shader_type spatial;
render_mode blend_mix, depth_draw_never;

global uniform float global_snow_start_time        = -1.0;
global uniform float global_snow_accumulation_speed = 0.1;
global uniform float global_snow_melt_time         = -1.0;
global uniform float global_snow_melt_speed        = 0.1;
global uniform float global_snow_threshold         = 0.5;
global uniform vec4  global_snow_color             = vec4(0.92, 0.96, 1.0, 1.0);

void fragment() {
    float snow_amount = 0.0;
    if (global_snow_start_time >= 0.0) {
        snow_amount = clamp((TIME - global_snow_start_time) * global_snow_accumulation_speed, 0.0, 1.0);
    }

    if (global_snow_melt_time >= 0.0) {
        float melt = clamp((TIME - global_snow_melt_time) * global_snow_melt_speed, 0.0, 1.0);
        snow_amount = snow_amount * (1.0 - melt);
    }

    vec3 world_normal = (INV_VIEW_MATRIX * vec4(NORMAL, 0.0)).xyz;
    float facing_up   = clamp(world_normal.y, 0.0, 1.0);
    float snow_factor = smoothstep(global_snow_threshold, 1.0, facing_up) * snow_amount;

    ALBEDO    = global_snow_color.rgb;
    ROUGHNESS = 0.15;
    METALLIC  = 0.0;
    ALPHA     = snow_factor;
}