sqrt(a^2 + b^2)
x^2 + y^2 = c^2
uniform vec2 resolution; void main() { vec2 coord = gl_fragcoord / resolution; if (distance(coord, vec2(x, y)) <= r) gl_fragcolor = vec4(1., 1., 1., 0.); else gl_fragcolor = vec4(0., 0., 0., 0.); }