H3 Window Seam (internal)

Another internal node, but this one does real work. H3 Window Seam lives inside H3 Window Expand and is what decides what happens where two regenerated windows touch. You won't usually wire it by hand - the expansion places it - but knowing what it does is how you read a windowed graph.

The two seam kinds

When a seam is cold - both sides of the boundary at hold 1, i.e. between bursts - this node is a passthrough. It just carries the planner's per-side crossfade widths into the splice map so the splice knows how wide to fade each edge.

When a seam is hot - a boundary forced inside a burst - it does the pin-and-trim trick. The next window regenerates an overlap of the already-recovered frames, uses that overlap as context, and then this node drops trim_head frames from the front of the recovered window and moves the splice start forward by the same amount. The previous window's output is kept; this window's second take of those frames is thrown away.

That's not a minor implementation detail. Crossfading two takes of the fastest motion in the clip dissolves between two independent samples of a moment, and on audio it doubles or smears a percussive hit no matter how wide the fade is. A hard cut at a pinned frame is the honest seam.

The inputs that do things

• segment - the recovered window on the world clock
• splice_map - the planner's map, which this node rewrites
• trim_head - how many frames to discard off the front at a hot seam (0 at a cold seam)
• fade_in / fade_out - the crossfade widths the splice will actually use (0 = hard cut). These are what make cold seams soft.
• segment_audio - trimmed by the same frame count, cut at the sample boundary

Outputs are segment, the rewritten splice_map, and segment_audio. When nothing else is wired it synthesizes a single sample of silence rather than erroring - the same convention the rest of the window family uses.

Why it exists

The whole windowed-regeneration family in ComfyUI-MAINodes (H3 Window Plan, H3 Window Loop, H3 Window Expand and friends, all alpha from 2026-08-12) is the pack's answer to one question: how do you de-rope a clip your card can't hold dilated all at once? Seams are the tax you pay for splitting. H3 Window Seam is the part that refuses to let that tax become a smear - video and audio. It's one of the three side-by-side window implementations the pack is deliberately comparing, so its placement may move before one of them wins, but the seam policy it implements is shared across all three.