Eyeon:Manual/Fusion 6/Cineon Log
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Cineon Log [Log] | |
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The Cineon Log Tool is used to convert image data from logarithmic to linear. Use this tool the log-lin conversion was bypassed in a Cineon Loader to return processing to Linear. For additional details on the Cineon Log file format, consult the chapter entitled Film In Fusion. |
Contents |
Controls
These buttons offer two options, one for converting log images to linear and one for converting linear images to logarithmic.
When selected, the settings in this tab will affect all of the color channels equally. De-select this control to convert the Red, Green and Blue channels of the image using separate settings for each channel.
Use this control to set the black point and white point in the log image before converting. Pixels with values in log space below the Black Level will become out of range values below 0.0. Pixels with values above the White Level will become out of range values above 1.0 after conversion.
When processing in floating point color space, the out of range values stored in the Cineon format are preserved through the conversion. In 16 bit or 8 bit mode the out of range values are clipped.
The Soft Clip control is used to draw values that are out of range back into the image. This is done by smoothing the conversion curve at the top and bottom of the curve, allowing more values to be represented.
Applying a soft clip of any value other than one will immediately cause the tool to process at 16 bit integer, eliminating all out of range values that do not fit within the soft clip.
These controls are used to set the response curves of the logarithmic data during conversion. In addition to the settings above, a custom ASCII file Look Up Table can be created with specific conversion values. The ASCII LUT file can be loaded using the file folder icon button.
A description of the ASCII LUT format can be found in the appendices.
Tips for Cineon Log (edit)
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 |
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