Artist time costs more money than software licenses do, and as the open source software isn't generally good enough for their needs, using better off-the-shelf software that have fairly decent and flexible plugin APIs is good enough for them.
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A lot of the code VFX studios have is proprietary (as in unique to the individual studio who wrote it) plugins for host apps like Maya, Katana and Nuke, and shaders/integrators for renderers like PRMan and Arnold.
Blender is the free and open source 3D creation suite. It supports the entirety of the. With Nuke Non-commercial, you get access to free, non-watermarked versions of Nuke, NukeX and even top-of-the-line Nuke Studio for as long as you like, so you can learn, explore and have fun on your own schedule. Video Compositing Green Screen Chromakey Software Windows Mac Natron Online Code Brand: the software dude Platform: Mac OS Sierra 10.12, Windows 8.1, Mac OS X El Capitan 10.11, Mac OS X 10.8 Mountain Lion, Windows 8, Windows 10, Windows 7, Mac OS X 10.10 Yosemite, Mac OS X 10.9 Mavericks. The Nuke family makes up a fundamental ecosystem of tools with a global, industry-wide reach and impact—powering up pipelines, talent pools, and studios large and small. Made up of Nuke, NukeX, Nuke Studio, Hiero and HieroPlayer and Nuke Indie, this family of formidable tools seamlessly tackles high-end compositing, conform, VFX editorial.
There are open source projects that are used for glue in the VFX industry:
But these have almost exclusively been internal proprietary projects started within studios that have been open-sourced, to help transfer assets between studios, and are mostly open file formats for images, volumes and geometry, as well as other intermediate tooling.