Create a moving mask

[ROBERMARTIN]

Greetings to everyone. 

I would like to know how can I do a moving mask on a layer. For exemple, if the screem format is square, a square mask, convering the entire format to decrease little  by little, and showing the image that is in the layer below. If somebody can help me, I will be very, very grateful. Thank you so much!!! 

[Monika Gause]

You mean you want to animate this? In Illustrator?

[ROBERMARTIN]

Thank you for your help. I have been able to solved it

[Ton Frederiks]

If you want to create an animation you can do that in After Effects or make a screenrecording while changing the size of the mask with the arrow keys in the Transform panel.

 Create a new top layer and draw a rectangle.

In the Layers panel menu with the rectangle selected choose Create Clipping Mask.

Shift select the underlying layers and move them below the clipping mask rectangle.

With the rectangle still selected, you can change the Width and Height in the Transform panel, using the up and down arrow keys. 

[ROBERMARTIN · Accepted solution]

Thank you so much for your help and your time. I am very grateful. I have used the mask in movement and all is ok.