red circle GLSL fragment shader

red circle shader

void main(void) {
    // the center of the texture
    vec2 center = vec2(iResolution.x/2.0,iResolution.y/2.0); 
	
    // current pixel location
    vec2 loc = gl_FragCoord.xy;
	
    // how far we are from the center
    float radius=length(loc-center);
	
    // if we are within our circle, paint it red
    if (radius<100.0)
        gl_FragColor = vec4(1,0,0,1); // red
    else
        gl_FragColor = vec4(0,0,0,1); // black
}