it's
the same as printer lights.
Power
Via power you can adjust the mid tones, similar to the Gamma parameter in video
color grading. A power value below 1.0 will increase, a value above 1.0 willl decrease
the mid tone brightness.
(Log) Saturation
This parameter affects the saturation of all color components in the Log domain. A
value of 1.0 represents the default saturation.
13.8.8.2 3D LUT and video look parameters
Via 3D LUT (lookup table), the camera generates a Rec 709 video image from Log
C data. Next to the ASC CDL parameters, which apply to the Log C image, the
camera also offers a set of video look parameters (VLP) for tone mapping and color
transformation of the rendering 3D LUT.
VLPs determine knee, black gamma and the gamma of the tone map curve, which
qualify the contrast of the output image. They also include values for saturation and
the saturation by hue for six color vectors (green, yellow, red, magenta, blue, cyan).
Whatever the adjustments, the underlying 3D LUT always converts Log C to Rec 709
color space.
Knee
The knee parameter controls the transition of mid-tones into highlights. Values below
0.5 (default) produce harder highlights, higher values soften them. Knee is applied to
all channels equally (master control). It has no effect on the mid gray level.
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