[Nick Wood]

One for the road…

For the sake of my sanity I’ll note that I’ve hopefully found the answer to my remaining confusion.

The problem I was having was a lack of clarity regards BT.709 and sRGB with regard to OETF vs EOTF. I read a lot about sRGB vs BT.1886 and a lot about BT.709 vs BT.1886. What I couldn’t grasp is that, with the gamma compression defined in BT.709 and the subsequant expansion standardized in BT.1886, what on earth is sRGB expanding? Well, is it that both opto-electrical and electro-optical transfer functions are in the sRGB spec!? Hopefully it’s that obvious, but for me it’s really the missing piece of the puzzle.

A couple of articles that I found interesting along the way:

https://www.colour-science.org/posts/the-importance-of-terminology-and-srgb-uncertainty/
https://bartwronski.com/2016/09/01/dynamic-range-and-evs/
https://www.glowybits.com/blog/2017/01/04/ifl_iss_hdr_2/

I’ll bog off now. Thanks Andrew for the time taken to put such great responses together.

[Nick Wood]

[Andrew Somers] “I think you had the right idea in paragraph two.”
“Wowzers”.

My confusing comment was largely a muddled take on your explanation that using the 2.4 curve in the output module is incorrect, given that the expectation of BT.1886 is a BT.709 transfer function. In fact, there is something fundamental about the relationship between gamma compression and subsequent expansion that I’m still trying to grasp. Is it that both BT.1886 and sRGB expect the same gamma encoding, but overcompensate to varying degrees due to different viewing conditions? Both curves “bend” the compressed values back out, but slightly past their original scene linear state?

[Andrew Somers]… rabbit hole …

Correct, but it’s surprising how fascinating the journey has become the further down I’ve fallen. It also strikes me that, for something as important as controlling the exhibition of colours should be so easy to ignore/ misunderstand/ etc. I don’t think many would first pick up AE thinking “Gamma, what fun…”, compared with those who are enticed by top-line motion graphics wizardry; though as time passes it becomes increasingly apparent that the former is required at the bottom to support everything ‘wizardry’ further up.

Finally, and only for clarity (on the off chance that someone other than me finds this thread useful):

[Me] “Overall, the big struggle for me has been keeping a handle on the actual coded values that are committed to computer memory at every step in the journey from (for me) initial 3D render to delivery… “

It’s was the journey from acquisition to delivery to which I was referring, not only the final output transform (…as if the last bit wasn’t confusing enough!).

All in all, huge thanks again.

[Nick Wood]

Actually, having thought further about the second paragraph, while the general principle may be correct (final adjustment to coded values before committing to disk), I’m not sure that my example of 2.2/2.4 is at all correct.

Another (another) area that I’m currently trying to separate out is encoding vs display, and I think I might be confusing the two. I fact, isn’t it that the two exponents assume the same footage coming in, only adjusting at the display for different viewing environments?

Sorry if I’m starting to veer off topic. My original aim appeared very similar to this thread’s creator; though I may now be deviating a little.

[Nick Wood]

Hi Andrew,

Thanks for the answers.

A big thing for me is the importance of AE’s footage interpretation rules (Q: “Why is it doing that?” A: Because you’ve not configured it to do differently.”). The exposing of AE config in a somewhat raw form led me to believe that it wasn’t for adjustment by monkeys (me), when even its existence explains that AE isn’t stating “THIS is de facto how things are”, it’s only working to rules that are a user’s responsibility to set.

Another big take away was that there is nothing preserved in the Quicktime file regards colour profile. This was admittedly something of an assumption (“Output profile” at first inspection feels like metadata). So the output profile is actually a final burnt in transform that occurs upon render, is this right? In the case of 2.2/ 2.4 gamma, you could use the output module to adjust the gamma for ‘typical’ HDTV/ sRGB targets?

Overall, the big struggle for me has been keeping a handle on the actual coded values that are committed to computer memory at every step in the journey from (for me) render to delivery, and separating them from any on the fly operations applied by software and display. It’s becoming clearer as I continue to hammer away, but’s proving to be one heck of a journey which more often than not leaves me feeling like a total child.

[Nick Wood]

Long time lurker…

A fantastic post indeed. In fact it’s one of the best on colour in After Effects, but I’m still unclear and I’ve been smashing away at this subject for a while now. My situation is extremely similar to the above, hence the following addition; though my main concern is consistency across multiple generations of post work:

I work primarily with CG image sequences. I have a 32bit linear project, with an sRGB working space. A single sequence of linear openEXRs resides on my only timeline. I make corrections, I create a master video (DNX RGB 444).

Q1. Upon export, the output profile defaults to my sRGB working space. Why does the same file import with profile Rec.709 Gamma 2.4?

I can return the clip to its original perceptual state in AE by either changing the interpreted profile, or by using the Change Colour Profile effect, but is there a reason why the profile is being changed for me? Is it a deliberate correction, or an error?

Q2. What should I expect to see from the same DnX 444 clip on the same workstation inside Adobe Premiere? What about if an export (again DnX 444) from Premiere made its way back to AE?

Q3. On other flavours of DnX, why is it again that the input profiles of such flavours aren’t adjustable? Is it as this page of the Adobe forums suggests?

It’s funny, colour. Two Charles Poynton courses later and I understand less about After Effects than I did when I started. ????

Massive thanks to any readers.