How Hard Is It To Create A Broken LUT?

How Hard Is It To Create A Broken LUT?

May 24, 2018

In an earlier Insight, you learned how to Stress Test your Look Up Tables (LUTs) to discover if they are introducing artifacts in your images. In this Insight, learn how easy (or hard) it is to generate 'broken LUTs' in DaVinci Resolve.


Series

Following up on the LUT Stress Test

Jason Bowdach’s recent Insight on the LUT Stress Test fascinated me. Test images with computer-generated ramps have always been a favorite tool of mine for helping us visualize the work of Look Up Tables. Combine them with scopes and you have everything you need to know about a LUT. Jason’s reference test images added a whole lot more depth to my toolset.

And in the process – Jason has me asking more questions, and after a few days work I’m still not finding the answers I’m looking for. But after reading and watching his Insight, I decided to start off with answering a simple question.

Using normal color correction tools, how hard is it to ‘break’ a LUT?

Let’s be clear here. I know how to break a LUT in about 3 seconds. Just apply a vignette and the exported LUT becomes worthless. In fact – DaVinci Resolve 15 ignores Nodes that have Power Windows. It also ignores HSL selections when exporting a LUT. (I’m not sure if this is a new behavior in Resolve 15, or not? I know when LUT exports was first introduced in Resolve, if you applied a Vignette you’d get some mighty nasty LUTs. But I can’t reproduce that behavior any more – now that Resolve is much smarter about it).

What I’m asking is: If I get aggressive, can I break a the LUT Stress Test in the manner that some commercial film-emulation LUTs do?

In this Insight, you’ll see my many attempts to break a LUT export out of DaVinci Resolve. What do I have to do to break a LUT? Here’s a hint: I had to get super-aggressive with the film emulation plug-in Film Convert to get any image that you could possibly say *might* induce unwanted artifacts. Personally, I think FilmConvert did a great job, even when I yanked every slider around – and it gives you options to export LUTs in several different cube sizes.

If you add LUTs to DaVinci Resolve then don’t miss the end of this Insight!

While spending many hours creating LUTs and re-importing them into DaVinci Resolve 15 Public Beta, I discovered some key new shortcuts that makes this process super-fast and way less annoying. Jump to 14:35 if you just want to see these new shortcuts in Resolve 15.

Coming Next In This Series

Since I’m having trouble creating my own ‘broken LUTs’ I’ve started looking at 3rd party solutions for evaluating and manipulating LUTs – to help us better understand what’s going on. Can we do anything to clean these LUTs up? I’m not sure what the answer is yet. But that’s what’s coming next in this series. And if you have suggested avenues of exploration for me, please let me know in the comments!

-pi

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Comments

8 thoughts on “How Hard Is It To Create A Broken LUT?”

  1. Patrick, I too value the information gathered from generated ‘charts’. A thought is that even though an image looks alright, upon closer scrutiny one might actually find banding or some other artifact. It makes sense to QC a LUT with a chart sensitive enough to show where to look for possible issues. And, of course, there is always the question of just how much might one get away with in their film. But then I seem to be one of those who value nuances which may or may not at first be noticed. To add to your beta15.3 tips at the end: If one selects the picture viewer in the LUTs pane on the color page and scrubs over the various LUTs they will be temporarily applied and seen live in the viewer making for a quick way to change through several LUTs (or stills from the gallery.)

    1. Jeffrey – I’ve taken to turning off the Live Preview if I’m comparing LUTs. There’s a momentary flash as the mouse moves moves from one LUT to the next that makes it impossible for my eyes to do a direct comparison. When testing, I prefer being more old-school and applying then versioning and stepping through the versions for nice clean comparisons.

      Actually – I’ve taken to turning off Live Preview everywhere. I just don’t like having my Viewers taken over without me actively telling the software to do so.

  2. I’m no expert, but I’d say the reason of the inconsistencies in LUTs come because “commercial LUTs” are created by sampling chips: the various inconsistencies from one chip to another (both from developing, they use to be film-based, or from the capture of this samples) make the response non-linear. This subtle inconsistencies are streched when we play with the image -any LUT will require grading before and after it IMO.
    Our grading tools affect the image “overall”. Lift/Gamma/Gain’s effect is very soft, affecting a wide tonal range: Artifacts are only added when playing too hard with Log controls, or by drawing curves with flat/too steep shapes: I think then artifacts will rise.
    Old professional LUTs by Autodesk and even the ones in Resolve proved pretty reliable to me, but others one can buy for 100€ often got me into trouble: I guess sampling the whole latitude in 17 or 33 steps/channel could be the reason…
    Food for thought, thanks for trying hard!

  3. I generated a Lut with Color space transformation and a lot of banding and artefacts appeared in the chart. I debayered BMD RAW as linear gamma, then a transformation to gamma Log C, then I worked in log mode, and finally, a transformation to Gamma and Color Space to REC 709 (BMD v3 to 709). I enabled only the transformation from Log C Nodes up to the end, and generated a Lut. When I tested, I saw a a destroyed chart. Is Color Space transf not allowed to be baked in a Lut?

    1. From my testing: The LUT only encompasses actions performed on the Color Page. If you’re doing the transforms either in the Project Settings or in an OpenFX plug-in – the LUT export ignores those operations.

      How are you executing those transforms? Using what tools?

        1. My understanding: Currently OpenFX nodes are ignored. I’ve not heard that Transforms operations are handled differently from other OpenFX nodes. But if you need this, test it with each new Resolve 15 update. It may be added at some point.

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