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 ( 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.

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.


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.


Natron’s Resize & Transform Nodes: The need for pixel/bitmap based filters

A NatronNation Tidbit,

Filters are very important and needed in compositing when dealing with pixel base or bitmap media. Not all the nodes has the same filters. They are process specific. The attached images are from the Resize & Transform nodes and help note from the Resize node to guide you are recommended filters. The best way to see how they process your pixel/bitmap media is to zoom in to sub-pixel level and switch from filter to filter. There is an a lot of math going on.

The Resize node has gain a few extra filters over the Natron V2 beta development. Study and apply them when needed or necessary.

See attached images below.



Transform & Cornerpin

Keyer Node or Chromakey Node: Colorlookup node tidbit

Here is a NatronNation tidbit,

This is concerning pulling a key using the Keyer Node or Chromakey Node. I have been practicing using the Colorlookup node curves to adjust my keys instead of using the Key Lift & Key Gain when pulling my keys. I find sometimes on certain material you have to crank the acceptance angle higher than I want to and trying to find the best center position in the Keyer node to get that good high contrast matte. Sometime the edges may suffer based on the hue you have to pull.
The Colorlookup node curves offers you more control over crushing your blacks and expanding your white to get that perfect contrast. You can create anchors and pivot points in the curve to limit which range you want to adjust, rather it be shadows, mid-tones or highlights. The attached image shows a very basic screen shot of what it looks like in action.


The Key Lift & Key Gain performs great but you don’t have that extra control on the mid tones that may effect your edges. Just remember to insert the Colorlookup node after your keyer/chromakey node and use the Alpha Channel curve to finesse your matte.

P.S. Also use your gamma switch control at the top of the viewer to do a final check and pass on all your keys. This process is called slamming your gamma. Most times to the necked eye we can’t see our mattes properly if our monitors are not calibrated for the right gamma display. This is most useful when using the curves from the ColorLookup node for final adjustments.

Natron Keyer Node Tutorial Demo

This is a Natron Keyer Node tutorial demo. I’d explained the difference modes that the Keyer has how to us them in basic scenarios. The Keyer node is similar to TheFoundry Nuke keyer node, but Natron Keyer has a couple more functions and features. Join me at NatronNation –