One of the most requested features for Godot 3 has been occlusion culling (in some form). It is on the roadmap to be added to the core of the engine in Godot 4.0, but until that is available, I figured that I could come up with an interrim solution that would allow the production of some of the games that need this.
There are several approaches to occlusion culling, but one of the oldest and tried and tested techniques is the use of rooms (or 'cells') and portals. Portal rendering is particularly useful in indoor scenes, especially 1st / 3rd person games. It is not so useful in outdoor scenes. There was some support for portal rendering in Godot 2 but I have implemented this new system from scratch. Portal rendering was particularly attractive as an interrim solution partly because it is easier to integrate with the existing engine, and partly because I have done it before(!).
As with most room portal systems, LPortal works better if rooms are convex. Most portal systems require you to specify the convex hull (bound) for each room. In LPortal this is optional. The convex hull is only required for calculating the 'start room' of a dynamic object when you either initially place it (dob_register), or teleport it (dob_teleport). As such, both of these functions have an alternative hint version where the user can specify the start room manually. This is possible in some simpler designs of games, and avoids the laborious need to model room bounds (convex hulls).
If you are exclusively using the hint commands, and not creating bounds, there is strictly speaking no longer a requirement for the rooms to be convex, they can be concave shapes. This allows considerable flexibility in level design. HOWEVER, the portals should be placed such that no portal is in front of the portal plane of another portal in the room. I.e. the portals themselves should form some kind of convex structure.
The first thing you may notice is that LPortal doesn't make much use of special features within the editor, and simply relies on careful naming of spatials (or Empties within Blender). There is one new node type, 'LRoomManager', which behaves pretty much as any spatial however it has some added functions and runs an entire visibility system under the hood.
The reason for not creating bespoke editing within Godot is twofold:
1) This is a non-official plugin to the engine, without modifying core, or creating a specific importer
2) Using the node names to denote rooms and portals allows the whole level to be edited (and more importantly re-edited) within a modelling package such as blender.*
Within your scene graph, to use LPortal you should add an LRoomManager node to the scene, e.g. as a child of the root node. You can reuse the same room manager as you load multiple game levels.
On another branch of the scene graph you should create a standard Spatial which will act as the root node for all the rooms and meshes, lights to be controlled by the LRoomManager. You can name this anything you like, but in the documentation I will refer to it as the 'roomlist'.
In order for the room manager to know which roomlist it should operate on, in the IDE you should _assign_ the roomlist to the 'rooms' property of the LRoomManager. You can do this by clicking on 'assign' in the LRoomManager inspector, and selecting the roomlist Spatial that you created.
The children of the roomlist should be rooms, or areas (which themselves contain rooms). The rooms contain the meshes, lights that you want to be shown and hidden by the room system.
Rooms are simply spatials, whose children should contain the mesh instances that are static (non-moving) within the room. The naming of the room spatial is crucial in order for it to be recognised. All rooms should start with 'room_'.
Areas are used to define groups of rooms, currently this is only used to determine which area global directional lights (such as the sun) should apply to. For example if your game level has an outside area and indoor where the sun cannot reach, you may want to create two areas.
When using areas, you would place these between the roomlist and the rooms, like so:
```
roomlist
area_outside
room_outside1
room_kitchen
area_inside
room_cellar
```
Note that a room can be a member of more than one area if desired. Simply make a child of the room with the desired area, and it will also be added to that area.
In order to calculate the visibility between rooms (and the objects within rooms) you need to manually specify the location and shape of portals that should join the rooms. These should be thought of as doorways, or windows between rooms (and often cover exactly these features).
Portals are mesh instances, but have very strict requirements, and again naming conventions.
* Each portal should be a single-sided CONVEX polygon, on a single plane.
* The portal should be a child of the room it links from.
* Single-sided means it can only be seen from one direction. It should face out from the room it is a child of.
* You only need to create one portal per opening between rooms, rather than one in each room.
The mirror portal will be created automatically. This saves on editing in the modelling package. You can create the portal in either of the adjoining rooms.
