Procedural Circle Mask

Description
Renders a circle in UV space and uses it as an alpha mask. Because it uses a gradient to define the circle it's very easy to get well defined circles with controlled levels of aliasing.

Parameters
Main Texture: Diffuse texture

Color: Color multiplied into diffuse & alpha

Distort: X/Y affect the circle's center, W/Z affect the circle's (inverse) scale horizontally and vertically respectively.

Outer Radius: Main radius of the circle

Inner Radius: Inner radius of the circle (used to create a ring)

Hardness: Flattens the gradient towards the radius distances