I was working on a project recently where I was bringing in some rendered After Effects layers with alpha channels. I layered the imported ProRes 4444 files in the FCPX timeline but I was getting a weird black line around the mask blur. It turns out the problem is way FCPX interpolates alpha channels. By default it’s set to ‘straight’. It needs to be set to ‘premultiply’, but the location of the settings are not immediately apparent.
To change to ‘premultiply’ you need to select the clip, and click on the info tab. Then at the bottom of the info tab you need to change from ‘general view’ to ‘settings view’. You’ll then be able to see the option to change the way alpha is handled for that selected clip.
