#version 330 core
in vec2 vUv;
in float vShade;
in float vWorldY;
in vec3 vWorldPos;

out vec4 FragColor;

uniform sampler2D atlas;
uniform float u_caveDarkStartY;
uniform float u_caveDarkEndY;
uniform float u_caveMinBrightness;

uniform vec3 u_cameraPos;
uniform float u_fogStart;
uniform float u_fogEnd;
uniform vec3 u_skyColor;

void main()
{
    vec4 texel = texture(atlas, vUv);

    // Y-based cave darkness.
    float t = clamp((u_caveDarkStartY - vWorldY) /
                    (u_caveDarkStartY - u_caveDarkEndY), 0.0, 1.0);
    float brightness = mix(1.0, u_caveMinBrightness, t);
    vec3 color = texel.rgb * vShade * brightness;

    // Distance fog to the sky color, applied after cave darkness.
    float dist = length(vWorldPos - u_cameraPos);
    float fog = clamp((dist - u_fogStart) / (u_fogEnd - u_fogStart), 0.0, 1.0);
    color = mix(color, u_skyColor, fog);

    FragColor = vec4(color, 1.0);
}