Rainbow_circle.c

// https://www.shadertoy.com/view/XdlSDs

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // make origin (0,0) in center of the screen
	vec2 p = (2.0*fragCoord.xy-iResolution.xy)/iResolution.y;
    float tau = 3.1415926535*2.0;
    float a = atan(p.x,p.y);
    float r = length(p)*0.55; // 1/radius of circle default = 0.75
    vec2 uv = vec2(a/tau,r);
	
    const float numberOfLoops = 1.0; // 1.0
    const float coef = 3.0; // 3.0
	//get the color
	float xCol = (uv.x - (iTime / coef)) * coef * numberOfLoops;
	xCol = mod(xCol, 3.0);
	vec3 horColour = vec3(0.25); // 0.25
	
	if (xCol < 1.0)
    {
		horColour.r += 1.0 - xCol;
		horColour.g += xCol;
	}
	else if (xCol < 2.0)
    {	
		xCol -= 1.0;
		horColour.g += 1.0 - xCol;
		horColour.b += xCol;
	}
	else 
    {	
		xCol -= 2.0;
		horColour.b += 1.0 - xCol;
		horColour.r += xCol;
	}

	// draw color beam
	uv = (2.0 * uv) - 1.0;
	float beamWidth = 
        
      (0.7+0.5*cos(uv.x*10.0*tau*0.15*clamp(floor(5.0 + 10.0*cos(iTime)), 0.0, 10.0)))
        
        * abs(1.0 / (30.0 * uv.y));
    
	vec3 horBeam = vec3(beamWidth);
	fragColor = vec4((( horBeam) * horColour), 1.0);
}