Hi there,
First of all, make sure that the Rotobrush has created the transparency you want by clicking on the transparency grid in the comp panel. You should see the grid where there is transparency. You can also use the Channel menu to view the Alpha before exporting.
Secondly, Prores 444 codec works with Trillions of colors so you need to be working in a 16 or 32-bit project to render that color depth. 8-bit’s won’t cut it I’m afraid, only Millions of colors!
Alternatives to try if you’re staying in an 8-bit project are the Animation Codec and the PNG codec, both of these render with Alpha and are lossless codecs.
Hope this helps.
cheers,
Angie
Angie Taylor animation & illustration for television, film, web and devices
https://www.angietaylor.co.uk
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