circle.glsl

#version 330
uniform vec3 color;
uniform float radius;
uniform float thickness;
out vec4 outColor;
void main(){
    float edge, t;
    vec2 tc = gl_FragCoord.xy;
    tc.x -= width/2;
    tc.y -= height/2;
    edge = float(length(tc) - radius);
    if(edge < 0)
        outColor = vec4(color.rgb, 1.0);
    else if(edge > thickness)
        outColor = vec4(0);
    else { /* 0 < edge < thickness */
        t = 1.0 - edge/thickness
        outColor = vec4(color.rgb, t);
    }
}