ray marching

.fs

precision mediump float;
uniform vec2  resolution;     // resolution (width, height)
uniform vec2  mouse;          // mouse      (0.0 ~ 1.0)
uniform float time;           // time       (1second == 1.0)
uniform sampler2D backbuffer; // previous scene texture

float EPSILON = .001;

mat3 rotY ( float rad ) {

	return mat3(
		cos( rad ), 0, -sin( rad ),
		         0, 1,           0,
		sin( rad ), 0,  cos( rad )
	);

}


float udRoundBox ( vec3 p ) {

	vec3  b = vec3( 1., .5, .5 );
	float r = .1;
	return length( max( abs( p ) - b, 0. ) ) - r;

}

float distanceFn ( vec3 p ) {

	return udRoundBox( p );

}

void main ( void ) {

	vec2 coord = ( gl_FragCoord.xy * 2. - resolution ) / min( resolution.x, resolution.y );
	vec3 color = vec3( 0. );
	mat3 rot = rotY( time * 4. );

	vec3 pos = vec3( 0, 0, -10 ) * rot;
	vec3 dir = normalize( vec3( coord, 1. ) * rot );

	for ( int i = 0; i < 32; i ++ ) {

		float d = distanceFn( pos );

		if ( d < 0.1 ) {

			color= vec3( 1., 0., 0. );
			pos += dir * .1;

		}

		pos += dir * d;

	}

	gl_FragColor = vec4( color, 1. );

}