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ARRI Alexa grading

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Sam Coward
ARRI Alexa grading
on May 12, 2015 at 1:43:03 pm

Hello there,

I'm working with ARRI Alexa footage for the first time. Its grey and flat as expected, but the ARRI specific LUTS aren't working well, even as a base to do different grading. I think the camera settings in the actual shoot were not the best they could have been. The image looks really grungy - the contrast is super strong.

I seem to be able to get a better looking image - and faster too - from working off the flat image, than trying to work off footage which has the ARRI LUT applied.

My question is, if I simply do grading from the flat image, is there some quality that won't emerge? Will I miss something technical or magical from not applying the ARRI specific LUTs?

Thanks,
Sam


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Joseph Owens
Re: ARRI Alexa grading
on May 12, 2015 at 4:09:31 pm

[Sam Coward] "Will I miss something technical or magical from not applying the ARRI specific LUTs?"

The key is always to make it look as nice as possible -- "Do No Harm."

Are you trying to work in a linear or log mode with footage that has been converted in some way?

jPo

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


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Mike Most
Re: ARRI Alexa grading
on May 12, 2015 at 4:39:19 pm

Saturation, particularly blue saturation, will not be correct. That's what the Rec709 color matrix in the Arri LUTs made using the "Rec709" setting provide, and what many users don't seem to understand. The purpose of the LUT is not just implementing a proper gamma curve, it is also applying a saturation matrix to yield proper color saturation across the color spectrum. Simply turning up the saturation control in a color grading program does not provide the same thing, although some seem to prefer that result on some material.


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Marc Wielage
Re: ARRI Alexa grading
on May 13, 2015 at 4:20:29 am

[Sam Coward] "I'm working with ARRI Alexa footage for the first time. Its grey and flat as expected, but the ARRI specific LUTS aren't working well, even as a base to do different grading. I think the camera settings in the actual shoot were not the best they could have been. The image looks really grungy - the contrast is super strong."
Arri lets you create custom LogC -> Rec709 LUTs on their own website, and it's pretty easy to use:

http://www.arri.com/camera/alexa/tools/lut_generator/

They also have a dozen different LUTs you can try, each of which affects the toe and shoulder response slightly differently.

It's always very helpful to have the camera crew shoot a known color reference chart, like one of the DSC's, so you can get an idea of where to go. Mike Most above is correct that Arri color science is not a linear saturation change, but you can get pretty close to it by hand. I know of quite a few post houses that just use a minor curve or mid adjustment for gamma and then some very slight hue/sat curve adjustments, and you can get very good results out of that.


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Marcus Smith
Re: ARRI Alexa grading
on May 13, 2015 at 11:43:17 am
Last Edited By Marcus Smith on May 13, 2015 at 11:45:03 am

It sounds to me like you either have:

1. The wrong footage. IE not the camera originals, maybe something with a lut baked in?

OR

2. The wrong lut. This is probably more unlikely though, as the standard ARRI lut works well and is clearly labelled in Davinci.


Arri log, ungraded, will have blacks sitting on the waveform from 256 and the highlights usually end somewhere around 680. That will usually take any log lut you throw at it without the intense contrast you are talking about. Arri's implementation of log is somewhat unique in that it requires a Reference Rendering Transformation (in the form of a 3D LUT) to bring the colours up to the saturation of the original scene shot. This means that most 1D log / SLOG luts thrown at Arri log will look desaturated as it doesn't factor in the RRT RGB transform that needs to take place to get the picture looking correct.

Also, ARRI log footage with the lut applied almost always gives you a low con starting position, with the blacks sitting around 100 and the brightest spots around 900.

Cheers,

Marcus.


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Sascha Haber
Re: ARRI Alexa grading
on May 13, 2015 at 2:09:52 pm

Upload a screengrab of your ungraded picture, or a splittscreen in Resolve with the LUT applied and your mouse hovering over the Lut selector setting in the node graph.

Resolve 11.3 - Smoke 2015 EXT1 - Sapphire 8
Colorist / VFX Guru / Aerial footage nerd
http://vimeo.com/saschahaber
https://dk.linkedin.com/in/saschahaber


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Mike Most
Re: ARRI Alexa grading
on May 13, 2015 at 6:57:31 pm

>>Arri's implementation of log is somewhat unique in that it requires a Reference Rendering Transformation >>(in the form of a 3D LUT) to bring the colours up to the saturation of the original scene shot.

That is not unique, it is completely normal and common with nearly every
RAW format sourced from a Bayer pattern sensor. The color gamut of nearly all digital cinema cameras is much wider than Rec709, so a simple deBayer to Rec709 colorspace is going to yield an image with a very low saturation. Getting the saturation back to where it "should" be, and getting it correct across the color spectrum is best done by a color matrix operation, which is what the Rec709 color matrix in the Arri lut builder does. For Red, the Redcolor settings are in fact settings for the color matrix. For Sony F55 and F65, the supplied LUTs from Sony implement a color matrix appropriate to the colorspace you capture in (SGamut2 or SGamut3). My point is that the use of a color matrix to correct for saturation when displayed on a Rec709 display is normal and common across virtually all Bayer sensor digital cinema cameras. It is not "unique" to the Alexa.

