Source: https://www.reddit.com/r/godot/comments/cp9pq2/fog_portal_shader/ https://twitter.com/lentsius_bark/status/1161353797056258048

fog_portal_shader.shader

    shader_type spatial;

    render_mode unshaded;

    render_mode cull_disabled;

    uniform float depth_distance = 1.0;

    uniform float beer_factor : hint_range(0.0,0.2) = 0.2;

    uniform vec4 fog_colour : hint_color;

    uniform float cam_distance = 1.0;

    uniform float cam_distance_multiplier = 1.0;

    uniform float gradient;

    uniform float distance_fade_min;

    uniform float distance_fade_max = 5.0;

    void fragment(){

    ALBEDO = fog_colour.rgb;

    float depth = texture(DEPTH_TEXTURE, SCREEN_UV).r;

    depth = depth * 2.0 - 1.0;

    depth = PROJECTION_MATRIX[3][2] / (depth + PROJECTION_MATRIX[2][2]);

    depth = depth + VERTEX.z;

    // beer's law

    depth = exp(-depth * beer_factor);

    depth *= depth_distance;

    depth = clamp(1.0 - depth, 0.0, 1.0);

    //proximity fade

    float depth_tex = textureLod(DEPTH_TEXTURE,SCREEN_UV,0.0).r;

    vec4 world_pos = INV_PROJECTION_MATRIX * vec4(SCREEN_UV * 2.0 - 1.0,depth_tex * 2.0-1.0,1.0);

    world_pos.xyz /= world_pos.w;

    depth *= clamp(1.0 - smoothstep(world_pos.z + gradient, world_pos.z, VERTEX.z), 0.0, 1.0);

    depth *=clamp(smoothstep(distance_fade_min,distance_fade_max,-VERTEX.z),0.0,1.0);

    ALPHA *= depth;

    }