Natron Motion Tracker & Roto Shape/Mask: Digital Compositor Tidbit

Hello Creative Intellect,

We are seeing more and more films rather they are short or full featured incorporating the use of green screen for background and cost free location replacements. In order to replace these green screens for background photography, the green surfaces needs a marker that a motion tracker can lock on to. These markers would be known as features which color is high contrast. This make it easy for the motion analysis engine to solve a reverse engineered motion.

The image below is a screen capture of an open source digital vfx compositor called Natron (www.natron.fr). It has the ability to composite hundreds of images and effects together using a node based pipeline. On the top left portion of the image you should a woman being filmed against a green screen with a blue marker on the surface. The blue marker provides a nice contrast against the green for Natron’s motion analysis node (aka Tracker) to lock on to. Once the tracker has down all the tracking it needs to do, I can export that tracking data or link to it via right clicking on other nodes that generates transformation or movement.

I am using the Tracker for two processes. One is for matching the movement of the marker for background/environment replacement. The second, is that I am using that same tracking data to mask out the markers so that it doesn’t show or bleed on to the background footage.  I am using a roto node to draw a shape/mask to cut out that marker. All I have to do is link mask my Roto shape to the tracking data and it matches every movement of every frame that the motion analysis node generated. This is a common and an effective way to clean up your shot when roto work is required in the visual effect world. It is a great time saver.
Tracker_Roto_Full

The image below shows a close up of the tracker and on the right side it shows the roto shape acting as a mask. Also in the window is a dialog box that allows you to choose which tracking data that provides the motion analysis.

When working with green screens requires at lease one to four markers for translation on the XY axis. That is known as a 2D track. 3D tracking requires more features to track, but that is another tidbit. I could have animated the roto shape using its own auto keying features, but that would have required more time. This image photo is a video clip with one blue marker to track. Natron gives you the option to link all of the mask points to the center of the marker. This way all the manual shifting of the roto shape/mask is completely unnecessary. The tracker in Natron allows you to export the tracking analysis into a corner pin node, stabilize your shots, and/or link your motion data between nodes.

Tracker_Roto_Full_closeup

This is very basic workflow for what you can do in any digital vfx compositor. Natron tracker is simple to use. You can control a multitude of effects using the Natron’s tracker. If you have more that one tracker to paint out, use the Natron’s tracker and attach a rotopaint node shapes over the marker and link to those trackers.

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