* The name of all portals must start with 'portal_' followed by the name of the room (not including the 'room_' prefix) that the portal should link to. e.g. 'portal_kitchen', 'portal_hall', 'portal_bathroom1'
#### Portal naming
There is a special case for naming portals - because Godot cannot handle more than one node with the same name!
If you had several portals named 'portal_kitchen', Godot would not allow it, and would add characters to the name, and screw up the system. To get around this, and allow unique names for each portal, LPortal will ignore anything in the portal name after a '*' character. e.g.:
* portal_kitchen*0
* portal_kitchen*from_the_hall
* portal_kitchen*it_is_sunny_today
You get the idea, you can use whatever scheme you want to make the name unique. Something like using the room the portal comes from, and a number is probably sensible, e.g. 'portal_kitchen*hall0' but it is totally up to you.
When you first add an object to a room at runtime, it has to decide which room it should be in.
You have three choices here:
1) Manually supply the convex hull (bound) for a room
2) Do not supply a bound, in which case LPortal will simply choose the nearest room centre to the object. _(this can choose the wrong room when rooms are different sizes, for instance a hallway by a large room, so it is not recommended)_
3) Use the dob_register_hint and teleport_hint versions, where the user specifies the start room
Approach (1) is the most powerful and flexible, but requires creating a bound, either in your modelling package or within Godot IDE.
The bound should be stored as a MeshInstance child of the room, with a specific naming convention, it should start with the prefix 'bound_'. You can put anything after the prefix (or leave as is).
Although the bound will be treated as a convex hull at runtime, you don't have to be perfect when creating it because the geometry will be run through the quickhull algorithm.
You can optionally prevent objects being added to LPortal internal room system (so they will not be culled to the planes). To do this, their name should begin with 'ignore_'. Note this is only relevant for objects derived from VisualInstance, LPortal ignores all non-visual instance objects. However note that these objects will still be culled when entire rooms are hidden by LPortal (_?is this still true?_).
While most of the world in the room portal is assumed to be static (non-moving), you will inevitably want some objects to move around in the scene. These objects that can move _between_ rooms are special, the visibility system has to keep track of which room they are in in order to render them correctly.
Players, boxes, props, physics objects are all examples of candidates for DOBs, as well as the most crucial example, the camera itself. Unless the visibility system knows which room the camera is in, the system will not know what to render! We will use the camera as an example as you will need to register this as a DOB.
Somewhat counter-intuitively, all DOBs should be maintained in your game __OUTSIDE__ the room list subtree. I.e., __NOT__ in a room. This is because the relationship between the DOB and the room is a soft one, the DOB can move between rooms. This is also handy because it makes it easier to use things like pools for your dynamic objects, because you don't have to worry about them being in different rooms in different places in the scene graph.
The number 0.0 is the radius of the DOB in the visibility system. All DOBs are managed as spheres. For a camera the radius can be zero because it will never be visible (however it DOES require to be a DOB so that the system can keep track of which room it is in).
If a DOB is being culled (popping out of view) when it should not, it is normally because the bounding radius needs to be larger. On the other hand, too large a radius will make the DOB render when it is not necessary, so it is a good idea to tweak this value. It is in Godot world units, so you may be able to simply measure you object in the IDE.
* When you have finished with a DOB you should call `dob_unregister(cam)` to remove the soft link from the system. This is more important when you are creating and deleting DOBs (say with multiple game levels). If you call rooms_release when unloading a level and want to keep DOBs in between levels, it is crucial that you do not update them until you have re-registered them after calling rooms_convert to create the new level (you will get an error message otherwise).
* If you are suddenly moving a DOB a large distance (rather than into a neighbouring room), you should call `dob_teleport(cam)`. This uses a different system to estimate the new room that the DOB is in. If you know in advance what room the dob will teleport to, there is a hint version of the function.
