Question about color-correcting log footage

A Quick Primer for "normalizing" Log Footage In Premiere

Remember, log encoding means the camera applied a strong and increasing logarithmic "downward curve" on the data recorded to the file, compared to the data from the original scene. This was done to reduce many stops of data into a few stops of file range.

For getting a mental image, think of a box graph, like the RGB Curves. Left to right is increasing brightness of the scene, the bottom to top is the amount of data recorded on the image in the camera. Let's say 1,000 points to the scales.

• Reality: a straight line from the lower left corner to the upper right.
• Log-encoding: starts straight in the lower left corner, but part way up begins to slightly curve "down" below reality. And the farther 'out' you go on brightness, the more sharply the line curves downward.

So to 'unpack' a log-encoded image means mostly sort of straightening out the curved data to a straight line. Kind of.

In Lumetri, the easiest way is to work with the Exposure and Contrast tools to start. A primer on the tools to use, and what they do:

Exposure changes all values, but higher values more, starting similar to a "gain" operation or grabbing the far-right end point of a Curves tool. But soon it starts rolling off values in the right-side Highlights area above 95 on the left side scope scale. Though it's totally straight-line when going down.

Lifting Contrast applies a curve that pushes data away from the middle/50nits(IRE/%) value on the scopes, but will never push something past 100nits (SDR/Rec709 space) nor past 0 into super-blacks. The farther the data from the 50% center, the more it's moved until it approaches 0 or 100.

Dropping Contrast applies an inverse curve pulling data towards the middle.

So expanding that curved data is best started with those two tools. In effect, you use the Exposure tool to 'center' the data on the 50% line of the scale, then the Contrast tool to expand the data to a more "normal" dynamic range visually and in the scopes.

By steps:

1. Look at the RGB Parade or Waveform (YC no chroma form) scopes to find where the center of the data trace lies.
2. Use the Exposure tool to place the center of the scope data on the 50% line.
3. Raise Contrast to expand the signal trace. The goal is to expand the total contrast to what the image needs visually, with nothing below about 1-5 nor clipping on the top either.
4. Check where the expanded signal trace ends. To adjust where the shadows and highlights fall if the image is a bit bright or dark, Use the Exposure tool to adjust the data to the scales.
5. Fine-tune using the other tonal tools as needed.

Do this a couple times, it quickly becomes fast and easy.

6. Now that the tonal range and "slope" are set, use the Saturation and Vibrance tools to increase color saturation as needed.

That's the basic process ... takes me about 20 to 45 seconds per clip, though I'm using a control surface rather than mouse.

If you have similar clips from one camera, it's easy to do this to one image, copy the Lumetri effect from the Effects Control Panel, then select all the images in the bin(including the one you just worked), and paste that onto them. Now delete the effect from the clip on the timeline's 'normal' ECP section so it's not double-applied.

And you've just normalized a bunch of clips fast. If any need small adjustments, use the Source tab of the ECP to get to that Lumetri effect and adjust. Otherwise, just grade your sequence as normal for now.

Neil

I'm a contributing author at MixingLight.com, a pro colorist's teaching site. For background.

Colorist's call LUTs "the dumbest math out there" because they are.

• Datapoint A: a LUT is simply a data matrix saying "take pixels with value XXX to value XYX" ... that's all.
• Datapoint B: manufacturer-provided LUTs for various cameras/settings are created typically in a "perfect" shooting situation. Highly controlled exposure, color temperature and contrast of lighting, perfect on-camera settings to match the scene.

Take those two bits, and you see where a problem happens ... no "real world" shooting situation will match that scene used to create the LUT. You may have highlights bright enough that the LUT simply clips them, or shadows dark enough they get crushed by the LUT. And ... you're stuck if you can't fix this, right?

So you always have to 'trim' each clip in the color controls, to fit within the parameters the LUT needs to work properly.

Meaning you apply the LUT, so you can see the effect it has on the clip. Then go to color/tonal controls applied before the LUT, to adjust the exposure/contrast/saturation/WB of the clip to fit what the LUT needs and give a best-case image on screen and especially in the scopes.

In Premiere, that means really ... applying the LUT in the Creative tab, and using Basic tab tonal/color controls to trim each clip.

Now ... you can as easily make a Lumetri preset for your shooting needs. Take a fairly standard clip or what you work with. mod the image in Lumetri so it looks good and doesn't blow out whites/blacks/saturation. Save as a preset, apply to entire groups of clips, right?

So if you start with a log-encoded image, and work with the tonal/color controls to expand that to mostly fill out the scopes and look "normal", you've just "normalized" the clip. Save as a preset and you can apply and then mod that Lumetri for any clip. Save as a LUT , and use the LUT approach above.

Your choice.

I rarely use LUTs that I haven't made, by the way. I can take my cameras, my situations, and normalize them then craft my own LUTs to get better results than with pre-built ones.

Neil

Hey,

Sorry for the late reply. There is no rule as such that you need to apply LUTs on log footages. LUTs are just an easy way to get to the finished product faster and save time. They do not set an accurate white balance for your footage automatically. If you were using the grey card before so that you get accurate white balance, then I suggest that you continue using it since LUTs do not serve the same purpose.

Regards,

Ishan