render_video_timeline

Edit assets the user already has into ONE finished video — sequence clips, add transitions, overlay
text or logos, layer audio. This is editing, not generating new content.

Estimate with `estimate_video_timeline_price` first. The render is asynchronous; check the returned
`generation_id` with `get_generation_run`.

Pass `timeline_object_id` whenever that board timeline already exists — one you placed with
`create_canvas_timeline` or read with `read_canvas_timelines` — so the render updates it in place
instead of placing a duplicate. It is the one input `estimate_video_timeline_price` does not take,
so add it back on top of the arguments you estimated with. Omit it only when no board timeline
exists yet.

Parameters

Parameter	Type	Default	Description
workspace_idrequired	string (uuid)	—	The user's workspace id.
session_id	string (uuid)	—	The Session ID to attach the results to.
session_name	string	—	Creates a new session with this name. One of `session_name` or `session_id` must specified, but not both.
templaterequired	TimelineTemplate	—	Timeline layout designed in the editor. Tracks, items, timing, and slot references.
tracks	TimelineTemplateTrack[]	—	Ordered list of tracks in the timeline. Composited in reverse order (Track 1 is on top). Do not overlap items within the same track; use multiple tracks for overlapping content.
idrequired	string	—	Unique identifier for the track.
namerequired	string	—	User-facing name of the track.
itemsrequired	any[]	—	Ordered list of items on this track.
transitions	any[]	—	Transitions on the seams between adjacent items on this track.
disabled	boolean	false	Whether the track is excluded from rendering.
fps	integer≤120, >0	30	Frames per second of the output video.
width	integer2–7680	1920	Width of the output video, in pixels. Must be even.
height	integer2–4320	1080	Height of the output video, in pixels. Must be even.
codec	string	h264	Video codec for encoding the output. Only h264 is supported.
crf	integer0–51	18	Constant Rate Factor controlling output quality. Lower values produce higher quality.
max_duration_in_seconds	number≤120, >0	—	Maximum allowed duration of the rendered video, in seconds.
playback_rate	number≤4, >0	1	Global playback speed multiplier. 1.0 = normal, 2.0 = double speed, 0.5 = half speed.
videos	object	—	Slotted video assets. Each slot name matches a slot reference in the timeline template.
images	object	—	Slotted image assets. Each slot name matches a slot reference in the timeline template.
audios	object	—	Slotted audio assets. Each slot name matches a slot reference in the timeline template.
timeline_object_id	string (uuid)	—	The board timeline this render writes to. Pass it whenever that timeline already exists on the board: the template alone does not identify it, so an id-less render places a second timeline beside the existing one. Omit it only when no board timeline exists yet.