thebookofshaders/examples/moon.frag

// Author @patriciogv - 2015
// http://patriciogonzalezvivo.com

#ifdef GL_ES
precision mediump float;
#endif

#define PI 3.1415926535
#define HALF_PI 1.57079632679

uniform vec2 u_resolution;
uniform float u_time;

uniform sampler2D u_tex0;
uniform vec2 u_tex0Resolution;

float speedMoon = 0.01;
float speedSun = 0.25;

vec3 sphereNormals(in vec2 uv) {
    uv = fract(uv)*2.0-1.0; 
    vec3 ret;
    ret.xy = sqrt(uv * uv) * sign(uv);
    ret.z = sqrt(abs(1.0 - dot(ret.xy,ret.xy)));
    ret = ret * 0.5 + 0.5;    
    return mix(vec3(0.0), ret, smoothstep(1.0,0.98,dot(uv,uv)) );
}

vec2 sphereCoords(vec2 _st, float _scale){
    float maxFactor = sin(1.570796327);
    vec2 uv = vec2(0.0);
    vec2 xy = 2.0 * _st.xy - 1.0;
    float d = length(xy);
    if (d < (2.0-maxFactor)){
        d = length(xy * maxFactor);
        float z = sqrt(1.0 - d * d);
        float r = atan(d, z) / 3.1415926535 * _scale;
        float phi = atan(xy.y, xy.x);

        uv.x = r * cos(phi) + 0.5;
        uv.y = r * sin(phi) + 0.5;
    } else {
        uv = _st.xy;
    }
    return uv;
}

vec4 sphereTexture(in sampler2D _tex, in vec2 _uv) {
    vec2 st = sphereCoords(_uv, 1.0);

    float aspect = u_tex0Resolution.y/u_tex0Resolution.x;
    st.x = fract(st.x*aspect + u_time*speedMoon);

    return texture2D(_tex, st);
}

void main(){
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    vec3 color = vec3(1.0);
  
    color *= sphereTexture(u_tex0, st).rgb;

    // Calculate sun direction
    vec3 sunPos = normalize(vec3(cos(u_time*speedSun-HALF_PI),0.0,sin(speedSun*u_time-HALF_PI)));
    vec3 surface = normalize(sphereNormals(st)*2.0-1.0);
   
    // Add Shadows
    color *= dot(sunPos,surface);

    // Blend black the edge of the sphere
    float radius = 1.0-length( vec2(0.5)-st )*2.0;
    color *= smoothstep(0.001,0.05,radius);

    color = 1.0-color;

    gl_FragColor = vec4(color,1.0);
}