Comparison of CC workflow’s: Film Log vs. Linear REC709

[Nick Shaw]

REC709 is a defined standard log curve for broadcast display.

I know what you are saying, and I know I'm being a nit-picker, but Rec709 is not a log curve. It is a power law curve.

The purpose is similar to log, but there are important differences. One of the most important ones is that each stop of exposure maps to the same number of code values on a log scale. This is 90 10-bit values for the standard Cineon curve. This means that to push a log file one stop you just add 90 to all the values, and to pull it one stop you subtract 90. This is not possible in the same way with Rec709 image data.

 

[Evangelos Achillopoulos]

Ok, so we are in a total agreement, lets straighten it up...

What is actually preferable is to use captured image data that is over-range and wide gamut, and use a non-destructive display LUT that limits the data range for display on a known standard monitor (Rec709 or P3 in all likelihood).

This then leaves the final image data as over-range and wide gamut (no clipping) such that any desired deliverable can then be generated as required.

Choosing to use a LUT with roll-off is a personal choice, but can just as easily be done as a creative process as part of the grading, rather than as being fixed within a LUT.

So, to sum up, you need a workflow that is non-destructive to the captured data - nothing more - and the correct data conversion LUTs to the desired final deliverables.

ALL this thread is about exactly that... non destructive colorgrading using viewing LUTs and conversion LUTs...!!! so please, if you like, go to the first page of this thread and realize what I'm trying to explain...

But, the biggest single confusion is saying using film LUTs is preferable...

...And this is probably the biggest issue with saying a film LUT is preferable - film profile LUTs have a toe compression region as well as shoulder roll-off, and this is actually not desirable for digital deliverables as it reduces shadow detail drastically...

This is all about aesthetics... If you read carefully the posts I'm explaining that it can be used a Film targeted viewing LUT if you want the character of a film stock OR an artificial target like Cineon 6500K to get the max, with no difficult grading but characterless...

Character is synonymous to beautiful imperfections... and I do propose the use of it because 95% of my partners (DI houses, small posts), clients (DoPs, Directors etc..) are preferring it... Since all the process is non destructive if you don't like it you can change it...


If you have a comment on the first post and we agree upon it, I will gladly add it...

 

[Graeme Nattress]

REC709 is a defined standard log curve for broadcast display. The target values of the color bar and gray scale reference test signals define a reference series of log visual values converted to linear voltage steps in the video signal. Regardless of work flow, if you are grading on a broadcast reference monitor, you are grading to this log curve.
Reference monitors have to match the color space of your delivery format.
The log value range of REC709 is less than 7 stops max.
Theoretical maximum film projection value range in a totally dark theater is about 9 stops. But most theaters don't come close to this.

As mentioned, REC709 is not a log curve. It's a modified power-law curve. It's also a monitor referred standard and tells you nothing about how the tonal values from a camera should be encoded for display, just what the display will do to those values. If you take camera data and encode with the REC709 display curve you'll find that the image you see is rather sub-optimal.

Graeme

 

[Nick Shaw]

It's also a monitor referred standard and tells you nothing about how the tonal values from a camera should be encoded for display, just what the display will do to those values. If you take camera data and encode with the REC709 display curve you'll find that the image you see is rather sub-optimal.

And more than that, not only do no cameras encode image data according to the Rec709 curve, no monitor displays image data according to it either. That is why grading software manufacturers often make their own display profiles that match what a "normal" broadcast monitor looks like. For example Truelight has a built in display profile called "Sony HD". If you calibrate your monitor to the Rec709 transfer function, what you see will not be representative of what end users (unless they have calibrated their monitors in the same way) will be seeing.

 

[Graeme Nattress]

And more than that, not only do no cameras encode image data according to the Rec709 curve, no monitor displays image data according to it either. That is why grading software manufacturers often make their own display profiles that match what a "normal" broadcast monitor looks like. For example Truelight has a built in display profile called "Sony HD". If you calibrate your monitor to the Rec709 transfer function, what you see will not be representative of what end users (unless they have calibrated their monitors in the same way) will be seeing.

Agreed. And indeed the system relies on the curve of the data being different from the display curve to add some contrast to the image too!

Graeme

 

[Evangelos Achillopoulos]

Agreed. And indeed the system relies on the curve of the data being different from the display curve to add some contrast to the image too!

Graeme

That's why I prefer to grade for a specific target other than REC709 relaying on my profiled calibrated monitor and then convert to it with LUTs... bypassing the problem...

Let alone what happens to TVs when the image is broadcasted... analogue... madness...

 

[M Most]

T

Let alone what happens to TVs when the image is broadcasted... analogue... madness...

There is no analog broadcast in the US any more. The digital broadcast transition was completed in 2009. I'm not aware of other territories doing the same thing yet, but in the US, analog TV broadcast does not exist.

 

[Evangelos Achillopoulos]

There is no analog broadcast in the US any more. The digital broadcast transition was completed in 2009. I'm not aware of other territories doing the same thing yet, but in the US, analog TV broadcast does not exist.

In the rest of the world is not... so madness... We do though have digital Mpeg4 DTV in Greece but not all TV stations... we expecting to have only DTV by 2015... If we are still up and running by then...

 

[Luis Otero]

You never know where your work will be broadcasted. I, for one, do not restrain my job to be US-Centric, so this is importand to many of us.

 

[Evangelos Achillopoulos]

Nothing specific to say... just that I was a bit busy lately and many PMs I didn’t get them on time...

Still around and helping with colour management and LUTs guys from around the world...

Instead of PMs better to send me emails at lakis@motionfx.gr

Keep up shooting beautiful images!!