Tips and Techniques/Workflow/Blurring Within Edges
From VFXPedia
Blurring an Image Area Within Contrasty Edges
Summary
The way to blur a part of the image enclosed into contrasty edges keeping the edges intact and avoiding the blurred spill from them.
Description
You may need to blur an image area surrounded by edges for different reasons. It can be a reflection which needs some defocusing while the reflecting object should remain sharp. It can also be dust, grain and other artefacts which are to be suppressed with blurring.
If you just apply a blurring tool with an effect mask, you'll get color spill from the edges to the inside parts of the mask.
To avoid this, perform a premasking with a MatteControl tool, making all but the area to blur transparent. Only the target area should remain, the edges around it should be gone. For this you will need to check the Invert Garbage Matte in the MatteControl.
Then apply the blurring and after that, add the ChannelBoolean tool which divides all the channels by Alpha. Thus, you'll get the separated blurred area with extended borders and no spill from outside the mask.
Merge this image over the original using the same mask.
