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Grading DPX Log Files

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Gray Marshall
Grading DPX Log Files
on Jan 2, 2013 at 6:04:48 pm

Hi,

I'm new to Resolve & finish grading, but I come from a long pedigree of VFX color work, going back to when Cineon files had to match back to film in Hazeltine color timing (Pre-DI).

My question is "what is the actual purpose of the Log controls"? Are they there specifically to manipulate Log image files, like Cineon, DPX, Log-C & others? I'm looking for the tools I am familiar with in VFX software which did specific things to a Log image to get it into a working "linear" environment. These included black point, white point, gamma, highlight roll off & offsets (before the gamma was applied). Sure, I can see how to make them work with the 3WCC & nodes, but I'm trying to understand the true/intended use of the Log controls. To read the manual & others sources, they all just seem to treat the Log tools as a form of "Secondary", providing control over narrow linear ranges. How is this Log? Also, I suppose a LUT can be used, but I've noticed that once a LUT is applied, image data is clipped and a generic Log to Lin LUT would not handle the fact that specific film stock/scanner combos treat each color channel in a slightly different way (producing slight color casts to lows, mids & highlights).

Thanks in advance for your answers.

Cheers,
Gray


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Alexis Hurkman
Re: Grading DPX Log Files
on Jan 2, 2013 at 7:40:35 pm

The name of the LOG controls is in my opinion misleading, because they're not specifically for making adjustments to logarithmic media, with the exception of maybe the contrast control, but the S-curve it applies isn't specific to linearizing log material, and you can do better creating your own custom curve.

You can either manually linearize log material using the custom curves, which gives you maximum control at the expense of being a bit time consuming, or you can use some means to create a series of LUTs that are specific to your project, and use these as a starting point (for example, creating a series of LUTs of varying contrast and applying the one that works best for a given scene). If you apply a LUT using a node that's "downstream" in your tree, then you can trim the data using an "upstream" node to account for any unwanted clipping.

If you're doing a LOG to LOG workflow, you can apply a LUT using the Project Settings Look Up Tables panel, either as a Display LUT or as an Output LUT, which will then effect the entire project as the last operation in the image processing pipeline. Display LUTs are never rendered into output, whereas Output LUTs are if you leave them selected. This way you can apply a universal linearizing LUT to your project, and then turn it off to render LOG output.

I don't do LOG to LOG workflows typically, so please someone chime in if I've omitted a step.

http://www.alexisvanhurkman.com | http://www.correctionforcolor.com


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Gray Marshall
Re: Grading DPX Log Files
on Jan 3, 2013 at 8:51:57 am

Well, THAT makes me feel better. Thanks, Alexis. I felt like I just wasn't getting the "Log" controls. Still, I'm surprised that the dominant form of high-end image representation from 1994-2010 isn't easily realized in this software. Resolve seems to be very much "about" the digital-origination formats, judging from my initial experience with it. Time & familiarity may change that opinion.

Cheers,
Gray


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Joseph Owens
Re: Grading DPX Log Files
on Jan 3, 2013 at 5:08:12 pm

[Gray Marshall] "Resolve seems to be very much "about" the digital-origination formats,"

Very much so.

Renaissance/8:8:8, 2K, 2K+... were about telecine and the naturally-log performance of OCN in the gate of a scanning device. Resolve was more or less shelved about 5 years ago, which was probably a mistake and led to the company's latest difficulties with the resulting change in controlling interest. At least that's my take, reading the various press releases... it was not a product with a future in the old daVinci organization. But then film died. And so it goes. Poo-tee-weet.

jPo

"I always pass on free advice -- its never of any use to me" Oscar Wilde.


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Mike Most
Re: Grading DPX Log Files
on Jan 3, 2013 at 5:23:38 pm

The log controls have nothing to do with linearizing the output. That is a step handled, as Alexis says, by either a LUT or a curve. The log controls are designed to manipulate information that is coded in a log format, with the expectation that you're viewing the result through a normalizing LUT. The values in a log coded image have different black and white points and a different gamma than video images, and controls need to be scaled differently to allow for manipulation of those images in a more controllable and predictable way. The most common way of dealing with this, based on the film timing approach, is to use a combination of exposure (implemented here as offset) and contrast to alter the color balance and overall level on the image as a whole and adjust contrast to better match any under or overexposed material to the video display environment. The shadow, mid tone, and highlight controls act on very restricted ranges that are based on the Cineon log black and white points of 95 and 685 rather than 0 and 1023. The result is that you can set up basic balance on an image using purely offset and contrast, then fine tune it using shadows, mid tone, and highlight adjustments. Because of the design of those controls, you'll find that the highlight control, while restricted to input values near or above 685, acts as a sort of variable soft clip rather than a gain control (the gain control has a pivot at 0) so it's sometimes useful even in a non-log grading environment. It is also important to note that the contrast pivot can be very useful in dealing with exposure issues, such as lack of fill or blown out highlights. By raising the pivot, the contrast control affects the darker areas, and begins to feel more like a "black stretch" control. By lowering the pivot, it affects the upper ranges, bringing out more detail in things like overexposed facial highlights and the like. On Resolve, you get a lot of corrective power out of these things because you can implement them separately on different nodes (provided all of those nodes are prior to the output LUT).