* DOBs should only move between rooms via the portals. In fact that is how their movement between rooms is defined. This is why a room's portals should form a convex space, never concave. In order to limit movement between rooms to the portals, you should use e.g. physics, or a navmesh.
* If you want to unload a level and load a new one, call `rooms_release()`, free the old level, load the new one, and call `rooms_convert()` again (which clears the old data), and repeat each step.
_Be aware that if you unload (using queue_free) and load levels with the same name then Godot can rename them to avoid name conflicts. The nodepath to the rooms assigned to the LRoomManager must be correct!_
* The most involved step is building your original rooms and portals, and getting the names right. Watch the debug output in the console when converting the rooms, it will let you know or give indications where it is having trouble converting.
A significant portion of LPortal is devoted to debugging, as without feedback it is difficult to diagnose problems that are occurring. The debugging occurs in 2 stages - the initial conversion, and at runtime, it will provide the visibility tree when you request debug output for a frame with rooms_log_frame().
A sample conversion is as follows:
```
running convert
Convert_Rooms
Convert_Room : room_myroom_x0y0
Found VI : WallsAndFloor
Found VI : Box
Found VI : Box2
myroom_x0y0 centre 0, 1, 0
Convert_Room : room_myroom_x1y0
Found VI : WallsAndFloor
Found VI : Box
Found VI : Box2
myroom_x1y0 centre 4, 1, 0
Convert_Room : room_myroom_x0y1
Found VI : WallsAndFloor
Found VI : Box
Found VI : Box2
myroom_x0y1 centre 0, 1, 4
Convert_Room : room_myroom_x1y1
Found VI : WallsAndFloor
Found VI : Box
Found VI : Box2
myroom_x1y1 centre 4, 1, 4
Convert_Portals pass 0
DETECT_PORTALS from room myroom_x0y0
detected to myroom_x0y1
detected to myroom_x1y0
detected to myroom_x1y0
DETECT_PORTALS from room myroom_x1y0
detected to myroom_x1y1
detected to myroom_x2y0
WARNING : portal to room myroom_x2y0, room not found
detected to myroom_x2y0
WARNING : portal to room myroom_x2y0, room not found
DETECT_PORTALS from room myroom_x0y1
detected to myroom_x0y2
WARNING : portal to room myroom_x0y2, room not found
detected to myroom_x1y1
detected to myroom_x1y1
DETECT_PORTALS from room myroom_x1y1
detected to myroom_x1y2
WARNING : portal to room myroom_x1y2, room not found
detected to myroom_x2y1
WARNING : portal to room myroom_x2y1, room not found
detected to myroom_x2y1
WARNING : portal to room myroom_x2y1, room not found
Convert_Portals pass 1
MAKE_PORTALS_TWOWAY from room myroom_x0y0
contains 3 portals
considering portal myroom_x0y1
creating opposite portal
considering portal myroom_x1y0
creating opposite portal
considering portal myroom_x1y0
creating opposite portal
MAKE_PORTALS_TWOWAY from room myroom_x1y0
contains 3 portals
considering portal myroom_x1y1
creating opposite portal
considering portal myroom_x0y0
is MIRROR, ignoring
considering portal myroom_x0y0
is MIRROR, ignoring
MAKE_PORTALS_TWOWAY from room myroom_x0y1
contains 3 portals
considering portal myroom_x1y1
creating opposite portal
considering portal myroom_x1y1
creating opposite portal
considering portal myroom_x0y0
is MIRROR, ignoring
MAKE_PORTALS_TWOWAY from room myroom_x1y1
contains 3 portals
considering portal myroom_x1y0
is MIRROR, ignoring
considering portal myroom_x0y1
is MIRROR, ignoring
considering portal myroom_x0y1
is MIRROR, ignoring
Convert_Portals pass 2
CONVERT_BOUND : 'bound_ofroom' for room 'myroom_x0y0'
contained 6 planes.
CONVERT_BOUND : 'bound_ofroom' for room 'myroom_x1y0'
contained 6 planes.
