Documentation for CIDepthBlurEffect "inputShape"

I'm using CIDepthBlurEffect to create a portrait mode effect on a rendered image. The effect is working as expected however I want to create the "bokeh ball" effect which is seen in the photos app. I see that the filter has a "inputShape" input of type NSString, however the documents do not specify what value this should be.

Any pointers are help is greatly apprecaited.

Documentation for CIDepthBlurEffect "inputShape"
 
 
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