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Understanding Log and Color Space In Compositing


Log footage is an important part of the post-production workflow. Here’s what you need to know.

As digital filmmaking becomes more and more affordable, technologies become increasingly available to colorists or post-production professionals. In this case, Log footage. The Log (logarithmic) color space has been around for quite a while. Initially high-end post houses used it with scanned film negatives in a color space called Cineon Log. Now, pretty much all camera manufacturers offer their own Log curve (or multiple). There is S-Log 2&3 (Sony), LogC (Arri), Canon Log, V-Log (panasonic), Red Logfilm, Blackmagic Log, etc. Each of them are different, usually tailored for the color science of the particular manufacturer’s products.

The biggest reason to use the Log color curve is how it retains the most dynamic range of information from the camera sensor (or film negative). It encodes what the camera sees logarithmically, meaning that the correlation between the exposure of the image (measured in stops) and the recorded image  is completely constant over a wider range. It utilizes more of the sensor’s information than a standard video curve because it’s saving as much data as possible rather than capturing specifically for the human eye or a video screen. This gives you much more color data to work with in post-production.

Log vs. Linear Color Space

Understanding LOG and Color Space In Compositing — LOG vs Linear

Above is a (not necessarily exact) representation of the differences between a linear color curve and a Log color curve. If you’re familiar with  curves, you know that the bottom left of the curve represents your blacks and shadows, and as you move to the top right of the curve, you have your highlights and whites. As you can see, a Log curve pushes the darker part of the image upwards to retain shadows. The top of the curve shifts down to retain highlights. Therefore, you retain more data from each side of the color curve.

Linear color space has constant and unchanged luminance values true to their exact mathematical values in the scene. To the human eye, this will look very dark and muddy in certain areas and overexposed in others because our eyes (and monitors for that matter) see colors differently than their exact luminance values and can see more detail in bright and dark areas. Therefore, we have to apply a different color space in order to see the color information in a more aesthetically pleasing way, as well as standardize color values for displays and monitors.

Log vs. Video (Rec. 709)

Understanding LOG and Color Space In Compositing — LOG vs Video

A CG Element in Linear color space on a Log background.

So, how does Log fit into a VFX workflow?

If you’re receiving Log footage to composite, more often than not, you’re going to want to leave the entire image of the final composition in a Log color space so that it will get color corrected along with the other footage and maximize its data. This can be a problem because, generally speaking, your CGI or video elements are going to be in a more linear color space, like sRGB, Rec. 709 (video), or linear. So you have to convert your elements to Log. We’re going to use a Rec. 709 color space (a color space created for televisions and computer monitors — usually the alternative to Log in digital cameras). Rec. 709 isn’t a completely linear color space, but it’s much closer to linear than Log.

In the example above, the CGI chess set (a model from the starter pack that comes with Video Copilot Element 3d) starts out much darker with a higher contrast than the background plate. This is because the footage plate was shot in Log, and the CGI element is in a more linear color space. How do you change the color space of your CGI element? You use a LUT.

A LUT (look-up-table) is essentially a color preset that you can apply to any video footage or element. LUTs create a stylized look, feel, or tone for an image, but there are also general utility and conversion LUTs. You can use these to change from one color space to another. IWLTBAP has a great free pack of conversion LUTS — check them out here.

Understanding LOG and Color Space In Compositing — LUTs

To apply the correct conversion LUT, you’ll want to convert your CGI element from Rec. 709 to Log. To do this in After Effects, you can use the effect Apply Color LUT (from the Utility section) and click Choose LUT to select a Rec. 709 to Log conversion LUT when the file select box appears.

Now, in order to composite properly and see what you’re working with, you can correct the entire image (including back plate) back into a Rec. 709 color space.

Understanding LOG and Color Space In Compositing — Rec 709

To do this, you’ll create an adjustment layer and apply the Apply Color LUT effect to it. From there, you need apply a Log to Rec. 709 LUT.  Now you can see a generally color corrected image to work with, and the two elements merge together from a color science perspective.

Understanding LOG and Color Space In Compositing — 709 LUT

Before you send your composited images back to a colorist or an editor for final assembly, you’ll want to make sure to turn off the Log to Rec. 709 adjustment layer so the image returns back to its original Log state. This way, the colorist or editor can use the higher dynamic range and color data when doing the grade.

Why It’s Important

Understanding LOG and Color Space In Compositing — End Result

Fire elements in Rec. 709 color space merged with Log footage.

Log is an amazing resource for filmmakers. The fact that you can now get it at a lower cost built in to consumer-grade cameras is just an incredible tool for all low-budget filmmakers. Just a few short years ago, there were only a handful of cameras in an affordable price range that had true Log picture profiles in them. Now, at just $1,400 Sony A6500 features both S-log2 and S-log3 (with an impressive 14-stops of dynamic range).

Although it can get confusing, it’s very useful to understand these color spaces. To quickly summarize: Log is flat with more data, and Rec. 709 is higher contrast and bakes away some of the data. If you’re going to composite into a Log background, you need to convert your non-Log elements to Log first. Simple as that. You’re happy, your colorist is happy, and your project looks much better.

Do you have advice for working in the Log color space? Let us know in the comments.