Compositing

A Merge node combines data that already belongs to the same compatible system. A Render node usually converts a richer system—Shape, particles, 3D or USD—into a pixel image that can re-enter the regular 2D pipeline.

That simple distinction solves a large share of confusing Fusion connections. Similar names do not mean interchangeable data types.

The normal Merge works with image layers: Background, Foreground and an optional mask. MultiMerge extends the same image-compositing idea when many 2D layers need to be managed together.

These nodes belong near the final image assembly stage, after any specialized Shape, Particle or 3D branches have been rendered to images.

Merge3D combines classic 3D scene elements. SMerge combines vector Shape elements. pMerge combines compatible particle streams. UMerge combines USD scene data.

Each one is useful inside its own world, but none of them automatically converts that world into a normal image. That is why a separate render boundary still matters.

SRender turns Shape data into pixels. pRender (2D) turns a particle simulation into pixels. Renderer3D renders a classic 3D scene. URenderer handles the USD-oriented scene output path.

After that conversion, ordinary image-processing tools can work again. This is the point where a specialized branch can be merged over footage, graded and delivered.

Render late when you still need to change object-level behavior inside the specialized system: particle forces, 3D transforms, Shape geometry or USD hierarchy. Keeping the native data alive preserves control.

Render earlier when the branch is finished and the rest of the work is purely 2D. This can simplify the Flow and sometimes improve performance, especially when a heavy subsystem no longer needs to be recalculated.

When a graph becomes complicated, identify every Merge-like node and ask what data it is combining. Regular Merge combines images, Merge3D combines scene objects, SMerge combines Shape data, and particle or USD merge tools operate inside their own systems.

Then locate the render boundaries. SRender, pRender (2D), Renderer3D and the USD renderer are conversion points where specialized data becomes an ordinary image. Everything after that point can usually enter the familiar 2D compositing pipeline.

This reading method is faster than inspecting every parameter. First determine the data domain, then the merge operation inside that domain, then the render boundary that hands the result to the next domain.

Imagine a shot with a Shape-based lower third, a particle spark pass and a 3D logo. Each branch is built in its own native system: Shape → SRender, particles → pRender (2D), and 3D scene → Renderer3D.

The three rendered images can then enter the regular 2D pipeline and be layered over MediaIn with separate Merge nodes. Color, blur, glow and grain matching can be applied after each render or during the final assembly as appropriate.

This structure keeps the graph modular. A change to particle behavior stays inside the particle branch, a camera adjustment stays inside 3D, and the final Merge chain remains responsible only for image compositing.

Ordinary Merge and MultiMerge accept images. SMerge combines Shape data, pMerge combines particles, Merge3D combines classic 3D elements, and UMerge combines USD scene data. pRender is a special case: 2D can go straight to Merge, while 3D first goes through Merge3D/Renderer3D. The word Render alone does not guarantee an image output.

For a 3D title, assign material to Text3D and join Text3D, Camera3D and a light in Merge3D. For a Shape panel, join SRectangle and SText in SMerge. These are parallel sources for assembly, not sequential Text3D → Camera3D or SRectangle → SText chains.

In an ordinary Merge, check Background, Foreground, Apply Mode, Operator, Blend and Effect Mask in turn. Standard compositing uses Over; Screen/Add modes do not replace correct alpha. If a layer disappears, inspect both sources separately and make sure the current frame lies within their valid ranges.

Rendering converts the scene into an image of a defined size. A downstream Transform cannot rotate its camera or automatically make separately rendered objects intersect according to 3D depth. If shared occlusion and perspective matter, assemble a compatible scene before Renderer3D. For separate passes, plan Z/matte data and the compositing method in advance.