GLSL talk

Sphere

float intersectSphere(vec3 origin, vec3 direction, vec3 center, float radius) {
  vec3 co = origin - center;
  float b = dot(co, direction);
  float a = dot(co, co) - radius * radius;
  float d = b * b - a;
  
  if (d <= 0.0) {
    return -1.0;
  }
  
  float t = -b - sqrt(b * b - a);
  return t;
}

float intersectGround(vec3 origin, vec3 direction) {
  return -origin.y / direction.y;
}

float intersect(vec3 origin, vec3 direction, vec3 sphereCenter, float sphereRadius, out float dist) {
  float ground = intersectGround(origin, direction);
  float sphere = intersectSphere(origin, direction, sphereCenter, sphereRadius);

	if (ground + sphere <= 0.0) {
	  return 0.0;
	}
	
	if(sphere <= 0.0 || ground < sphere) {
		dist = ground;
		return 1.0;
	}
	
	if(ground <= 0.0 || sphere < ground) {
		dist = sphere;
		return 2.0;
	}
}
 
void main(void) {
  vec2 position = gl_FragCoord.xy / resolution.xy;
  vec2 p = -1.0 + 2.0 * position;
  p.x *= resolution.x / resolution.y;
	
  
  vec3 origin = vec3(cos(mouse.x * 2.0 * 3.1415) * 5.0, 1.0, sin(mouse.x * 2.0 * 3.1415) * 5.0);
	
  vec3 ww = normalize(vec3(0.0) - origin);
  vec3 uu = normalize(cross(ww, vec3(0.0, 1.0, 0.0)));
  vec3 vv = normalize(cross(uu, ww));
  vec3 direction = normalize(p.x*uu + p.y * vv + ww);
  
  float sphereRadius = 0.5;
  vec3 sphereCenter = vec3(0.0, sphereRadius, 0.0);
	  
  float dist = -1.0;

  float id = intersect(origin, direction, sphereCenter, sphereRadius, dist);
  
  vec3 color = vec3(0.0);
  
  if (id > 0.0) {
    vec3 hit = origin + direction * dist;
    vec3 light = normalize(vec3(-1.0, 1.0, -1.0));
	  
	  if (id == 1.0) {
		  color = vec3(1.0, 1.0, 1.0) * (length(hit.xz - sphereCenter.xz) / sphereRadius);
	  }
	  else if(id == 2.0) {
		float diffuse = dot(light, (hit - sphereCenter) * (1.0 / sphereRadius));
    color = vec3(1.0) * diffuse;
	  }
  }
  
  gl_FragColor = vec4(color, 1.0);