Sunday, April 24, 2016

Matching colors across camera brands with Resolve

By Guest Blogger: PastramiSwissRye

No matter the size of your budget, there will come a time when you need to match different cameras. It could be a GoPro crash cam in a stunt scene, pickup shots on a DSLR after a rented RED has been returned, or just borrowing a B-cam from a friend to cover an interview. 
While you can always try to match your settings on location, no two camera manufacturers use the same color science.  As a result, your shots won't match exactly and the final intercut product may seem a little "off" to the viewer no matter what you do on set.

My initial reaction to mismatched shots across cameras was to carefully color-match each shot in post.  This cost me a lot of time and money - until a co-worker and I devised a quicker method.

The cameras we used were the Panasonic G7 and the Blackmagic Production Camera 4K, which both record at 3840x2160p Ultra High Definition - but this technique should work for any camera brand and at any resolution.

Panasonic DMC-G7
Blackmagic Production Camera 4K
Over time, we found that under most conditions, we were making similar corrections to match the BMPC4K and Panasonic G7 on every shot.  Eventually, we asked ourselves, "What if we could quantify the differences between the cameras? What if we could create some sort of tool to apply precise, consistent corrections based on those differences?"

X-Rite ColorChecker Passport
Our first step was to order a color chart. We chose the X-Rite ColorChecker Passport for its durability, excellent reviews and competitive price. We set it up under the most color-accurate light source we could find:

Generic Main Sequence Star
Once that was done, we set the BMPC4K to its native ISO as it is more sensitive to noise than the Panasonic, and matched exposure by varying shutter speed since motion blur did not affect the test. We used the same lens at the same t-stop to take identical short clips of the color chart with each of our cameras, then we loaded the clips into the free and powerful Davinci Resolve color grading and editing suite from Blackmagic (if you want to export at 4K, you'll need the $299 paid version - you can download it here).
Panasonic G7 test shot
Blackmagic Production Camera 4K test shot
From here, there are a couple of options. Davinci Resolve actually has a built-in correction tool that when directed toward the color chart, automatically performs a correction for you. You can find it by clicking “color match” in the bottom left panel of the Resolve “color room”.   In some cases, this may be good enough.

But we wanted a tool with more versatility and more precise control. It took us a few more days of work to create it, but it has been well worth it.
First, we chose to leave the BlackMagic footage untouched and instead matched the (objectively poorer) Panasonic footage to it.
Next, we used the curves tool and the waveform monitor to match the exposure of each of the grayscale frames on the color chart. This was made slightly easier by rotating the clip 90 degrees and masking around the grayscale frames so they showed up clearly on the scopes. The process was then as simple as creating a point on the curve for each frame and adjusting it up and down until the waveform from the G7 matched that of the BMPC4K.

 We then used the “hue vs hue” and “hue vs sat” controls and the vectorscope to match the hue and saturation of each color frame on the X-rite card. This process was made easy by using the eyedropper tool to create a point for each frame, then moving the points up and down to adjust the hue and saturation until the vectorscope from the Panasonic matched that of the Blackmagic.

Next, we used the “hue vs lum” tool to match the exposure on the color frames in much the same way as we did the color adjustments. Finally, we shot the color charts under a few other light sources and reviewed our corrections a few times to be sure they remained reasonably accurate. This final “dialing in process” can take as long as you like, depending on how precise you’d like your LUT to be and how comfortable you are with the DaVinci Resolve toolset.
When we were satisfied, we exported the corrections as a LUT via “File>Project Settings>Color Management>Generate LUT From Current Grade”.
 Original Panasonic G7 CineD shot
Original BMPC4K shot
LUT-matched shots ready to be graded
Now, assuming our cameras are reasonably well white-balanced and exposed, we can easily match a day’s worth of shots in minutes with only minor corrections needed before moving on to grading.
The time saved and the quality of the results has easily been worth the cost of the color chart and the time invested. I highly recommend that anyone who regularly needs to match cameras as part of their job - or who wants to realize a creative vision without spending hours matching cameras in post - give this workflow a try.

What do you think? Do you plan to use this technique to match colors between cameras? We would love to hear from you. Please post your thoughts in the comments section below.
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