Why is Camera Matching with Color Charts So Hard?

December 24, 2014

If you've tried to match multiple cameras together using color charts, you've likely found it difficult. What's the problem you're having?

Day 24: 25 Insights in 25 Days Holiday Marathon

Why is Camera Matching with Color Charts So Hard?

In  Resolve 11, Blackmagic released a heavily hyped new feature: Automatic Color Match. From their website:

Simply use the chip grid to identify the color chip chart and Resolve will automatically balance the images, even if they were shot on different cameras, under different lighting conditions and with different color temperatures!

The promise is simple, shoot a color chart and have your Primary Corrections set with technical precision and all your different cameras matching each other.

What’s the problem?

The problem is: Automatic Color Match rarely matches

The intertubes are  littered with forum and blog posts of professionals who have been taking the Marketing copy at its word… and are finding it sorely lacking.

In fact, many pros are finding it so frustrating they no longer trust the tool.

If you don’t trust the tool, you stop using it

In this short series we’re going to first frame the problem: Why is the Automatic Color Match tool not working the way editors, DPs and colorists want it work?

Then we’re going to help you figure out a workflow that is repeatable and workable.

Automatic Color Match works if you use it properly…

But what’s the proper use of Automatic Color Match? The manual has a few things to say about it, but nothing that solves the problems that seem to be plaguing people.

What we need to do is more precisely define the problem that Automatic Color Match is trying to solve. Then we need to test to see what the tool is actually doing.

And then, finally, we need to develop a workflow that bridges between our expectations of the tool and the reality of how it works (we’ll tackle this in the final part of this series).

Special Thanks to MixingLight Member Greg Greenhaw!

Over in our series on working in LAB Colorspace, Greg offered to shoot me a test scene with a color chart (with a Blackmagic 4K camera).

Since I rarely ever see color charts in my professional work, I jumped on the opportunity to work with his footage. What’s even better – he mistakenly first shot his test scene to Rec. 709, not to BMDFilm (a flat, log-profile image). He then re-shot the scene to BMDFilm and provided me both files. This turned out to be the Perfect Camera Matching Test ™.

We start by matching the same camera to itself

Since Greg provided both the Rec 709 recording and the BMDFilm recording of the exact same scene with the exact same camera using the exact same lens… we can remove those variables from our experiments and figure out how to get Resolve’s Auto Color Match tool to work.

We refine using the DSC Labs SMPTE OneShot

At the time of this writing, DaVinci Resolve supports four different color charts for automated shot matching, Greg’s test image included two of them! He gave us the X-Rite Color Checker and the DSC LABS SMPTE OneShot.

In this series, I’ll be using the OneShot for evaluating this tool. Why?

The OneShot is based off the work and ideas of Cinematographer Art Adams. And Art has a great article on how he designed the OneShot, with a full explanation of exactly the targets he designed the chart for and—most important—how the chart is designed to be used.

Using Art’s explanation of the OneShot, we can deconstruct Resolve’s Auto Color Match feature

While reading how Art uses the OneShot, I had an Ah-HA moment of my own when he said this:

It’s more important to ensure the colors fall on the correct “vectors”, which are lines drawn from the center of the vectorscope through each color box. If the colors fall on the proper vectors they are at least accurate, even if they aren’t fully saturated.

This is the key to understanding how to properly use a color chart – and what precisely Resolve’s Auto Color Match feature is doing to our images.

What you’ll learn in this Insight…

By the end of this Insight, you should be up to speed on how to define the problem we’re all having when camera matching with color charts.

In the next part… I’ll offer up a workflow solution for getting much more consistent camera matching results.

Other Resources

A lot has been written on using color charts. I’ve pulled a few URLs that I thought you might find interesting:

Art Adams: Rough Guide to Color Grading with the DSC Labs SMPTE OneShot – The man who designed this chart takes it out into the field to share his thoughts on how to color grade.

Curtis Judd: Resolve’s Auto Color Match – This YouTube video has some good things to say about different light sources. Plus you’ll get a good idea how to use the Color Match feature.

Blackmagic Forum: Recommended Color Chart for DaVinci Resolve? – A forum thread with lots of interesting advice, reflections and thoughts on how and when color charts are useful to a colorist.

– pat

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