Masks can be applied on the composition, layer or clip level.
To create a mask, drop an image file from the file browser onto the area of the Composition, Layer of Clip tabs where it says 'Drop effect or mask here'.
If a .png file with an alpha channel is used, the alpha channel is used for the mask.
If another image format is used, the red channel is used for the mask.
The -I- button inverts the mask, this will make what was visible become invisible and vice versa.
You can use the -B- button to temporarily bypass a mask, the -X- button removes the mask completely.
Just like with effects, the render order of masks and transformations can be swapped.
By default a mask is applied before any transformations, so you can move, scale and rotate the content, while the mask stays locked to the content and moves with it. This way you can make slight adjustments to your mask to account for lens distortions or misaligned projectors.
Mask applied before transformations and effects
If you want to apply a mask and reposition the content independently from the masked area, you can drag the Mask tab below the Effects and Transformation tabs. This way you can animate or apply effects to content within a mask, without affecting areas outside the mask.
Mask applied after transformations and effects