EyeonTips:Manual/Fusion 6/Cineon Log
From VFXPedia
Black Rolloff
Since a mathematical log() operation on a value of zero or lower will result in invalid values, Fusion will clip values below 1e-38 (0 followed by 38 zeros) to 0 to ensure correct results. This is almost never an issue, since values that small have no visual impact on an image. To see such tiny values you would have to add three brightness contrast tools, each with a gain set to 1,000,000. Even then the values would hover very close to zero.
We have seen processes where instead of cropping these very small values, they are instead scaled. So values between 0.0 and 1e-16 are scaled be between 1e-18 and 1e-16. The idea is to crush the majority of the visual range in a float image into values very near to zero, then expand them again, forcing a gentle ramp to produce a small ramp in the extreme black values. Should you find yourself facing a color pipeline using the process, here is how you can mimic it with the help of a custom tool.
The process involves converting the log image to linear with a very small gamma and a wider than normal black level to white level. (e.g. conversion gamma of 0.6, black of 10, white of 1010) This will crush most of the images range into very very small values. This is followed by a custom tool (described below), then by a linear to log conversion that reverses the process, but uses a slightly higher black level. The difference between the black levels defines the falloff range.
Since this will lift the blacks, the image is usually then converted back to linear one more time, using more traditional values (i.e. 95-685) to reset the black point.
The custom tool should use the following equation in the Red, Green and Blue expressions.
if (c1< 1e-16, 1e-18 + (c1/1e-16)*(1e-16 - 1e-18), c1)
 The falloff from the native Fusion process |
 The falloff from the ramped clipping process |
