Canon Log to Loc C LUT

[Policar]

Does anyone have such a thing? Would be really useful.

[Luke Mason]

Yep, download OpenColor IO and ACES Config for After Effects and you'll be able to generate your own, you can do cross conversions between almost any modern gamma curve.

You are trying to do shot matching I believe? if you use Resolve you can grade in ACES log, if you have to use an LUT it's better to convert Log C to Canon Log, compressing DR instead of expanding, which may introduce artefacts.

[jcs]

If you have access to both cameras, you could create a 3D LUT (this is much more than a simple gamma transform):

Using a black-body emitter / continuous spectrum light (tungsten etc.),

Using the same lens at the same position and camera settings,

Shoot a color chart with both cameras (Log for Canon, Log C with ARRI),

Create a match transform from one chart to the other (using Resolve or similar- this may be the tricky part),

Save the final transform as a 3D LUT.

[Policar]

Create a match transform from one chart to the other (using Resolve or similar- this may be the tricky part),

Save the final transform as a 3D LUT.

How do you do that?

Would the match transform adjust both gamma and color? Gamma is quite different–annoyingly, Canon Log behaves badly compared with Log C–but color is definitely quite different in the reds and blues.

[jcs]

I'm not a Resolve expert, however there's a feature called "Shot Match to This Clip". If it works OK on the two color chip shots (perhaps with additional manual tuning with scopes, etc.), Resolve provides a means to save the current grade as a 3D LUT.

Conceptually, as a graphics developer, it wouldn't be too hard to directly create this process using two color charts as input and a 3D LUT as output. Just use the average color in each sample square to create the look up transform (input/output) to apply to the 3D color cube. I wrote code to do something similar years ago (create a 3D LUT to transform key colors in one image to the same location in another image, then apply the final 3D LUT to the entire first image (all intermediate color values are interpolated by the 3D LUT (color cube) either with a trilinear or tricubic function). It worked really well).

This type of function might be directly built into other higher-end coloring tools (or could perhaps be created with their scripting languages).

There might be a way to do this with Photoshop too. The basic idea is to transform the color chips in the first shot into the color chips of the second shot, then save the 3D transform as a 3D LUT.

[Policar]

But this would't do a thing for gamma, would it? Just color... Still useful. You'd have to shoot the chart once under daylight and once under tungsten.

[jcs]

This process would make an image shot in Canon Log look like it was shot in ARRI Log C, this includes both color and gamma: it's a full 3D non-linear transform. You're right in that you would need to shoot charts for each color temperature. Shooting a variety of color temps could create an array of 3D LUTs, which could then be interpolated to any desired color temperature.

[Julian Krause]

Maybe have a look at LUTCalc. With it you can create your own conversion LUTs. It provides different gammas and gamuts for all famous cameras including Canon and Arri.

https://cameramanben.github.io/LUTCalc/LUTCalc/index.html

So I think for your example you would select Canon C-Log as Rec Gamma and Canon Cinema Gamut as Rec Gamut. And for Out Gamma it would be Arri Log-C and Out Gamut would be Arri Wide Gamut. Then just hit the create LUT button.

If you give it a shot, let me know how well it works.. 

[jcs]

Very cool Julian. A quick test shows Canon Log2+CinemaGamut+Production Camera matrix is very close to ARRI LogC (gamma curve is almost identical). I can test Canon Log to LogC along with LogC to 709 later.

[jcs]

I created a Canon Log Rec709 to ARRI LogC Widegamut LUT, then created an ARRI LogC to Rec709 LUT and the results looked pretty good. Curious to hear from Policar if this works OK to match Canon footage to actual ARRI footage.

Also tested A7S II footage shot in SLog2+SGamut3.cine, converted to ARRI LogC, then from LogC to 709- looked pretty good. Lots of interesting LogC LUTs out there- this can be useful.

He also has an OSX desktop app for $2.99 (pretty fair price if it works for you): https://cameramanben.github.io/LUTCalc/

[Policar]

Still trying to figure out how to get this working but it's really promising. Might be useful! Reds stay a little too magenta and I have some issues that might just be white balance–but they match really closely overall.

Only issue is the gamma curve seems close but the black and white clip points definitely aren't right. And while I tried to set that when generating the LUT I had to change it manually in Resolve in the curves, dragging 0 IRE to 10 IRE, 100 IRE to 95 IRE. The rest of the curve seems close. This is likely user error, my mistake.

Colors are nicely desaturated as they roll off into the highlights, as they actually are in Log C.

I think the Canon footage I'm working with is 2/3 stop underexposed. Surprisingly grainy and surprisingly good highlight detail, maybe a t stop f stop thing. Anyhow, very promising!