uniform float m_Interval;
uniform float m_LineWidth;
uniform float m_Opacity;

in float vWorldY;

out vec4 outColor;

// Maps height to a blue→green→yellow→red gradient
vec3 heightColor(float t) {
    t = clamp(t, 0.0, 1.0);
    vec3 c;
    if (t < 0.25) {
        c = mix(vec3(0.0, 0.0, 0.8), vec3(0.0, 0.7, 0.7), t * 4.0);
    } else if (t < 0.5) {
        c = mix(vec3(0.0, 0.7, 0.7), vec3(0.2, 0.8, 0.1), (t - 0.25) * 4.0);
    } else if (t < 0.75) {
        c = mix(vec3(0.2, 0.8, 0.1), vec3(0.9, 0.8, 0.0), (t - 0.5) * 4.0);
    } else {
        c = mix(vec3(0.9, 0.8, 0.0), vec3(0.8, 0.1, 0.0), (t - 0.75) * 4.0);
    }
    return c;
}

void main() {
    float minH  = -20.0;
    float maxH  = 300.0;
    float t     = (vWorldY - minH) / (maxH - minH);
    vec3  band  = heightColor(t);

    // Contour lines: sharp pulse near each interval boundary
    float phase = fract(vWorldY / m_Interval);
    float fw    = fwidth(vWorldY / m_Interval);
    float line  = 1.0 - smoothstep(m_LineWidth - fw, m_LineWidth + fw, min(phase, 1.0 - phase));

    vec3 color = mix(band, vec3(0.0), line * 0.75);
    outColor   = vec4(color, m_Opacity);
}