And just to use the terminology correctly, the term Reference Rendering Transform is usually used when talking about ACES, whose developers invented that term to refer to a specific combination of lookup tables and matrix transforms that are used to yield an "idealized" image from values stored in ACES space. A color matrix is a color matrix. The Reference Rendering Transform is something else that is much more complex than that.


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Marcus Smith
Re: ARRI Alexa grading
on May 13, 2015 at 11:57:57 pm

Hi Mike,

Thanks for jumping in here and clearing up my terrible terminology.
I'm just learning about ACES and its clearly not the appropriate use of RRT here, and its great to learn from someone with your experience.

In regards to Arri log, I was referring to Arri's implementation of log, for Post Production / grading purposes, not its full colour pipeline from raw to Rec709. With Arri log in Davinci the colour matrix and the log to video transform happens in one 3D lut. You can break the two up, but standard workflow is all in one.

It my understanding this is different compared to the Red log pipeline, or the original Sony Slog pipeline, where the colour matrix is applied and included in the original log format. The only thing is required in the signal chain is to add a 1D RGB curve to bring it back to a video 2.4 gamma viewable image. So when comparing these two log formats with ARRI log they are different conceptually (and visually if compared). Its still the same colour pipeline just different from the log part up, which is why I mentioned it here in this post. Is this correct - or have I misunderstood something?

I've also noticed with Arri log to REC709 3D LUT is that it appears to be bending and changing colours quite dramatically, more than just a colour saturation boost. Do you think its just a straight forward 3x3 colour matrix that's being applied here? Or is something more complicated going on?

Thanks again!

Marcus.


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Mike Most
Re: ARRI Alexa grading
on May 14, 2015 at 2:36:47 am

All of these things are choices. Red chose to make the choice of color matrix part of their overall conversion routines, but that is in part because they also chose to make available color paths within that conversion setup that do not require any further manipulation, at least for Rec709 display. So they give you a choice of multiple color matrices (or none, which is what Camera RGB is) and a choice of multiple gamma curves that include Rec709 compatible output (Redgamma 1, 2, 3, 4, etc.) and one that puts you into log space (RedlogFilm) that is compatible with a pipeline that would normally include a LUT to get you to your chosen display gamma. Sony puts the color matrix in the LUT path, much the same as Arri. But both Sony and Arri offer LUTs that do not implement a color matrix, so that the user has both flexibility and responsibility in these things. Also, Arri allows you to apply a P3 targeted color matrix directly to LogC Prores files internally, but most users prefer not to do this and apply it as a post process (or not) instead.

As for Arri's math, I have no direct knowledge of what they're doing, but my guess is that it's a fairly straightforward 3x3 matrix, developed with deep knowledge of the sensor characteristics and what their color scientists feel is the best representation of the captured color. This is supported by the fact that if you use their ACES IDT and look at the scene referred result through a Rec709 RRT/ODT, you get a very similar color pallette to that obtained from the Rec709 targeted LUT approach.


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Charles Haine
Re: ARRI Alexa grading
on May 26, 2015 at 1:50:06 pm

Have you tried any of the non-Arri LUTs?

I have one client that shoots a lot of Alexa and for some reason the Arri LUT's look terrible on it. No idea why: I've tried to find out (they've been a client a long time), not sure if it's rental house issues, or something the DP does in camera, but LUT's I use on other clients footage that looks great just doesn't work on their footage. Specifically the red color channel goes super duper pink.

I graded around it for a long time, but during a lunch break a few years ago I played around and discovered that for some reason Sony SLog2 (I think) works perfectly on their footage.

No idea why.

CharlesHaineColorist.com


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Marc Wielage
Re: ARRI Alexa grading
on May 27, 2015 at 1:21:45 am

[Charles Haine] "I have one client that shoots a lot of Alexa and for some reason the Arri LUT's look terrible on it. No idea why: I've tried to find out (they've been a client a long time), not sure if it's rental house issues, or something the DP does in camera, but LUT's I use on other clients footage that looks great just doesn't work on their footage. Specifically the red color channel goes super duper pink."
I have never found the Arri LUTs to produce pink results -- if anything, 90% of what I see is a gamma curve and a selective chroma increase. Note there are LUT-creation tools on Arri's website that allow you to customize their Rec709 LUTs quite a bit. In general, though, I just start from Log and do it myself -- it's not that hard to come up with a setting within Resolve that will make perfectly usable pictures.

If you want to determine if there's a legitimate issue with those cameras, have the DP shoot a DSC color chart and look at the results. If anything bad is going on, you'll see it very quickly. There's also a procedure in the Alexa manual to do a "first birthday" reset on the camera, which will restore all the settings back to the factory defaults. That will eliminate any possible corruption that might be screwing up the settings in some way, unless of course the camera is just defective.


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Sam Coward
Re: ARRI Alexa grading
on May 27, 2015 at 5:10:55 am

So many useful replies! Thank you. I guess my initial question was if, by doing a DIY job, I'd miss some invisible technical step specific to Arri... From all the responses, I guess that to convert footage using Arri LUTs simply IS just that - their suggested LUTs - as pointed out, those can be customized as well. So there's nothing wrong with saying "Erk, I don't like that result. I'm going to use another LUT or create my own option". Especially if in the circumstances as the cameraman having made some 'interesting' decisions! :)


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