Is it possible to "filter" our media that is already on the timeline?
eg: If I import 100 clips, and have used 25 - I would expect to filter out 25 clips and only view the 75 that have not been used yet.
I don't know if you can "filter out" used media, but what I would do to keep things organized would be to create a folder called "Used Media" and move them into that folder as I use them. You might even create subfolders inside the main folder categorizing each asset even further. You could categorize based on media type (i.e. pictures, video clips, etc) or maybe based on what part of the project the media is being used in. Whatever you do, it should be easy to find the asset in case you need it again.
Certain assets, like planes for example, can be used over and over again in different parts of a project, but you wouldn't want to adjust the properties (i.e. color, size, etc...) of the plane because that would cause it to be adjusted throughout the project. Instead, you would want to add an effect to change the plane (i.e. Color Fill to change the color, etc...)
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