Gen-4.5 true HDR (BT.2020) vs a Sume MP4 finish

If your deliverable must be HDR, Runway's documentation describes Gen-4.5 HDR outputs as true HDR renders, while Sume's documentation describes MP4 outputs and a Timeline render with no HDR or BT.2020 claim. Do not assume HDR metadata survives a Sume finish; treat Sume results as ordinary MP4 files.

What Runway documents

The Runway models guide says Gen-4.5 and Aleph 2.0 both offer ProRes and PNG-sequence containers and 10-bit SDR, and that Aleph 2.0 does not deliver native HDR. For Gen-4.5 it states that "Gen-4.5 HDR outputs are true HDR renders - graded into BT.2020 with measured HDR10 metadata".

What Sume documents

Completed Sume video jobs return artifacts hosted on media.sume.com with content_type: video/mp4. Timeline 1.0 compiles ffmpeg on Sume's worker and returns one MP4; callers never send filtergraphs, codecs or shell fragments. Video trim's exact mode is a frame-accurate re-encode with libx264 and yuv420p. The docs do not describe HDR, BT.2020 or 10-bit handling for any of these.

Re-encoding steps are where color metadata can be lost, so if HDR matters, finish in your own grading tool.

A workable split

Use Sume for what its docs support: generate clips, trim ranges, assemble a cut with a voice spine in Timeline 1.0, and burn captions. Timeline's default output is 1080 by 1920 and accepts output.width, output.height and output.fps of 24, 25, 30 or 60.

If the source material must keep HDR, grade and deliver outside Sume and use Sume only for non-HDR variants such as social cutdowns. Say that up front to whoever signs off on the deliverable.