Rescaling Coordinates

    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    st = st*2.-1.;

Trigonometry

$$x=\mbox{cos}\theta$$ $$y=\mbox{sin}\theta$$

$$\frac{y}{x}=\mbox{tan}\theta$$

Polar Coordinates

$$\theta = \mbox{tan}^{-1} \frac{y}{x}$$

$$r = \sqrt{x^2 + y^2}$$

    float theta = atan(st.y, st.x);
    float mag = length(st);