Within the effects tab of a source, there’s a masking tab. From here you can choose to hide specific parts of a source and create effects like a circle webcam. However, there may be times where you want to customize what can and can’t be seen in a source. This is where the File option comes in.
The file option allows you to specify your own mask file in the form of a black and white image (luma). Black indicates the parts of the source you want to be transparent and on the opposite side, white will be the parts of the source you want to be visible or opaque. This also means that gray will be semi-transparent. Depending on what end of the spectrum the gray is(closer to white or black), it will impact how transparent that part of the source is.
Lets start with an example. I’ve created a white star on a black background. Based on the description above, any parts of the source in the white star will be visible.
So when we apply the image we created, we can see that this is the case and only the star area is visible.
An added option is the ability to mask to stage. This means the mask we created will behave like we applied it to the whole stage. However, it will still only apply to the source we added it to. This means when we move the source around, the mask stays in place.
Because the mask is just an image, we could actually get fancy and create a black and white animated GIF and then apply it in the same way. Now we have an animated mask applied to the source.