LOG DATA

 

Differences between operating on data before and after the LUT

In this example, we use a normal film emulation 3D-LUT taking 10-bit Cineon Log data to P3 RGB. If you operate on the data before the LUT, the color corrector is changing the Cineon Log data which is then put through the LUT to determine how it will display. If you are color correcting after the LUT, you are operating on gamma 1.0/2.6 P3 data and the look on the screen would be whatever the colorist desires with the changes. The controls will act differently since the metric of the data is different. 

Operating on data after the LUT will break the "calibration" 

Also, the look via after the LUT, in this case, will not translate to the film out process since the film emulation “calibration” afforded by the LUT is broken. By film emulation “calibration”, I mean that the image (Cineon Log) going through the 3D-LUT displays the look (exact match of the digital image on a digital projector to the film image on a film projector) that would be apparent if the Cineon Log DPX code values went to a recorder and then printed and displayed on a film projector. 

When grading after this LUT, any changes will be apparent on the screen and if only a digital release is required, that’s fine. However, the Log DPX values did not change and if they are sent to the recorder, the film image would not look like the graded after the LUT image.

From P3 back to Cineon

If, however, the graded P3 image was then put through a P3 to Cineon DPX LUT then the DPX values sent to the recorder will reflect the grading changes. That is why when using film emulation LUT with the anticipation of going to film, the grading is done on the DPX data. 
Any grading after the LUT can also produce colors that are not attainable on film, grading before assures that the color on the screen will match the film out. 

Use the control settings that matches the data type

Both Primary adjustments and Log controls will work on ANY type of data. The only caution is that the RESULTS will be dramatically different. This is the domain of the color corrector.

If it is set up to control lift, gain, and gamma, then it will be most effective on linear images (either “true” linear or gamma linear where the “true” linear data has the inverse of the display gamma applied. This will result in a linear light output on the display. 

If the color corrector is set up to control Log data, then it will be most effective on Log images. The overall brightness of the image is then controlled by simply adding or subtracting a constant on each channel, analogous but not identical to lift. The “gamma” of the image is controlled by a multiplier by the color corrector, since the Log data is already in a gamma metric, multiplying by a constant will change the overall gamma of the image, analogous to the gain control. 

The gamma control operating on a Log data image actually preserves the overall gamma between black and white but repartitions it between the highlights and the shadows differently. Numerically, applying a gamma control greater than 1.0 will actually lower the gamma in the shadows (and darken them) and raise the gamma in the highlights (and brighten then). Gamma values less than 1.0 does the opposite. What we consider normal highlight and shadow control becomes a little different than the lift, gamma, and gain model but fairly easy to implement in hardware or software. 

Bottom line is that it is best to use the control settings of the color corrector that matches the data type of the image. Using other settings may get you to a desirable look, but the colorist must be aware of the consequences. A colorist is an artist and may use any tools available as long as they are fully understood.

 

Mitch Bogdanowicz

 

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Mitch,

Great information, but I need a bit of clarification! 

You have a LUT for your camera original footage (whatever log to lin flavor) and an (Image Transform/LUT) for your grading projector (Lin->P3) and a Cineon LUT for filmout.

Where does the Cineon LUT reside?  Do you actually run the P3 colorspace adjusted image THRU the Cineon LUT to the film recorder, or do you run it in parallel and take the Linear data prior to the P3 Image transform (which should just be a color space adjustment for the projector)? 

Could you do both to grade theatrical and home video releases at once?

Cam orig --> Log to Lin LUT --> Filmstock Emulation LUT --> P3 IT/LUT -- Cineon LUT --> film recorder

or

Cam orig --> Log to Lin LUT --> Filmstock Emulation LUT --> P3 IT/LUT

                                                                                      ^   --> Cineon LUT -- film recorder

Also, could you speak a bit about the film recorder calibration process?

How do you calibrate the recorder to the grading system?  LAD values?

Thanks!

Frank

 

 

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Hi @Frank Wylie

There are a few points to consider in the image chain. First, what is the camera data metric? Is it a standard format such as Rec-709 with an implied tone scale or a manufacturer specific metric i.e., Arri RAW log-C or other standards such as ACES. And in ACES is it AP0 (full ACES, original) or ACEScg which is a reduced gamut, close, but outside of Rec-2020.

Anyway, for most color correctors (not the special effects programs like Nuke etc., which deal mainly at its core in true scene linear data), the data from the camera should be transformed into either some sort of log data or gamma-linear data.

There is a bit of confusion about “linear”. True linear is scene referenced and is intrinsically gamma 1.0. What many people call linear is actually gamma-linear, which means that the inverse of the display device gamma is applied to the data with the hope that after it is displayed, then the output of the display is linear. Actually for perceptual reasons, the gamma of the output of a monitor/projector is somewhere between 1.1 and 1.2 to account for the human perceptual preference.

That brings us to the LUT. One rule of thumb for any reasonable size LUTs < 256^3, is to never push through true linear data because odd artifacts will occur. For example, if a LUT is designed for log Cineon input (by the way, never use a LUT with a data metric that it was not designed for) in 10 bits, it is 0 – 1023. If the output of the LUT is P3 then it would display correctly on a P3 monitor/projector if the image was truly in Cineon Log.

In your 1st image chain, you indicate a Log-lin LUT, actually, you would need a camera color space to Cineon log conversion, then go through the film emulation LUT. If the P3 will then go to a recorder, it must go through a P3 to Cineon log LUT which is the INVERSE of the Cineon film LUT.

LUTs could be built/tailored for other input metrics (except true linear for normal sized LUTs), one could create a LUT that inputs gamma 2.6, or gamma 2.2 etc., in these cases, the tone scale is sufficiently compressed to work in a 3D LUT. But it would be different than a normal Cineon film LUT.

If you desire a film look then the camera data must be converted into a log data or into a gamma data (usually gamma 2.0 – 2.6) Then this data is passed through a 3D film LUT designed for the exact data metric you have. The output of the LUT is usually P3 since it is a straight forward conversion to Rec-709 which is a smaller color space. You also must gracefully remap any out of rec-709 gamut colors from the P3 color space. This allows grading for theatrical release and then in a color consistent way go to home video release. This P3 output could go directly to a projector or if a filmout is needed, then a P3 to Cineon log LUT is used.

If the data is Cineon log, then the color correction is on that metric and the log to P3 LUT is only used for display or P3 deliverable, then the Cineon log data can go directly to the recorder. If it is any other format, gamma etc., then there must be a robust (color wise) transform to Cineon log to go to the recorder or use a P3 to Cineon log 3D LUT.

The film recorder is usually calibrated such that the input code values (Cineon log) are treated as digitized code values representing printing density. Usually code values of 445 RGB on input are mapped to the 18% gray density position of the intermediate film stock. This “LAD” is similar but not equal to the camera film LAD density position. There are Kodak articles online that talk about film recorder calibration, but if they are no longer there or hard to find I could at a later time expand on that.

For an ARRI Alexa output the RAW data from Arri’s software to log-c then apply a normal Cineon log LUT and grade the log data to display in P3.

For Red, output from REDCine-X Pro in red log and probably rec-2020 color space then apply a normal Cineon log LUT and grade the log data to display in P3. For the same scene, both of these will be quite different but then it is the job of the colorist to make the scene look as the project desires. Whichever way, if you grade the log data through the Cineon LUT, the log data could go directly to a recorder.

If your camera’s metric is rec-709, then it must be transformed to true linear, then to Cineon log (or a direct transform) for use with the LUT.

I hope this helps, I did not have a lot of time to respond, many things going on. If there are any other questions / clarifications let me know.

Cheers Mitch

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