CONVERT_BOUND : 'bound_ofroom' for room 'myroom_x0y1'
contained 6 planes.
CONVERT_BOUND : 'bound_ofroom' for room 'myroom_x1y1'
contained 6 planes.
```
You can see that conversion takes place in several stages:
1) Finding the rooms and visual instances (meshes etc)
_(Note that if you are reading the log in the Godot IDE you may need to bump up the value of network/limits/debugger_stdout/max_chars_per_second in the project settings, to prevent getting an output overflow message)_
Visibility starts from the room the camera is in, checks the objects within against the clipping planes (just the view frustum at first), then checks each portal to see if it is visible. If it is visible it checks the room on the other side of the portal, and continues the process recursively. Each time it moves through a portal it adds clipping planes from the camera to the edges of the portal. This ensures that objects that are hidden by portal planes are not rendered.
You can show the portal culling graphically by calling `rooms_set_debug_planes(true)`, and show the room bounds (if defined) by calling `rooms_set_debug_bounds(true)`.
Portal polygons with fewer edges are also faster to cull against. So usually an axis aligned quad will make a lot of sense, even when covering an irregular opening (say a cave).
Although deciding what is in view of the camera is relatively straightforward, what complicates matters is that objects in view may be lit by lights that are not in view. Even worse, objects in view may be shadowed by objects that are NOT in view!
### Realtime lighting
Godot's realtime lighting works using shadow mapping, that is, before rendering the view from the camera, it first renders the view from each light affecting the scene, drawing each object to a _shadow map_. During the camera render it then uses the shadow map (or maps) to determine which pixels are in shadow.
It should be obvious that this is potentially very expensive, both the extra steps to render from each light, and the lookup at runtime to determine whether pixels are in shadow. As a result, realtime shadows are not a good option on some low powered devices.
If we cull our visible objects using LPortal but still render all objects when rendering the shadow maps from each light, we would be missing out on a lot of the benefits of our rooms / portal system. Due to this, I have made considerable effort to allow LPortal to cull not only the objects in view of the camera, but also to cull objects that are in view of the lights, and ONLY those lights that are relevant to the visible scene (a game level may contain 100 lights, but if only 2 are lighting the current view, those are the only 2 that are needed).
This is still a work in progress and there are some limitations. There are 3 light types in Godot :
Spotlights and Omnis are treated in a very similar manner within LPortal. You should place them within your rooms, in a similar manner to meshes. If these lights are static (non moving), that is all that needs to be done, and they should work automatically.
#### Dynamic Local Lights
Making these lights dynamic (movable) is possible too. Place them in a room as normal, but make sure to give them a unique name (e.g. 'kitchen_light'). From gdscript or similar you will want to retain a reference to the light after loading the level.
E.g.
```
var m_node_KitchenLight = get_node("/root").find_node("kitchen_light", true, false)
In practice, spotlights and omni lights are treated in a similar manner within LPortal, and can be placed within rooms. For directional lights (think the sun) the exact position of the light is unimportant, only the direction. For this reason, directional lights are handled slightly differently
Directional lights such as the sun are great for lighting an entire level, but naively, they cast shadows from EVERY object in the level. Rendering the shadow map can be very expensive. Of far more interest in LPortal (for now) are the local lights, because they offer good opportunities for culling, both of the light itself, and of the shadow casters.
For now, if you place static spotlights within the rooms, they will be detected during conversion and used for realtime lighting with culling. Omni lights will be available soon via the same method. Directional lights I haven't come up with a good solution yet, I am working on this. You should place directional lights outside the room list somewhere else in the scene graph and register the light with LPortal by calling `register_light`, and it will do its best to cull shadow casters but this is still a work in progress.
### Baked Lighting
Often a far better fit for lighting with occlusion culling systems is the use of baked lighting, such as lightmaps. Lightmaps can store the entire static lighting for a level in precalculated form, as one or more textures, called lightmaps. This has pros and cons.
