1080p in a 4K timeline?

[Joe Marler]

Yes, 1080p in a 4k timeline is fine. In the inspector pane, by default FCPX uses spatial confirm>fit. There is a scaling issue that may slightly degrade resolution IF using 4k in a 1080p timeline and exporting as 1080p, vs using 4k in a 4k timeline and exporting as 1080p. The severity of this seems to vary based on the source codec.

Here is a test I previously did on this issue: https://joema.smugmug.com/Image-Tests/n-d3hVjt/

There is limited performance cost to using mixed 1080p and 4k in a 4k timeline for 1080p export. The internal render files are bigger but in most cases the performance cost is small.

For Youtube export at the best quality, I’d suggest this preset: File>Share>Master File>Settings, Format: Computer, Video Codec: H.264 Better Quality, Resolution: 3840 x 2160.

Most Youtube playback will be at 1080p or below, but starting with a 4k upload can give a better result.

Another common quality issue is mixed frame rates. Slow motion is OK since it’s played back slow, but if you have mixed 23.98 and 29.97 material played at 1x rate this can cause jerky motion cadence. The solution is select the clips that don’t match the timeline frame rate, then in the FCPX video inspector, scroll to bottom and under Frame Sampling, select Optical Flow. Then select that clip in the timeline, render with CTRL+R, and examine the playback smoothness.

If after re-encode on the Youtube server, there are any visible issues during streaming playback such as banding, you could try a short test exporting as ProRes and uploading that. Another possibility is exporting as ProRes then using Handbrake to transcode to 10-bit HEVC, which is much smaller and preserves more tonal data, then upload that. Unfortunately current versions of FCPX do not use hardware acceleration for 10-bit HEVC, so are impractically slow. You must use some other software for ProRes to 10-bit HEVC transcoding.