Film DI work has been done using exposure/contrast and log scaled range controls for a long time, in fact, Lustre was designed around it, as was the Film Grade strip in Baselight. Because you're not pulling the picture apart by separating the brightness ranges, the offset/contrast control lets you stay much more true to the original image because once you get the balance right (usually by setting up flesh tones), all of the colors in the scene become what they're supposed to be based on the original photography. It might be counter-intuitive to those who have only used lift/gain/gamma control systems, but it's much simpler to come up with something representative of the photography using only one trackball (the offset trackball) than it is using three. You get better color separation and thus a lot more depth. I would suggest experimenting with the log grading controls using this description rather than trying to use them as you would lift, gain, and gamma. Think of the offset control as your Hazeltine knobs, and the Contrast as a display gamma correction and it will make more sense. I think you'll be surprised at the clean results you can achieve.


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Alexis Hurkman
Re: Grading DPX Log Files
on Jan 3, 2013 at 5:36:24 pm

This is the best explanation of the corrective use of the Log controls for logarithmic media that I've ever read or heard. Thanks, Mike!

http://www.alexisvanhurkman.com | http://www.correctionforcolor.com


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Joseph Owens
Re: Grading DPX Log Files
on Jan 3, 2013 at 6:43:06 pm

Mike, does this, or would this help with what are otherwise linear files that have been "locked" that way in a Quicktime .mov but were exported straight from, for example, Log-C Canon where blacks/whites are showing up between 30-80 IRE? Seems like the 8-bit XDCam wrapper that most assistants default to really defeats the whole purpose... or does the pivot help to ameliorate the "ripping apart" phenomenon?

Or is this only of benefit in a 10-bit (or better) RGB environment?

jPo

"I always pass on free advice -- its never of any use to me" Oscar Wilde.


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Mike Most
Re: Grading DPX Log Files
on Jan 3, 2013 at 8:05:52 pm

Canon Log is not LogC. LogC is a particular log curve similar to but not exactly the same as Cineon Log, that is used by Arri in the Alexa. Just because something is log coded doesn't make it any kind of a standard. Cineon Log is a standard because it was developed by Kodak and is based on print density. LogC is a variant that is very similar but slightly adjusted due to the specific response of the sensor that Arri is using. RedLogFilm is an application of the Cineon Log curve that is used by Red. Unlike any of those things, Canon Log is more like Panalog was, a "mild" log curve that is used for the sole purpose of retaining more information than you would get if you coded to Rec709 type video space. It is not designed to work with controls that are scaled for Cineon type log curves and Cineon type white and black points.

Now, having said that, log coding is nothing magical and the container that the image exists in has nothing to do with the values in the image. So the answer to your question - at least I think the answer to your question - is that the controls in Resolve are designed to be used with "standard," Cineon type log curves and their use with other gamma curves is not impossible, but it is not what they were designed for. The shadow and highlight controls might be of some use because of their restricted range when compared to lift and gain, but in the cases you're talking about, you're probably better off using "traditional" controls. In fact, one of the design criteria for both Panalog and Sony's Slog was the ability to be corrected directly in a video environment with "normal" video scaled controls, but easily converted via a 1D LUT to "standard" Cineon log for a DI environment.


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Joseph Owens
Re: Grading DPX Log Files
on Jan 3, 2013 at 10:01:16 pm

[Mike Most] "the answer to your question - at least I think the answer to your question - is that the controls in Resolve are designed to be used with "standard," Cineon type log curves and their use with other gamma curves is not impossible, but it is not what they were designed for. "

Thanks for that. It explains a lot. I did run some experiments this morning after posing the question, and although there is the facility to establish a different response, the net difference was almost nil. I have worked with PanaLog, and this particular project started getting that feel to it; the difference being that the Panalog project was a Sony shoot, recorded on SR Dual-Link, so the digitization was worlds ahead of Canon's skeletal remains.

jPo

"I always pass on free advice -- its never of any use to me" Oscar Wilde.


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