Simple ray marching

rm.frag

precision mediump float;
uniform float t; // time
uniform vec2  r; // resolution
 
vec2 v=vec2(0.,1.);
 
float f(vec3 p)
{
	return length(p)-.6;
}
 
void main(void)
{
	vec3
		uv=vec3((gl_FragCoord.xy-r/2.)/r.x,1.),
		eye=v.xxy*3.,
		cen=v.xxx,
		dir=normalize(vec3(cen-eye)),
		air=v.xyx,
		sid=normalize(cross(dir,air)),
		top=normalize(cross(sid,dir)),
		ray=uv.x*sid+uv.y*top+dir,
		p=eye,d;
		
	for(int c=0;c<8;c++)
	{
		d.x=f(p);
		d.y+=d.x;
		p+=ray*d.x;
	}
	
	if(d.x<1E-1)
	{
		vec3 col=vec3(ray.x*.8,ray.y*.8,ray.x*.4+ray.y*.4)*4.0;
		gl_FragColor = vec4(col,1.);
	}
}