I'm new to blurring and I'm about to start working on a crime-based reality show so I'm brushing up. 

I have a shot of a car driving into the frame from a distance and I need to blur the license plate for the duration. My workflow is: Add blur effect to clip, create shape at first frame, and motion track. I need to make the blur rectangle expand over time but I can quite figure it out. Any workflow suggestions would be greatly appreciated. 

Daniel,

You need to use "Corner-pin Tracking"

That will track the 4 corners of the plate and expand or contract as necessary

Regards,
Douglas

Daniel

This old tutorial of mine may help you follow the workflow

http://www.screencast.com/t/kxjysFestZoy

Regards,
Douglas

Thanks, Douglas

This works great for the license plates, but what if i have a non-square target like someone's face? I have a similar clip where a person walks into the frame and I need to make an oval shape grow over the face. 

the_don:

Thanks, Douglas

This works great for the license plates, but what if i have a non-square target like someone's face? I have a similar clip where a person walks into the frame and I need to make an oval shape grow over the face. 

 

Daniel, You have to shoot people with rectangular faces!

Sorry, couldn't resist that.
Now the real answer

I haven't made a tutorial on this one myself, but there are a few others who have.

Here are a couple of links that may help you

http://borisfx.cachefly.net/borisfx/download_files/Witness_Protect_FCP.pdf

http://library.creativecow.net/ezzy_paul/witness_protection/1

They are using BCC Witness Protection filter.

Don't worry if you don't have the BCC AVX filter pack, the filter is also included in Avid FX!

Good luck

Regards,

Douglas

This is an old Tracker Tutorial but believe it will be helpful. This is another a good blur tutorial.