LOG DATA

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

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