Rec709, practical definition

[Thomas Singh]

I'm struggling a bit to understand what is the practical definition of rec709. When starting from log, how much change is enough before you can call it rec709? In most tutorials, the tutor defines the image rec709 as soon as an random s-curve or a normalization LUT is applied. What is correct? When can you define the image as rec709?

[Nikola Stefanovic]

You don't need to do anything to image to call it rec 709, just flag it in metadata like rec 709

[Thomas Singh]

Ok, so why is log to rec709 used as a phrase all over the place?

[Bruno Mansi]

Rec709 was set up by the ITU over 25 years ago as a set of standards for HDTV.

The part of the Rec709 standard I think you're referring to is the 709 gamut, which defines the colour space of the RGB primaries as well as gamma curves, although these curves were really designed for CRT displays. I think the newer BT.1886 standards have modified the gamma curves somewhat, to suite modern display technologies.

As the name implies, cameras which record log images are just applying a logarithmic curve to better retain the low & high level detail. A log to 709 transformation simply applies the necessary corrections to bring the image back to a 'normalised' image as defined by the 709 standard.

[Thomas Singh]

That tells me the log to rec709 translations are a highly inaccurate guideline. It's not exactly an rec709 image after applying i.e an rec709 Lut or normalizing the image with an s-curve. It is simply just stretching the signal to get closer to what is expected from the rec709 standard. Is that right?

[Nikola Stefanovic]

I think you overthinking this

Look at rec 709 as a viewing conditions and delivery guideline. LogC or any other log flavour is on acquisition side and still to be interpreted  in rec 709 (or P3 or rec 2020) environment. How you going to interpret it its completely up to you as long as you and your clients are happy with the result.

[Bruno Mansi]

I'm by no means an expert in all this colour science, and I'm sure there's others on this forum who'll give you a more informed break-down of what's going on with log to 709 conversions.

Rec709 is a standard in terms of the range of colours that are permissible within the prescribed gamut. It's a standard that was chosen for the reason that it would be achievable by the majority of the display technologies that existed at the time. If you use a standard digital camera that records in Rec709, then you would expect that with grading/editing software that is setup as a Rec709 project, your camera rushes would look roughly the way you'd expect, and that gamma and colours should be roughly within spec. What you do with the image after that is up to you, as long as you stay within the limits defined by the standard, otherwise you'll end up with illegal levels/colours etc.

With high-end cameras that record wide colour gamuts and log levels, you'll need some kind of transform (or LUT) to get it to fit into the Rec709 colour space. How that's done is open to some interpretation, but the camera manufacturers provide their own conversion LUTs to best utilise the abilities of the camera and produce the most pleasing image. Sometimes they may tweak these LUTs to (say) push a little more green into flesh tones or play around with highlight/lowlights to give the sort of look that people often associate with Arri or Red cameras. I don't think they would regard these as highly inaccurate, as I'm sure they've put a fair amount of effort into their design, but rather subtle changes to a purely technical LUT in an attempt to give the end user a choice in the starting point for their images before any further grading is done.