This is a nice video that quickly goes through four ways you could mask portions of your video during the editing process. Masking can be used for a variety of things from covering up something you don’t want the viewer to see to lightening a specific portion of your video.
From a journalistic standpoint, you shouldn’t be removing parts of your video and replacing it with another part, that’s changing what actually happened and that’s a journalistic no-no. However, these techniques could be useful for a PSA or some promotional video that you’re making.
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