How to make a shape that blurs the image behind it in illustrator?

Question

Im still familiarizing with illustrator here and I have an image on the canvas. I need to make an image (drawn using paths/pen tool) that ideally wherever I put it it blurs out the image behind it.

I am trying to achieve the effect that I have the image that is sharp everywhere EXCEPT for that one shape - that shape is the blurred version of the part of the image underneath it, similar to apple's alerts on the iPhone.

Think of that box being the blurred version of the red image:

I know how to blur the ENTIRE image with transform, but how can I blur JUST where a certain shape is?

Answer 1

You can try using an opacity mask.

1. duplicate your image (keep original on bottom of stack)
2. apply a blur effect to your duplicate image
3. draw a shape, using white fill and no stroke
4. select both the shape and the duplicate image (by clicking the selection bubbles on the right side of the layers panel)
5. in Transparency panel, click Make Mask
6. to move the mask, click the mask thumbnail (in transparency panel) to make it active and then, in the active art click the center point of the shape and drag it to a new position

Answer 2

Is this something that is absolutely critical to do in illustrator?

You could duplicate your entire design, place it in a clipping mask on top of the original design and apply a gaussian blur to the layers in that clipping mask accordingly.

This type of task however is ideal for photoshop to handle rather than illustrator.