Compositing: Combining Rendered Passes in the Compositor

Chapter 8's own tip box pointed here directly: simulated results often get rendered as separate passes, combined afterward in the Compositor. This chapter covers exactly that — Blender's own node-based post-processing system for combining, adjusting, and finishing a render after the fact.

What Compositing Actually Solves: Combining Elements After the Render

Compositing combines separately rendered elements — a foreground character, a background environment, a simulated cloth or particle pass — into one final image or sequence, after each element has already been rendered on its own. This separates the heavy computational work of rendering from the more flexible, iterative work of adjusting how those rendered pieces actually fit together.

Render Passes: Splitting a Single Render Into Separate Layers

A render pass isolates one specific component of a scene's own final appearance — the raw color, the shadows alone, reflections alone, a specific object rendered against a transparent background — as its own separate output, rather than baking everything into one single, inseparable final image.

The Compositor: Blender's Own Node-Based Post-Processing

The Compositor uses the same node-based interface as Chapter 5's own Shader Editor — data flowing through connected operations — but works on already-rendered images and passes rather than a live 3D material. A render's own output feeds in as an input node, flows through whatever adjustment and combination nodes are needed, and comes out the other end as the finished result.

Combining Passes: Mix Nodes and Alpha Over

A Mix node blends two image inputs together according to a chosen blend mode; Alpha Over specifically layers one image on top of another using its own transparency information, placing a foreground element convincingly in front of a separately rendered background — the same underlying idea as layering elements in Raster Editing Fundamentals, applied here to rendered 3D output instead of raster images.

Color Grading in the Compositor

Beyond combining separate elements, the Compositor also adjusts overall color and tone after rendering — brightening, adding contrast, applying an overall color tint — the same underlying color-grading concept Video Editing Fundamentals' own Chapter 6 covered for video footage, applied here to a Blender render's own output.

Blender's Own Compositing vs. Compositing Elsewhere on This Site

"Compositing" means genuinely the same underlying thing in each place it's used across this site — combining and adjusting already-produced visual elements after the fact — even though the specific tool and the source material differ each time: Raster Editing Fundamentals' own imageedit2 combines separate photographs, Video Editing Fundamentals' own color grading adjusts edited video footage, and this chapter combines Blender's own rendered 3D passes.

Hands-On Exercises