Log / Lin Workflow

Many digital cameras now record in a log format (Alexa LogC, REDlogFilm, Sony S-Log, etc.). This is a quick overview of dealing with the log images through a visual effects & color grading pipeline.

The following footage is from an early RED Scarlet prototype:
http://red.cachefly.net/Chicken.zip

lion_original

Log2Lin – Visual Effects

One main benefit of the Cineon log format, is that it can easily be converted into linear light for compositing / fx work. In linear light, the math works correctly, and pixels can be realistically blended together.

Linear light doubles the pixel value for every 1-stop increment. The data is also in “floating-point” (the decimal moves depending on how large the value is), so bright areas can end up being greater than 1.0 (white) – surpassing what a display device is capable of showing.

NOTE: Many people mistakenly refer to a video gamma encoded image as “linear”. True linear light data looks like the image below.

lion_linear

After the compositing / fx work, the linear data can simply be converted back to log for exporting and color grading.

lion_log

All the major compositing programs have tools for converting between log and linear.

Nuke’s log2lin node works out of the box:

nuke_log2lin

After Effects’ Cineon Converter needs some slight adjustments to get it working correctly (After Effects’ project space also needs to be changed from the default 8bc to 32bpc).

Gamma needs to be at “1.0”, and Highlight Rolloff at “0”:

AE_log2lin

Lin2Log – Color Grading

During the grade, an “s-curve” is applied to the log footage to bring it back to a video space. All color operations are applied underneath the curve, thus preserving the highlight / shadow data straight through to the end of the process.

s_curve

Often the curve is saved in the form of a LUT (Look-Up Table). The LUT applies a saved color transform, so it doesn’t have to be re-created from scratch every time.

lion_grade

NOTE: One should avoid using the common Lift, Gamma, and Gain functions. They were designed for linear data, and can clip the highlights / shadows fairly quickly.

Instead, it’s better to stick with the log grading tool-set: Contrast, Offset, and Pivot. Contrast does what the name implies. Pivot changes the center point of the contrast adjustment. Offset, when applied to log footage, acts like an exposure change. It can also uniformly shift the colors, resulting in a more natural adjustment.

resolve_log_grading

Most color grading programs include log grading tools, you just have to look for them.

There is of course much more to the subject of a log post-production workflow, as this is just the tip of the iceberg.

Resources:

You can read more about log grading here:

http://mikemost.com/?p=243

http://mikemost.com/?p=251

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One response to “Log / Lin Workflow”

  1. STARS Pictures says :

    Thank you so much, extremely helpful! I’d been trying to figure this out for a while now.

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