Do i need my .DPX files as linear or Log

[Darby Edelen]

I don’t know if I’ll be able to help much, as color spaces are one area that I’d definitely like to improve in myself.

My understanding of the log vs. linear issue is a little underdeveloped, but essentially film negatives have a much higher dynamic range than video. If we imagine black in video as having a value of 0 and white as having a value of 1 then film can store values at least as bright as 13. In addition to this, film negatives have a logarithmic response curve to light. This basically means that film is much more sensitive to differences in light in the 0 to 1 range and much less sensitive, say, in the 12-13 range (asymptotes anyone?).

Cineon (and .DPX files as I understand it) are 10-bit color and can store 1024 unique values per channel (2^10). If these 1024 values were distributed linearly you would have about 75 values for the 0 to 1 (video) range and 75 values for the 12 to 13 (overbright) range. This would not be ideal. Film is not as sensitive to this overbright range and our eyes are not as sensitive to this overbright range. So the distribution of color values in Cineon (and .DPX) follows a logarithmic curve as well: giving the darker 0 to 1 range more data than the overbright ranges.

This is a good thing as it more faithfully represents the native color of the film negative. However, you should never composite with a log file as the operations that we are accustomed to (blurring, opacity, blending modes) will behave much differently in log color space. It is ideal to composite in a linear color space. This is where my understanding begins to break down some.

A linear color space is one in which the gamma is 1.0. Compositing done in normal video space (where the gamma isn’t 1.0, more like 2.2) is actually not representing color values the way that film would.

There’s a lot of information on this phenomenon out there, most of which I haven’t read and so I feel uncomfortable getting too deep into it here. Most of what I know comes from Mark Christiansen’s excellent “After Effects 7.0: Studio Techniques” in the chapter entitled “Film, HDR, and 32 Bit Compositing,” but even that is a primer on the subject.

Without being able to tell you the precise workflow, I believe the way you’d want to approach this is as follows:

Have them provide log .DPX files (to properly preserve the film negative’s color). Convert these to linear color space. Composite.

And that’s as much as I can say on the subject honestly, I wish I could give you more specific details but I haven’t been through this process myself. This is a topic I’m very interested in exploring, but I’ve barely begun to.

I hope more people have input on the subject as I’m interested in learning as well.

[Andrew Shanks]

Yes, get them as LOG space, to preserve the dynamic range, if you get them to scan the footage and provide it as linear you can end up with having all sorts of problems in regards to a greenscreen shot (it ranges from the screen being over or under exposed, to the helpful people that scanned the film doing a bit of color correction before saving the clip, …not great when you want pure well exposed green for the key).

So yes, you get a log dpx sequence, you then will convert that to linear within after effects.
The beauty of doing it this way is that you can drop it straight into float colorspace (meaning you have more dynamic range to play with compared to a 16bit (or worse, an 8bit) linear scan they might have provided you), and if it is a problem key, you can do custom log-linear conversions to build up a better exposed plate for pulling the key, then another more straight linear conversion (a beauty version if you will) to which the matte we have just pulled can be applied to. I am guessing with the fact this is going out to digibeta, that it will be getting a full grade after its cut back into the sequence, but it might pay to see if you can get a reference frame from clips either side of your effects shot(s), as converted for the edit, just to make sure your colors match on a basic level for when your fx shot is cut back into the scene.

Goodluck,

andrew

[Darby Edelen]

[Andrew Shanks] “if it is a problem key, you can do custom log-linear conversions to build up a better exposed plate for pulling the key”

Teach me O master…

=)

[Chris Huggett]

Dammmmmm

The film has now been sent away and is being processed as we speak into Linear space. I have been trying to find info on this subject all day, i was unsuccesful, and because i had read about AE7s new linear colour space, i thought this would be the best way, so i told the business to do it Linear…..

I feel quite sick now, coz it costs alot of money to convert it. Having never done this before, theres alot of pressure on me using the files. Will they still easy to work with? What should i do when i recieve them?

THanks for all your input.
Chris