Procedural City: Stairs, Elevators, and Room Lights for Buildings

I've made lots of progress on building interiors since the last post two weeks ago. I started working on adding stairs and elevators to connect the floors together, but then I got distracted trying to add lights to room ceilings to improve the visuals. I was able to complete stairs, and they look much better when lit with shadows. I didn't get very far with elevators yet. The placement code is written, and the basic drawing code for elevator shafts is written, but I haven't gotten to drawing elevator interiors and details.

I'm going to show what I have at this point because I'll be flying to PA next weekend to visit my parents for Christmas and won't be able to work on 3DWorld for a few weeks. I can't really do graphics development on my old work laptop.


Room Lights and Shadows

It wasn't too difficult to get lighting and shadows to work for building interiors by using the existing lighting system I had in place for city blocks, which itself was adapted from fixed (non-tiled) terrain lighting for gameplay mode. I used the same lighting settings from the city config file, which is currently set to up to 1000 light sources and 40 shadowed spotlights. That was enough for city blocks, but is somewhat short of the number of lights and shadows needed inside buildings. Well, at least it's enough for the interior when the player is inside a building. It doesn't work as well when the player is outside looking in through the windows.

A rectangular light is placed in the center of the ceiling of each room. 50% of the lights are on, based on a random number. Rooms with stairs and elevators have a higher probability that the light is lit. These lights illuminate their respective rooms and sometimes the light extends into adjacent rooms through open doors. Here is a screenshot of a lit room with a table and some chairs, where everything is casting shadows. I use a 9-tap percentage closer filter with Poisson sampling to achieve smooth shadow outlines.

Room with a table and chairs, lit by an overhead light with soft shadows.

If you look closely you'll notice that the ceiling isn't very well lit. That's because I've modeled this light source as a spotlight pointing down. It's a bit misleading drawing the light as a rectangular cube on the ceiling. The problem is that 3DWorld doesn't support area light sources that have shadows. I only have shadowed spotlights and point lights. Point lights need six shadow maps, one for each cube face, which is a bit too much for these scenes. Spotlights can use a single shadow map (shadow plane), but the trade-off is that they're limited to somewhat less than a 180 degree (hemisphere) field of view. That means the light can only point downward and can't illuminate the ceiling.

My fix is to add a second light that has a wider field of view. To save on performance, this second light is smaller in radius and doesn't use a shadow map. I can't set it to more than 180 degrees because it will shine on the bottom surfaces of any objects on the floor above. But I can set it to right around 180 degrees so that it at least illuminates the ceiling somewhat. I need to be careful to make it small enough that it doesn't light the walls of adjacent rooms. That's why the light on the upper walls and the ceiling is somewhat dim. Well, the middle part of the wall is also light by both lights.

There's no real indirect lighting yet. I do have code to calculate the average ambient lighting in each room as a function of room light on/off state and incident sun/moon light through the windows, normalized to the room's surface area. So far I haven't figured out how to use this properly. The shaders I'm using don't have any easy/efficient way to set per-object/room material properties. So I use a simple constant ambient lighting term as a hack to avoid having the unlit surfaces appear black. It works well enough for now, but it's likely something I'll have to get back to later. At least the ambient lighting varies with the time of day so that it's darker at night.

Here's another screenshot showing some connected rooms with multiple lights and objects. This is the sort of geometry generated in buildings formed from overlapping cubes. The wall placement algorithm can't quite figure these out because of the irregular floorplans produced by the overlaps. It ends up with walls that end in the middle of rooms. It's not entirely wrong - I've seen offices with partial walls like this. It's not what I was intending to produce though. Oh well, there are more important problems that I need to fix, so I'll have to live with this for now. Just one more minor todo item for my list. At least this problem is pretty rare. This is the only building near the starting location where I've seen it.

Multiple rooms in an open floorplan with tables and chairs, lit with shadows.

