Mask Blur uses the alpha of an incoming layer as a per-pixel blur strength map. Wherever the layer is opaque, the underlying image stays sharp; wherever it's transparent, it blurs — and everywhere in between gets a smooth falloff. It's the cleanest way to drive a depth-of-field, vignette, or selective focus pass off a matte you've already painted, tracked, or generated.

This is a GPU accelerated plugin, meaning if you are on software only mode, it won't work properly.