Onion skinning

Formo's onion skinning isn't a simple "show one frame before and after" toggle; it's a configurable system: how many frames back and forward, whether it shows keyframes only or every interpolated in-between too, and which of two render styles you're reading it in.

Turning on onion skinning

A dedicated onion-skin button sits in the timeline panel. Click it to toggle onion skinning on or off. It highlights when active, so you can tell its state at a glance without opening anything.

Seeing the frame before and after

With it on, the frames around your current one show up as ghosts, layered right behind whatever you're currently drawing — past poses in red, future poses in blue. They're built from your stroke geometry, traced in the same paint order you drew them in, so the ghost is the shape of what's on the frame before or after.

It stays fast. Onion skinning runs on every redraw — every frame, every drag, every scrub through the timeline — without lagging behind your hand. This isn't accidental: a dedicated performance pass brought the ghost-rendering cost down from 2.31ms to 0.07ms per redraw at 600 animated nodes, alongside a separate improvement to pose evaluation itself (3.43ms down to 1.50ms). This is the kind of headroom that's specifically what keeps onion skinning invisible-until-you-need-it rather than something that makes the canvas feel sluggish once it's on.

Click the gear icon next to the onion button to open its settings popup.

Choosing color or outline mode

A Style control switches between two ways of rendering the ghosts:

- Color (internally called "tint"): a solid, filled silhouette in the past/future color. Fast to read at a glance, especially good for judging a big pose or a silhouette shift.

- Outline: just the linework, stroked instead of filled. Useful when you need to see through the ghost to whatever's underneath it, rather than having it block your view of the current frame's own drawing.

Setting how many past and future frames to show

Click the gear icon next to the onion button to open its settings popup. Two numeric fields, Past and Future, each ranging from 0 up to 10, control exactly how many frames back and forward get ghosted. Set Past to 1 and Future to 0 for a classic "just the previous pose" reference; push both up toward the higher end to see a fuller stretch of motion at once, useful for judging the arc of a longer action rather than just the immediate neighbor frame.

Choosing keyframes or interpolated mode

The same settings popup has a Mode toggle most onion-skinning implementations don't offer at all:

- Keyframes: only actual, hand-set keyframes get ghosted. In-between frames that are just holding or interpolating don't show a ghost of their own.

- Interpolated: every frame in range gets a ghost, including the computed in-between positions between two keyframes, not just the keyframes themselves.

This matters specifically for keyframed properties (not just hand-drawn cels): if you're judging the arc of a tweened motion rather than just comparing drawn poses, Interpolated mode shows you where the motion actually passes through frame by frame, not just where you deliberately placed a key.