[difficcd]

Continuing the overhead work (오버헤드 더는것을 최우선목표로). This is the one behind "선이 30획 이상이 되었을 때".

Cause

Committing a stroke re-bakes the layer's cache canvas, and bake() drew every stroke the layer holds from a cleared canvas. So the Nth stroke of a drawing costs N times the first, and a session's baking grows with the square of the strokes drawn.

Measured on a 1920×1080 layer, one full re-bake:

strokes	1	10	20	30	40	60
brush	23ms	179ms	1031ms	1603ms	2068ms	2985ms
pencil	38ms	203ms	310ms	325ms	613ms	840ms

And the total cost of actually drawing 40 strokes, one at a time:

	before	after
brush	37160ms	2113ms	17.6×
pencil	15436ms	742ms	20.8×

Skia on the CPU, so the absolute figures sit far above a browser's — the ratio is the point, and it is inherent: drawing N strokes costs N times drawing one, on any renderer.

Change

When a layer's strokes are the ones already on its canvas plus more on the end, only the new ones need drawing. They paint on top, which is the order they would have been drawn in anyway.

appendedAfter(stored, layer, rough) recognises that case by asking what the signature would have been when the layer was shorter (strokeSig/layerSig take an optional count). Everything else — a stroke edited or removed, the layer dragged, a merge, a canvas resize, a boiling phase — still bakes in full.

Why this is safe

It leans on the invariant the cache already needs, and no more. strokeSig has only ever looked at the stroke count and the last stroke's identity, so an edit that changes neither is invisible to it — and rev exists precisely to cover that. offsetLayers says so in its own comment:

Only coordinates change, so the cached canvas signature — built from stroke count and the last stroke's identity — does not notice, and the layer would keep drawing at its old position. Bumping rev is what invalidates it.

A prefix that changed under a cache that did not notice is already a stale cache today. This reuses that assumption rather than widening it.

Two further guards in bake: the stored signature is read before sizeCanvas, because resizing clears the canvas and then nothing is drawn however much the signature claims; and boiling layers are excluded outright, since each phase keeps its own canvas and an append would have to reach all of them.

Tests

test/core/appendedAfter.test.js — 11 cases, and nearly all of them are refusals, because saying "appended" when something else happened leaves stale pixels with no symptom:

• a bumped rev refuses the append even though the strokes only grew — the safety argument, pinned.
• the last stroke growing points - the eraser still drawing - is not an append
• a signature claiming more strokes than the layer has is refused
• a boiling layer signs per phase, so a different phase is not an append

And the claim itself, checked on bytes rather than on reasoning, in test/canvas/pixels.test.js:

• drawing only the new strokes gives the same pixels as redrawing the layer — every split point of a four-stroke layer, with an eraser among the appended part, because destination-out composes against what is already on the canvas and is the case most likely to come apart. Byte-identical at all three splits.

1292 tests pass; npm run check green.

To try

Draw thirty or forty strokes on one layer and keep going. The pen should stay as responsive at the fortieth stroke as at the first — before, each commit got steadily heavier. Nothing should look different; if a drawing ever renders wrong after an undo, a layer move, or a merge, that is this change and worth reporting.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated
easy-mv-maker	Ready	Preview	Oct 1, 2026 6:28am UTC