How do add a mask to a layer?

Question

It may be basic but it seems confusing, please I need help

Answer 1

Assuming that the variable myLayer is a layer object, the collection of all its masks is: myLayer.property("ADBE Mask Parade");
and its children are the actual masks groups (which can be renamed). To add a mask to the collection you do:
var myMask = myLayer.property("ADBE Mask Parade").addProperty("ADBE Mask Atom");
Then you can set its name, color, rotoBezier attributes etc (optionnal):
myMask.name = "My mask";
myMask.color = [1,0,0];
myMask.rotoBezier = false;
myMask.maskMode = MaskMode.ADD;
To set its keyframable properties: the maskPath property has special propertyType (SHAPE).
You need to create a Shape() object like this before setting the value:
var myShape = new Shape();
// set the vertices (coordinates of the vertices in layer coordinates, not comp)
// + the inTangents (coordinates of the incoming Bezier handles with respect to the the corresponding vertices)
// + the outTangents (coordinates of the outcoming Bezier handles with respect to the the corresponding vertices)
// + the closed attributes
var w = myLayer.width, h = myLayer.height;
myShape.vertices = [[w/2, 0], [w,h], [0,h]];            // here: array of 3 "points"
myShape.inTangents = [[0, 0], [0, 0], [0, 0]];        // array of 3 "vectors"
myShape.outTangents = [[0, 0], [0, 0], [0, 0]];        // array of 3 "vectors"
myShape.closed = true;
// now set the value:
myMask.property("ADBE Mask Path").setValue(myShape);
In this example all tangents are 0 which gives a mask with straight contour.
Unless you already have your shape data ready, inserting more complexe values for tangents is not so straighforward.
Xavier.