Shape layers that are created with everything except the pen tool are defined by rules that don't apply to masks. The size, radius, number of points, number of sides are defined by numbers. These numbers create the vectors that create the shapes. Masks, even when created with the shape tools, are converted immediately to paths. Once a mask is drawn you cannot change the number of points in a star by simply adjusting a number. A shape layer, on the other hand, that was created using the shape tools (Keyboard shortcut q) can be edited after the shape is created by simply adjusting the number of points, inner radius, outer radius and so on. You'll find the shape properties in the timeline under Contents, then Polystar, Rectangle and so forth. If you want to take a mask and convert it to a shape layer then all you have to do is select the mask path, copy that, then use the pen tool (keyboard shortcut g) to draw any path in the comp window. You then spin down the shape layer's Contents, drill down to Shape>Path, then select the path and paste. Your mask is now a Shape Path. When copying and pasting shapes I always find it easiest to set a keyframe for the mask path, select the keyframe, copy that, then go to the Shape Layer >Contents>Shape>Path>path and set a keyframe there, highlight the keyframe and paste. This assures you that you're moving the path info and it always works. That's also how you would turn an animated mask into an animated shape layer. I hope this helps.
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