Log conversion LUTS always pixelate my footage

[RamseySaleem]

 

 

Not sure why but I have this problem whenever I try to use a log conversion LUT in premiere. All of the colors crunch and I get blocks of color pixelation. I always end up having to manually grade it out of Log. I had this problem previously with the Sony A7s2 and been trying out the Fujifilm XH2s and having the same problem. Any insight? Am I doing something wrong in camera or what? Been having this problem for years and just working around it cause I have never found anything on the internet with the same problem. Please help 😭

[Shebbe]

Hey Ramsey,

There could be some factors at play here. 

1. Camera capture format. If this is 8bit footage it is not really enough to capture a log encoding such as FLog or SLog3. To extend this to a presentable Rec.709 image the bits are getting streched out too much and create banding in slowly transitioning color information such as a backdrop or sky. Make sure you always shoot 10bit for log.

2. Recording bitrate. If the data rates of the captured file is too low the compression artifacts will reduce the overal image quality by grouping similar pixels to decrase the amount of information for smaller filesizes. Even with 10bit this could lead to issues similar to what you show above.

3. LUT interpolation. You pretty much always want Tetrahedral LUT interpolation to ensure that you achieve the highest transform resolution possible. Especially for LUTs that move colors far from their origin like log to display conversions you will start seeing issues otherwise if the lower res method 'Trilinear' is used. You can change this interpolation method in the menu File->Project Settings->General bottom of the window.

[R Neil Haugen]

(Nice to "see" you here, Shebbe!)

To add to Shebbe's comments, I'd first note that LUTs are often called "the dumbest math out there" by pro colorists ... exactly because of issues like this. Or clipping or crushing or weird color or ... you get the picture.

LUTs are simple "look up tables" that just say X value becomes Y. No if-then or any other mathematical tool applied.

So ... if your media exactly matches the media used to create the LUT ... meaning the camera settings, exposure, lighting contrast, and the scene general saturation as shot with that camera ... match the original scene used to create the LUT ... then it works ... probably. 

However, if you are the slightest bit underexposed, overexposed, or too much contrast in your scene ... it ain't gonna work right.

So colorists always use normalization LUTs in a place where they can apply the LUT, then, in steps processed before that LUT, 'trim' the clip for exposure, contrast, white/black points, and saturation. You adjust those so that the clip looks good after the LUT is applied.

In Lumetri, the Basic tab Input LUT slot is processed prior to anything else ... which I've told their scientist, is simply wrong. So if the LUT damages the file (like in your examples) rather than fixes it, you're ... stuck in a Bad Place. (It was a fascinating discussion.)

For a proper example, if you apply a LUT in a node in Resolve, all color corrections you apply in that node are processed before the LUT. Correct behavior.

For all LUT based normalization routines in Lumetri, I recommend using the Creative tab's "Look" dropdown slot instead of the Basic Tab.

Why? Because then you can use the Basic tab's Exposure control to lift or drop the entire image, so when you apply the Basic tab's Contrast control, which works by pushing out from the 50IRE midline, it applies the contrast you want to see from where you want it "pivoted".

And use Shadow and Highlight controls, then Black and White,  to complete the normalization process.

Further ... most of the colorists who I work for/with/teach, and whom I learned from, don't use very many manufacturer LUTs. They figure out what's done (not that hard with scopes and test patterns) ... and then build several variants of their own to use with that particular media depending on the scene involved, or what the exposure/contrast of the scene of the project was.

[urbo73]

Another way is to use an adjustment layer for the Log -> 709 LUT, as that's also processed after the Lumetri adjustments on the video layer.