While it was relatively easy to make room lights work, it was quite difficult to make them work efficiently. The first major problem to solve was the interaction between building lights and city lights such as streetlights and car headlights. I was originally combining them both together into a single 2D lighting grid. This lead to poor performance because the different sets of lights had very different scales and densities. Streetlights were spread out over many city blocks far into the distance. Each light covered the area the size of a small building. In contrast, room lights are much smaller and placed very densely. In particular, they're vertically stacked on each floor. It's not uncommon to have several hundred of them, maybe even a thousand, inside a single city block. This same area is shared by no more than a dozen streetlights. The uniform grid structure didn't perform well when the distribution of lights was this nonuniform, where all of the nearby building lights were crammed into a handful of grid cells. In addition, cities and buildings were accessing each other's lights when they didn't need to.

The fix was to split out the lights into two different systems, one for city streets and the other for building interiors. This took considerable time and effort. There were many global functions and variables that had to be moved into their respective city and building manager classes. Of course, it's good to remove global variables. This change resulted in both cleaner (though more complex) code and much faster draw times. Frame rates more than doubled in some of the worse cases. Now the framerate was usually above 30 FPS. That's an improvement, but still not good enough.

The next problem to tackle was related to overdraw and fill rate. The GPU's fragment shader has a ton of work to do with all of these light sources. I'm using a 2D spatial grid data structure to limit the number of light sources contributing to each pixel, but the room lights still overlap a lot in 3D space. Walls and floors block most of the actual light, which is something the shader has to figure out using shadow maps. If you were to stand on the bottom floor of a 20 story building and look up, there would be 20 ceilings drawn above you. Ceilings are added to the list of vertices/quads to draw starting with the first floor and working upwards. This means that the first ceiling will be drawn first and will occlude the others. However, the floors are ordered the same way, so a player standing on the top floor looking down would have to wait until the fragment shader drew all 20 floors. That's terrible for the frame rate! (I guess I should go back and reorder floors. Update: I fixed it but it didn't seem to help.) Even if I can order everything properly, there are still the walls, doors, and other objects to draw.

The solution here is to do a depth pre-pass before the main draw pass to pre-populate the depth buffer with z-values and ensure that each fragment/screen pixel is drawn at most once. This makes a huge difference, nearly halving frame times (again) in some worse case view angles. It did take a lot of work though. I had to add a lot of complexity to select and draw individual tiles of buildings that were near the player. Together these two optimizations made enough of an improvement that walking around inside of buildings ran consistently above 60 FPS, and in most cases above 100 FPS.


Stairs and Elevators

This next image shows a set of stairs that was placed between two floors. Stairwells are generated as vertical slices in the floorplans that extend through all stories of the building. They're cut into the ceilings and floors of buildings using constructive solid geometry operations. Each part of a building has a set of stairs, except for houses, which only have one set of stairs per house. For each set of stairs, a room is selected where stairs can fit in the longer dimension of the room. They're placed so that none of the interior or exterior doors are blocked. Lights are placed only if they don't overlap stairs. Stairs ensure that buildings are fully connected across all floors.

Stairs between building levels, lit with shadows.

Some office buildings have elevators as well as stairs. Elevators are only added to multi-part buildings that already have stairs placed. [To comply with fire code, all buildings must have at least one set of stairs.] Elevators are incomplete at this stage. I have the elevator shafts cut into the ceilings and floors, the wall textured exteriors, and wood panel interiors. The wood paneling is really supposed to only be for the elevator cars, which don't exist yet. I haven't added elevators doors or buttons yet either. I do plan to complete them later, and maybe even add working elevators that the player can use. Here is a screenshot of an elevator tucked into the corner of a building. Elevators can also be added to central hallways of office buildings.

An office building room with an elevator in the corner. Maybe I should make it larger?

Drawing Building Exteriors

The previous screenshots showed buildings viewed from inside. That's actually the easier case to handle. Floors and ceilings are opaque and solid, except for cutouts where stairs and elevators are placed. That means that in most cases, the lights on floors above and below can be ignored because the player can't see them. The exception is rooms that have stairs, in which case we need to include the lights from the rooms one floor below and above as well. This is needed to allow light to shine up or down the stairs. This works well most of the time. There are rare cases when light should be visible from a room on a floor above or below that's adjacent to the stairs. I'm not sure if I'll fix this or not. These optimizations allow the majority of building lights to be excluded when the player is indoors, which reduces draw time considerably.

