Complex question about masks - help needed please

Question

Hello folks,
new to this forum and new to After Effects.  I hope you can help me achieve a particular animation.
The result I want is to move a pair of glasses across the frame and through the left lens of the glasses, the background image is made clearer.
I have 3 layers.  The bottom layer has one image (the image in focus), the middle layer up has another image (the same image as the bottom layer but out of focus), the top layer has an image of a pair of glasses.
I want to, first of all, select the area within the left lens of the glasses and make this a mask and have it move when the glasses move across the frame.
Is there an easy way to select a complex area like you have in Photoshop?
I then want this mask to subtract the middle layer image to reveal the background layer.
The mask would move with the pair of glasses.
How do I make the mask apply to the middle layer to cut it out to reveal the bottom layer?
Thanks in advance.

Answer 1

While you are reading the links Szalam gave you (I recommand you to read them all), here is how you could achieve this.
Create a new solid on top of your glasses, roughly the size of your lense (just a little bigger than the area you want to mask out).
Then use the pen tool to draw a mask with the shape to be used as a "hole" for your second layer. Be sure to have your solid layer selected before drawing the mask.
You will now a have a solid layer on top of your glasses with the shape of the lens. To make it follow your glasses, parent it to the glasses using the parent dropdown menu in the timeline, next to your layer's name.
Now in the stack, move your solid on top of your out of focus layer. Then select your out of focus layer, display the matte/mode column and in the matte dropdown select alpha (or alpha inverted). AE will then use your solid layer as a mask to cut out your out of focus layer.