[Theo Burtis]

Solved:

//This mask expands left to right from a point designated by a slider effect in a seperate null object layer.

var StartPosition_Slider = thisComp.layer(“Write-on_Controller-RectangularMask”).transform.position
var StartPosition_CompSpace = thisComp.layer(“Write-on_Controller-RectangularMask”).toComp(StartPosition_Slider);
var StartPosition = thisLayer.fromComp(StartPosition_CompSpace);
//space transform from slider position in null object space to to mask layer space.

var Start_Time = thisComp.layer(“Write-on_Controller-RectangularMask”).effect(“Start_Time”)(“Slider”)
var End_Time = Start_Time + (thisComp.layer(“Write-on_Controller-RectangularMask”).effect(“Duration”)(“Slider”));
var maskWidth = thisComp.layer(“Write-on_Controller-RectangularMask”).effect(“Mask_Width”)(“Slider”);
var maskLength = thisComp.layer(“Write-on_Controller-RectangularMask”).effect(“Mask_Length”)(“Slider”);
var Expansion = ease(time, Start_Time, End_Time , 0, maskLength);
//defines mask expansion parameters.

var topRight = [StartPosition[0],(StartPosition[1] – (maskWidth/2))]
var bottomRight = [StartPosition[0],(StartPosition[1] + (maskWidth/2))]
var bottomLeft = [StartPosition[0] – Expansion ,(StartPosition[1] + (maskWidth/2))]
var topLeft = [StartPosition[0] – Expansion ,(StartPosition[1] – (maskWidth/2))]
var maskVertices = [topRight,bottomRight,bottomLeft,topLeft]
//defines mask vertices based on expansion parameters.

createPath(maskVertices, [], [], isClosed = true);

[Theo Burtis]

Update:

This version is a lot closer and cleaner, but the start point is still in the wrong place:

//This mask expands left to right from a point designated by a slider effect in a separate null object layer.

var StartPosition_Slider = thisComp.layer(“Write-on_Controller-RectangularMask”).effect(“Start_Point”)(“3D Point”)
var StartPosition_CompSpace = thisLayer.toComp(StartPosition_Slider);
var StartPosition = thisLayer.fromComp(StartPosition_CompSpace);
//space transform from slider position in null object space to to mask layer space.

var Start_Time = thisComp.layer(“Write-on_Controller-RectangularMask”).effect(“Start_Time”)(“Slider”)
var End_Time = Start_Time + (thisComp.layer(“Write-on_Controller-RectangularMask”).effect(“Duration”)(“Slider”));
var maskWidth = thisComp.layer(“Write-on_Controller-RectangularMask”).effect(“Mask_Width”)(“Slider”);
var maskLength = thisComp.layer(“Write-on_Controller-RectangularMask”).effect(“Mask_Length”)(“Slider”);
var Expansion = ease(time, Start_Time, End_Time , 0, maskLength);
//defines mask expansion parameters.

var topRight = [StartPosition[0],(StartPosition[1] – (maskWidth/2))]
var bottomRight = [StartPosition[0],(StartPosition[1] + (maskWidth/2))]
var bottomLeft = [StartPosition[0] – Expansion ,(StartPosition[1] + (maskWidth/2))]
var topLeft = [StartPosition[0] – Expansion ,(StartPosition[1] – (maskWidth/2))]
var maskVertices = [topRight,bottomRight,bottomLeft,topLeft]
//defines mask vertices based on expansion parameters.

createPath(maskVertices, [], [], isCLosed = true);

Despite all the tutorials I’ve watched I still don’t quite understand what combination of toComp and fromComp I need to make this work. If you’ve got any ideas I’m all ears!

Thx,

-Theo B.