DPX transfer characteristics code 3 (logarithm)

Does any one know the formula for DPX transfer characteristics code = 3?
There’s a cryptic message in the SMPTE 268 that defers to I23 subgroup for a definition.

Is it this log10 function implied by this CTL script ?

I am trying to decypher the signal container used in these HDR clips captured from Sony F55 & F65:

http://medialab.sjtu.edu.cn/HDR/

The “Camp Fire" DPX files are here (33 Mbytes each).

ftp://202.120.39.226/dpx

Regards,

–Chad

Unfortunately you cannot reliably infer anything from the DPX Transfer metadata. Frequently the data in there may be wrong, and the code 3, meaning “log”, if used accurately at all, just gets used to indicate that one of many possible log transfer functions has been used.

The DPX spec was defined back when the only common log format was Cineon log from film scans, and it does not have the flexibility to indicate whether the log transfer function used is Cineon, LogC, S-Log3 etc. Accurate notes, or other “human” communication is required!

I agree, it is hard to tell what the intent of the party writing this file from that field.
In general, for scans, 0 is often used. 1 was intended for Cineon style curves (based on a
printing density for film). 3 was intended for camera log formats (Viper, et.al.), However the
committee referenced in the document never came to a consensus on this subject so all that you can
say is that the file is logarithmic (with a toe or not is unknown ). It is a problem to put things in standards
that are incomplete.

There is currently a SMPTE Constrained DPX standards group working on putting HDR encodings at the end of the table (PQ,etc…)

Because you know the source, I would look at these files with an assumption that they are SLOG3.
(though SLOG2 is possible)

Jim

Hi, bumped into this thread while researching about DPX transfer functions; I need to properly interpret 10bit DPX scan files of old 8mm film from the 50s/60s/70s, within a ACES color managed workflow; I’m unsure of how interpret them, as I can’t reliably tell by eye; I managed to print the DPX metadata with ExifTool, and the transfer function is code 1: Printing density; if this is indeed a log-type function, should I interpret them as log? Using OpenColorIO/After Effects, manually managing color, applying the Cineon Converter effect and setting it “log to log” makes no effect; which makes me think they are indeed log DPX; log to linear and linear to log make both visibily wrong results; problem is I need to adapt them into a linear color environment as I need to do some compositing work; any suggestion? Is the Printing density transfer function in the metadata a reliable indicator for the DPX to be log?

Thanks!

Can you expand on this?
How are you viewing the data once it’s in it’s linear state?