add snow management for materials

core/daynight/snow.gdshader

@@ -1,19 +1,30 @@
 shader_type spatial;
-render_mode blend_mix, depth_draw_opaque;
+render_mode blend_mix, depth_draw_never;
 
-global uniform float global_snow_amount;
-
-uniform vec4 snow_color : source_color = vec4(0.95, 0.98, 1.0, 1.0); 
+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() {
-    vec3 world_normal = (INV_VIEW_MATRIX * vec4(NORMAL, 0.0)).xyz;
-    float up_dot = dot(world_normal, vec3(0.0, 1.0, 0.0));
-    float snow_mask = smoothstep(1.0 - global_snow_amount, 1.1 - global_snow_amount, up_dot);
-    snow_mask *= step(0.01, global_snow_amount);
+    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);
+    }
 
-    ALBEDO = snow_color.rgb;
-    ALPHA = snow_mask;
-    
-    ROUGHNESS = 0.3; 
-    SPECULAR = 0.5;
-}
+    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;
+}