// ----- Vertex Shader -----
#version 330 core

layout(location = 0) in vec3 position;
uniform mat4 projection;
uniform mat4 model;

void main(){
  gl_Position =  projection * model * vec4(position,1);
}

// ----- Fragment Shader -----

#version 330 core

uniform vec2 resolution;
uniform float time;

out vec3 color;


#define MAX_STEPS 100
#define MAX_DIST 100.
#define SURF_DIST .01

float GetDist(vec3 p) {
	vec4 s = vec4(0, 1, 6, 1);
    
    float sphereDist =  length(p-s.xyz)-s.w;
    float planeDist = p.y;
    
    float d = min(sphereDist, planeDist);
    return d;
}

float RayMarch(vec3 ro, vec3 rd) {
	float dO=0.;
    
    for(int i=0; i<MAX_STEPS; i++) {
    	vec3 p = ro + rd*dO;
        float dS = GetDist(p);
        dO += dS;
        if(dO>MAX_DIST || dS<SURF_DIST) break;
    }
    
    return dO;
}

vec3 GetNormal(vec3 p) {
	float d = GetDist(p);
    vec2 e = vec2(.01, 0);
    
    vec3 n = d - vec3(
        GetDist(p-e.xyy),
        GetDist(p-e.yxy),
        GetDist(p-e.yyx));
    
    return normalize(n);
}

float GetLight(vec3 p) {
    vec3 lightPos = vec3(0, 5, 6);
    lightPos.xz += vec2(sin(time), cos(time))*2.;
    vec3 l = normalize(lightPos-p);
    vec3 n = GetNormal(p);
    
    float dif = clamp(dot(n, l), 0., 1.);
    float d = RayMarch(p+n*SURF_DIST*2., l);
    if(d<length(lightPos-p)) dif *= .1;
    
    return dif;
}

void main()
{
    vec2 uv = (gl_FragCoord.xy-.5*resolution.xy)/resolution.y;

    vec3 col = vec3(0);
    
    vec3 ro = vec3(0, 1, 0);
    vec3 rd = normalize(vec3(uv.x, uv.y, 1));

    float d = RayMarch(ro, rd);
    
    vec3 p = ro + rd * d;
    
    float dif = GetLight(p);
    col = vec3(dif);
    
    col = pow(col, vec3(.4545));	// gamma correction
    
    color = col;
}