gistfile1.glsl

 vec2 uv = gl_FragCoord.xy;
    
 //uv = fragCoord.xy / iResolution.xy;
uv.y -= u_RenderSize.y / 2.0;
uv.x -= u_RenderSize.x / 2.0;
    

    vec2 coords = uv;
  	vec4 color = vec4(1.0,1.0,0.0,1.0);
  	float radius = 20.0;
  	vec2 center = vec2(0.);
  	float sD = distance(coords, center);
	  float a = 1.0 - clamp(sD - radius + .5, 0.0, 1.0);
    
    gl_FragColor = mix(gl_FragColor,color,a * color.a);