#### Pros
* Lightmaps are pre-calculated ahead of time, so can use far more complex and realistic models of lighting than would be possible in realtime. Techniques such as radiosity, and multiple bounces of light rays are possible.
* Lightmaps are very fast at runtime. There is no need to render a shadow map for each light, and when rendering from the camera, instead of an expensive lookup into shadow maps, simple texture mapping can be used to lookup the lightmap texture. As such they can produce fantastic lighting even on low powered machines such as mobile phones. This is the reason many of the first action 3d games used lightmaps, such as early quake and unreal.
#### Cons
* Lightmaps are limited in what kind of lighting information they store. Classical lightmaps only store the diffuse component of the lighting, and ignore the specular component. There are some more complex types of lightmaps but only the diffuse type is directly supported in Godot so far.
* Lightmaps only deal with static lighting. As the lighting is baked it gives a snapshot of lighting in one particular arrangement. If you want moving lights, or lights that change in brightness, you will need to either use other techniques or combine them with lightmapping. (One way of getting some simple variation is to bake more than one lightmap for a level, then blend the lightmaps on each frame prior to rendering, however this feature is not standard in Godot)
* Another big consequence of lightmaps being static is they don't show shadows for moving objects. This can be acceptable for the best performance on low end machines, but in practice, many games combine lightmaps with other techniques to get some kind of shadows on moving objects.
This may mean using lightmapping in combination with traditional realtime lighting, but for example, only rendering dynamic objects to the shadow maps. You can also render the realtime lighting as normal for all objects, but only put indirect lighting into the lightmaps. This gives some increase in visual quality compared to fully realtime lighting but does not help performance wise.
I have been preparing two workflows to simplify lightmapping a level with LPortal.
1) Internal workflow - Uses Godot to UV map the objects and the BakedLightmap node to create the lightmaps
2) External workflow - Allows UV mapping and rendering lightmaps in an external 3d modelling program such as Blender
The first workflow is fast to use but the end results are not as high quality as using an external program. There are also currently (as of 3.2 alpha) bugs in the Godot uvmapper and lightmapper, which can result in visual anomalies.
I am still polishing the external workflow, so will only describe the internal workflow here. It can be a good idea to examine the [BoxRooms demo](https://github.com/lawnjelly/lportal-demos/tree/master/BoxRooms), which shows the process in detail. Note that all this can be done manually, or with your own code (it is not even a core part of LPortal, more convenience functionality), the workflow is just a way of simplifying this for most people.
1) Create your level as normal with rooms and portals.
2) Add spotlights as desired within the rooms (omnis may also work but I am still working on them).
3) You can also use directional lights (outside the room list).
4) Make sure each light has the property set 'Use In Baked Lighting'.
5) The workflow allows having one shared lightmap for the entire level, instead of one per object. To do this as well as running the game as normal you must also have a 'preparation' phase where the level will be UV mapped (a second set of UV coordinates, which are used for the lightmap). The preparation phase can be a separate godot project if desired, the only output required for a game level is (1) the uv mapped level and (2) the lightmap.
6) For the preparation phase instead of calling `rooms_convert` you should call:
This will save 2 files, a .tscn containing a merged mesh (proxy) of the entire level, and a .tscn containing the entire level, but with a second set of UV coordinates matched to the proxy.
7) You now have to create the lightmap. Make sure there is a BakedLightmap node in the scene, and add the proxy .tscn to the scene using the 'link' button in the Godot IDE. Hide any other geometry aside from the proxy. Make sure the original level roomlist is in the scene, but hidden. The original level contains the lights that are needed for baking.
8) When all is set up, select the BakedLightmap node and click 'Bake'. If all is well it should bake a single lightmap, and save it in the folder specified in the BakedLightmap path.
9) The only 2 things of interest at the end of the preparation phase are the final level .tscn file, and the lightmap. You should copy these to your main project (if you are using 2 separate projects, one for the game and one for preparation / baking).
10) The only other thing necessary for your game to use the lightmap is to use a shader that blends the lightmap texture with the level textures. Here is a simple example:
