Ray marching.

demo.glsl

#ifdef GL_ES
precision mediump float;
#endif

#extension GL_OES_standard_derivatives : enable

uniform float time;
uniform vec2 mouse;
uniform vec2 resolution;

vec3 dup(vec3 x) {
	return mod(x - vec3(time, 0, 0), 4.0);
}

float dist(vec3 pos) {
	pos = dup(pos);
	return length(pos - vec3(2, 2, 2)) - 1.0;
}

vec3 norm(vec3 pos) {
	pos = dup(pos);
	return normalize(pos - vec3(2, 2, 2));
}

vec3 march(vec3 origin, vec3 ray) {
	for (int i = 0; i < 100; ++i) {
		origin += ray * dist(origin);
	}
	return origin;
}

vec2 trans(vec2 p) {
	vec2 r = resolution;
	float m =min(r.x, r.y);
	return (p  * 2.0 - 1.0) * r / m;
}

void main() {
	vec2 pos = trans(gl_FragCoord.xy / resolution.xy);
	vec2 m = trans(mouse);
	vec3 screen = vec3(pos.x, pos.y, 1.0);
	vec3 camera = vec3(0, 0, 0);
	vec3 light = vec3(m.x * 2.0, 0.0, m.y * 5.0 + 5.0);
	vec3 ray = normalize(screen - camera);
	vec3 hit = march(camera, ray);
	float d = dist(hit);
	if (abs(d) < 0.01) {
		vec3 n = norm(hit);
		vec3 l = normalize(light - hit);
		float a = length(light - hit);
		gl_FragColor.xyz = vec3(1, 1, 1) * dot(n, l) / a * 5.0;
	} else {
		gl_FragColor = vec4(0, 0, 0, 1);
	}
}