What causes digital artefacts in Final Cut Pro while exporting videos?

You may have out-of-gamut material which reacts differently on various monitors. In the FCP viewer, do View>Range Check>All and make whatever adjustments are necessary to avoid illegal luma or saturation levels:

Detect out-of-gamut colors in Final Cut Pro for Mac - Apple Support

As a stopgap, you can apply the built-in FCP "Broadcast Safe" filter. Increase the slider if needed until no luma or saturation zebra pattern is showing.

Then export the video in that state and test it on various monitors. This assumes you are in an SDR library and the timeline is Rec.709.

Just looking at a photo of the screen doesn't give us the needed troubleshooting info. As a first step, we need to get our hands on a piece of your exported clip. That need not be the entire timeline. If a 3-sec piece of it displays the problem on your end, just export and send us 3 seconds of it. Use the same export format you normally use.

To export a (for example) 3-sec section of your timeline, press the "R" key to select the range tool, drag that over the 3-sec timeline region, then export as normal. It will only export the highlighted range of the timeline. Even though only the primary storyline layer is highlighted, all layers above and below that range will be composited together, rendered and exported.

Also, please export and send a project XML. Even without the underlying media, we can examine the effects and other attributes.

As Louis said, it would also be useful to see a corresponding piece of the original source clip. You can use Quicktime Player to do a non-encode trim of the original camera clip, producing about the same 3-sec range as above. Despite not having any effects and not including layers above or below that, it's still a key item we need to examine.

Quicktime Player User's Guide on trimming: Trim a movie or clip in QuickTime Player on Mac - Apple Support (JO)

For a video tutorial, go to Youtube and search on this phrase: "Easily Trimming Video Files With QuickTime Player"

Mike-DS wrote:

Hi Louis - so how can I share a Source clip - its several overlaid using masking.

Thanks

Mike

By source clip I mean the clip as you imported into FCP, without any masks, etc.

You can select the clip in the browser, control-click and choose Reveal in Finder.

Then you can send that to yourself using wetransfer, and post the link you receive here.

It looks like these are artifacts that may have been introduced at some point, probably already present in the original file, and that become more pronounced when you compress the file using a lower bit rate (which is not surprising, of course).

It is hard to say without at least a frame from the original, or better yet a short video clip.

It may be that one can apply a mask and place a generator below the clip, but some care must be taken to make it plausible looking.

What are the original clips? In what format/codec?

Do you see anything like that in the viewe in FCP timeline, if you zoom in?

Would it be possible to share a small example source clip so we can have a look and test?

It could be some weird encoding.

I am interested in seeing how the original clips look, and what the codec is (maybe H264, or maybe some older codec like MotionJPEG or something - since you mentioned these were made long ago). If you imported into FCP, you can locate it in the Finder and make a copy, then send that.

One way would be to send one such file to yourself using wetransfer, and posting the link here; or putting it on some online service such as onedrive, and posting the link here. It is possible that the link is removed, but those of us following will still get it in the e-mail notifications.

Without a short clip it is impossible to tell what you mean . . . a picture is worth a thousand words so a video clip . . .

Mike-DS wrote:

Hi Louis - your suggestions all make sense but the problem has a more basic explanation or solution I think : I’ll upload a photo from my monitor today - When I stand up and look down on the monitor the problem is visible right there. Its a black rectangle with an image within laid on a black background....I get notifications you and another have replied, but I cant see you posts.

Make sure that you are viewing the thread in chronological order.

Right before the initial post, near the right side, you can see

Sort by: .....

Make sure that it says "Oldest".

Someone decided that the default sort order should be determined by upvotes, and not based on order of posting. This makes a total mess.

To prevent having to do this all the time, you can go to your profile and set the default order as Oldest. That (provided that you are logged in when perusing the forums) will make sure that sensible order of posting is maintained.