Vectors and Builtins

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OK, we've drawn a square — next, a circle.

You can check if a point is in a circle by seeing if its distance from the circle's center is smaller than the circle's radius. In GLSL we measure distance using the length() function:

float d = length(p2 - p1)

Much more concise than it would be in JavaScript!

All that's left to do is compare that distance to the radius and you should be able to draw that circle out to the screen. Good luck :)