shader_type canvas_item;

// 鱼眼效果强度控制
uniform float fish_intensity : hint_range(0.0, 2.0) = 1.0;

void fragment() {
    // 获取纹理分辨率
    vec2 iResolution = vec2(textureSize(TEXTURE, 0));

    // 基础UV坐标
    vec2 uv = UV;

    // 宽高比计算
    float aspectRatio = iResolution.x / iResolution.y;

    // 计算强度参数
    float strength = fish_intensity * 0.03;
    vec2 intensity = vec2(
        strength * aspectRatio,
        strength * aspectRatio
    );

    // 坐标转换到[-1, 1]范围
    vec2 coords = (uv - 0.5) * 2.0;

    // 计算坐标偏移量
    vec2 realCoordOffs;
    realCoordOffs.x = (1.0 - coords.y * coords.y) * intensity.y * coords.x;
    realCoordOffs.y = (1.0 - coords.x * coords.x) * intensity.x * coords.y;

    // 应用偏移
    vec2 fuv = uv - realCoordOffs;

    // 边界检查
    if(fuv.x < 0.0 || fuv.x > 1.0 || fuv.y < 0.0 || fuv.y > 1.0) {
        COLOR = vec4(0.0, 0.0, 0.0, 0.0); // 超出范围显示透明
    } else {
        vec4 color = texture(TEXTURE, fuv);
        // 应用RGB/BGR混合效果，基于距离中心的距离
        COLOR.rgb = mix(color.rgb, color.bgr, length(fuv - 0.5));
        COLOR.a = color.a;
    }
}