When viewing buildings from outside, occlusion culling doesn't work as well. It's not easy to determine which room lights are visible. The set of possibly visible lights for a single building can range in the hundreds, which makes performance (frame rate) suffer. I've written an algorithm that tries to approximate which lights are the most visible and uses a priority system to assign shadows to the 40 most influential lights. It works in most cases, but not always.

There are several optimizations that can be made here, which work well in most cases. (These optimizations are in addition to the simple and obvious distance based light filtering and view frustum/visibility culling.) First, only lights in rooms that have windows to the side of the building facing the player are drawn. Rooms behind them are usually mostly occluded by walls and are of less importance. This causes some drawing artifacts though, most notably when a back room's lights shine through a doorway into a front room with no lights on that would otherwise be dark. To help with this problem, this optimization is disabled for building floors that are on the level of the player. This means that if a player looks directly into a window from outside, all lights on that floor will be drawn.

The second optimization is related to viewing angle. If the player is close to a building, then they're unlikely to be able to see anything significant in rooms many floors above or below their position. This is because the exterior building walls block the line of sight through the windows. Fortunately, I haven't added interiors to all-glass buildings, so there's a good amount of occlusion. I use a test of vertical distance divided by horizontal distance from the light source with a viewing angle threshold. This optimization has a minor impact on visual quality but isn't very noticeable to the player compared to the previous optimization.

Finally, the third optimization is to disable the small unshadowed lights that illuminate the ceilings when the player is outside. These lights cost about a third of the total lighting time, but provide only a small fraction of the visible light in the rooms. This can be seen in the screenshot below, where the ceilings are darker. I might add a config file option to always enable these smaller lights later if it seems to be needed.

View of stairs, tables, and chairs through the building windows. Many of the rooms have their lights turned on.

One great thing is that the room lights are all dynamic. I could probably get away with static lights, but I want to allow the player to turn on and off room lights in the future. Or maybe lights can go on and off randomly as time advances to simulate people using the rooms. I guess at some point I need to add people to these buildings as well. I don't think it takes much frame time to redraw the small shadow maps each frame anyway.

Here is a particularly large office building, one of those types that are formed by gluing together different overlapping cube sections. Actually, this is the exterior of that building with the odd half walls shown in the second image from the top. It's 20 stories tall and has 27 rooms in the floorplan of the main section, for around 500 total rooms. If half of them are lit, that would be 250 lights just in this one building! This is the largest building in the starting area around the player and the one that causes the lowest framerate. I was able to improve the framerate from a low of 38 FPS to a new low of 74 FPS using a variety of optimizations. At this point I'm pretty happy with the 74 FPS. Buildings are now taking similar draw times compared to grass. For reference, this view runs at 134 FPS with building interior lighting disabled. Also, the smaller buildings can be drawn at over 100 FPS.

Looking into the windows of a large 20 story office building. Room interiors are lit.

Here is a wireframe view showing just how many walls, ceilings, floors, doors, tables, chairs, stairs, and lights it has. There are thousands of objects placed in this one (out of 14,000) building. It probably has a thousand stairs alone. However, most of  the frame time is determined by the number of lights and shadow maps used. Even though the building is drawn once for each shadow map, that doesn't contribute very much to the total frame time. I should be able to add several times as many room objects without too much of a framerate hit. I guess we'll see.

Wireframe view of the previous screenshot showing the large number of placed objects and lights in this building.

Unfortunately, room lights don't match up with lit vs. dark windows at night. Rooms are lit on the CPU side and statically cached in VBOs. Windows are lit dynamically on the GPU inside the fragment shaders by using the window IDs as random number seeds to determine which windows are lit and which are dark. The GPU doesn't even know which windows along a building exterior wall belong to the same room. I have no idea how to solve this problem. It's not so bad though, at least the window lights fade out when the player approaches the building, making the problem less obvious.

What's next? I have several items on my todo list. I need to finish elevators. I would like to add some better form of indirect lighting for rooms using per-room or per-object ambient values, or something better. I also need to revisit some of the light occlusion culling methods to see if I can come up with something that has a better performance vs. quality trade-off. Finally, I need to place more smaller detail objects into rooms.

As usual, all of the code can be found on my 3DWorld GitHub project here and here.