#version 300 es
precision highp float;

uniform sampler2D u_tex;
uniform sampler2D u_lut;
uniform float u_opacity;

in vec2 lonlat;
out vec4 fragColor;

const float PI = 3.141592653589793;

const vec4 bound = vec4(65.24686922, 13.12169419,
                        138.85867996, 55.34323806);


void main() {
    vec2 uv = vec2(
        (lonlat.x - bound.x) / (bound.z - bound.x),  // 经度映射到 u
        1.0 - (lonlat.y - bound.y) / (bound.w - bound.y)   // 纬度映射到 v
    );

    // 调试：可以取消注释下面的行来可视化坐标
    // fragColor = vec4(uv.x, 0.0, 0.0, 1.0); return; // UV坐标可视化
    // 显示原始经纬度值（归一化到0-1范围用于可视化）
    // fragColor = vec4((lonlat.x - 60.0)/100.0, (lonlat.y + 10.0)/70.0, 0.0, 1.0); 
    // return; // 经纬度可视化
    
    if(any(lessThan(uv, vec2(0.0))) || any(greaterThan(uv, vec2(1.0)))) {
        discard;
    }

    vec4 texColor = texture(u_tex, uv);

    if (texColor.r <= 0.0196 || texColor.r > 0.29411) {
        discard;
    }

    float value = texColor.r * 3.4;
    value = clamp(value, 0.0, 1.0);

    float alpha = smoothstep(0.07, 0.12, value) * 0.9;
    if (alpha <= 0.001) discard;

    vec4 lutColor = texture(u_lut, vec2(value, 0.5));
    fragColor = vec4(lutColor.rgb, alpha * u_opacity);
}