What Is CGI in Visual Effects and How Does It Fit Into Compositing?
CGI (computer-generated imagery) is any image or moving image element created with software rather than captured with a camera. In visual effects, CGI supplies the parts of a shot that cannot be filmed — a digital creature, a destroyed building, a set extension — and compositing is the stage where those rendered elements are combined with live-action footage so they appear to share the same camera, light, and space. If you are trying to understand where a node-based compositor such as Natron sits in that process, the short answer is: CGI is generated upstream (modeling, animation, rendering), and compositing is where it is integrated, corrected, and finished.
CGI vs. live-action vs. practical effects
These three terms describe how an image element originates, not how good it looks.
| Element type | How it is made | Typical role in a shot |
|---|---|---|
| Live-action footage | Captured with a camera | The base plate the shot is built around |
| Practical effects | Physical objects, makeup, miniatures, pyrotechnics, filmed on set | Real light, real texture, real interaction with actors |
| CGI | Built and rendered in software | Anything unfilmable, too dangerous, too expensive, or impossible to repeat |
A single shot usually mixes all three. A practical explosion might be filmed on set, a CGI building added behind it, and both combined over live-action actors. The audience only sees the final composite.
Where compositing enters the pipeline
CGI does not arrive at the compositor as a finished picture. It arrives as rendered layers — often many of them — with separate passes for color, shadows, reflections, and transparency. Compositing is the step that:
- Places CGI elements into the correct position, scale, and perspective relative to the plate
- Matches color, contrast, and lighting direction between CGI and live action
- Removes unwanted parts of the plate so CGI can replace them
- Adds atmospheric effects, grain, lens artifacts, and motion blur so the join is invisible
This is why compositing is often described as the last major creative stage: it is where separately produced pieces become one believable image.
The core tasks that connect CGI and compositing
Most CGI-integration work reduces to a handful of recurring operations. Natron's feature set maps directly onto them:
- Keying — pulling a matte from a green- or blue-screen plate so a CGI background can replace it. Natron lists "industry-proven chroma keying and matte refinement tools."
- Roto — hand-drawing mattes around objects the keyer cannot isolate. Natron provides a "multi-layered rotoscoping toolset with per-vertex feathering."
- 2D tracking — following features in the plate so CGI elements stay locked to the camera's motion. Natron offers a "fast, pixel-perfect tracking toolset."
- Matte creation — building the masks that control where CGI and live action mix. Natron's node workflow is aimed at creating "pixel perfect mattes, set-extensions, and more."
- Plugin-based operations — many CGI-integration tools ship as OpenFX plugins. Natron supports "industry-standard OFX plug-in support."
How node-based compositing fits
In a node-based compositor, each operation is a node and the image flows through connections between them. Natron's page describes this directly: "No more hunting through nested pre-comps! Nuke and Fusion users will be right at home with Natron's node-based workflow. Group, merge, and tinker to achieve just about any effect."
For CGI work this matters because integration is rarely linear. You might need the same keyed matte feeding three different branches, or a tracking node driving both a CGI insert and a grain match. Nodes make those shared dependencies visible and reusable, and Natron lets you "build, group, and save your own node snippets to create and share custom tools and presets," with drag-and-drop installation of community-made PyPlugs.
A concrete example
Say a shot needs a CGI tower added behind an actor on a rooftop.
- The plate is loaded and a 2D track follows fixed points on the building so the camera move is known.
- A roto node draws a matte around the actor so the tower can sit behind them.
- The rendered CGI tower is placed and aligned using the track data.
- Color and light are matched to the plate, and grain is added so the CGI does not look too clean.
- The matte controls where the tower is hidden by the actor and where it is visible.
Every one of those steps is a compositing operation, and each is a node in the graph.
Where Natron itself fits
Natron is an open-source, node-based 2D compositor for VFX. Its stated toolkit is the "standard 2D compositing toolkit," with OpenFX plugin support, 2D tracking, roto, keying, curve and dope-sheet editors, and headless rendering for "smooth headless integration with render farms for distributed network rendering."
Practical details if you want to try it: the latest version listed is 2.5.0, released November 25, 2022. It runs on macOS (Intel; the latest stable release runs on Apple Silicon via Rosetta 2, with a native alpha build available), Windows 8.1/10/11 on 64-bit x86, and GNU/Linux on any x86_64 distribution with X11, glibc 2.17+, and libgcc 4.8+. Installers and portable builds are offered, plus a Flatpak for Linux. The site also points to community plugins, community scripts, sample projects, and support through Discord, the PIXLS.US forum, and a Facebook group.
The page does not state pricing or licensing terms beyond describing the project as open source, so check the project's own documentation before assuming how it can be used commercially.