#version 330 core

in vec3 direction;

out vec4 frag_color;

uniform sampler2D equirect;

const vec2 inv_atan = vec2(0.15915494309, 0.31830988618);

vec2 sample_spherical_map(vec3 v) {
    vec2 uv = vec2(atan(v.z, v.x), asin(clamp(v.y, -1.0, 1.0)));
    uv *= inv_atan;
    uv += 0.5;
    uv.x = fract(uv.x);
    return uv;
}

void main() {
    vec2 uv = sample_spherical_map(normalize(direction));
    uv.y = 1.0 - uv.y;
    vec2 size = vec2(textureSize(equirect, 0));
    float epsY = 0.5 / max(size.y, 1.0);
    uv.y = clamp(uv.y, epsY, 1.0 - epsY);
    vec3 color = texture(equirect, uv).rgb;
    frag_color = vec4(color, 1.0);
}