Building Window Generation

I'm continuing to work on 3DWorld's procedural cities, in particular the buildings. I want to make them look more interesting and realistic. Some of the building textures are plain brick or concrete block. There are no windows, doors, or other features. I've disabled these in many of my city screenshots because they look out of place next to the office buildings and other types that do have windows. It's time to fix this.

I decided to generate windows for buildings. Windows are just drawn as a texture in a postprocessing pass on top of the original geometry for the day time scenes. I draw the geometry of these buildings a second time, which applies the windows as decals using an alpha mask to let the bricks show through non-window areas. However, at night, building lights come on and the windows are emissive. These are drawn in yet a third pass with a different shader. I'll describe this more later.

I began by adding uniform 2D grids of windows to the previously windowless brick and concrete block buildings. I scaled the windows to approximately match the vertical floor spacing of the office buildings that already had windows in their textures. It makes sense that the various building types would be at the same scale. I made the center of windows a dark gray with a light gray window frame around them. There is only one window texture, and it contains exactly one window. The texture is generated based on user-specified config files that control window width, height, x-space, y-space, and border width. Window spacing and grayscale color can also be specified per-building material to help them match the surface textures with good contrast. Here are two screenshots of buildings with windows added. Note that I also added sloped roofs to some of the larger rectangular buildings.

Brick and concrete block buildings with generated windows in a uniform grid.

More brick and block buildings with generated windows. Window colors have been adjusted for good contrast.

The next step was making these windows look better at night. Previously, buildings were all a uniform grayish color in the dark. Car headlights and city streetlights definitely improved the look of the city, but buildings were still dull and boring. Night lighting really made buildings come to life.

This part was inspired by Shamus Young's Pixel City project from 2009. Apparently, this project has been restarted. I'm not going for the same look as Pixel City though. I want cities to look good in daytime, night time, and in between. I want them to be viewed from street level instead of just from above. I haven't added the window shape/color variation, lighted signs, and other detail lighting and geometry yet. I don't think my city lights look as good as Pixel City, but I'm still working on them.

I started by adding lights to some of the windows that were generated by the procedure above. These include smaller brick and block buildings that were placed outside the main city. I had them shoved out of the way to keep the windowless buildings out of my screenshots. There are no roads, cars, or streetlights out here. It's easier to experiment with lighting with the other light sources turned off anyway. Here is how it turned out.

Buildings with generated windows lit up at night in shades of white, yellow, and blue.

Each building has a single window light color that's randomly generated to be in the bright yellow to light blue spectrum. It's a little difficult to see unless you click on the image to zoom in. Too much color variations looks unnatural. This allows for various light styles including incandescent, fluorescent, and LED. It's the same technique I used for car headlights. In addition, each building has a random number of lit windows, between 10% and 50% of the total.

You might wonder how I'm able to light a random subset of the windows efficiently when there's only a single texture. All of the magic is in the fragment shader. We have access to two important parameters here: the texture UV coordinates and the per-building emissive light color. Texture coordinates are generated by scaling the building {x,y,z} vertex positions in world space. This ensures that textures align correctly across building faces that are split into multiple quads/triangles. Unfortunately, this also causes windows to sometimes wrap around the corners of buildings. I can't think of an easy fix for this.

The window texture contains a single window + frame within the texture's UV range of [0.0, 0.0] to [1.0, 1.0]. We can split the texture coordinates into an integer and a floating-point fractional component using the fract() GLSL function. The fractional components tell us where this texel is within the window. If it's around 0.5, the texel is near the center of the window. If it's close to 0.0, it's on the bottom/left, and if it's close to 1.0, it's near the top/right. The integer components give us the world space index of the window (u=x or y, v=z). We therefore have two pieces of data here that can be used to add variation to windows:

• The window color, which is per-building
• The integer texture coordinate (window ID), which is per-window

These values can be used as seeds in a pseudorandom number generator within the shader. The window color is hashed to produce a lit window density value between 10% and 50%. This will determine what percentage of windows in the current building are lit. The integer texture coordinate can be used to generate a random number to compare against this lit threshold. Together, these variables will determine which windows are lit and which ones are dark. Fragments from dark windows are discarded (not drawn) by the shader. Since all fragments (pixels) of a window have the same window ID, they will all be either lit or dark.


Let's move on to the inner city, which is full of tall office buildings that already include windows in their textures. I decided to create a larger city this time. The city is a single flat rectangular area of several square km with a total of 3700 buildings. Each building uses one of 12 unique side textures. Here is how the city looks in the day time. I get around 92 FPS with this view. All windows are part of the original building textures; none of them are generated by the new algorithm.

A large city that stretches to the horizon. These office buildings all have windows as part of their 12 different texture maps.

Here is how it looks at night, before I made any modifications. The buildings are the same bland grayish colors as the brick and stone buildings. All the texture detail is muted by the dark lighting, except at ground level where the streetlights and headlights illuminate it. The moon is out here. Without the moon, the scene is even darker gray.

A large city at night. The cars and streetlights produce light, but building windows are all dark. This will have to be fixed.

Here is the same scene with window lights enabled. I think they look pretty good for medium to far distance buildings. This addition makes the scene look much more interesting. Note that distant building lights are obscured by fog, allowing them to blend into the background. This helps to hide aliasing for sub-pixel windows. Also, I moved the moon below the horizon for this image so that the lights stand out more.

The same city as above, but with lit windows added to the office buildings. Looks much more interesting.

This screenshot shows just how large the city is. There are thousands of buildings stretching to the horizon.

Smaller city with cars, headlights, streetlights, and building window lights.

Here are some images showing how window lights look at a street level view. I don't think the nearby windows look very good because they aren't perfectly aligned with the building textures. I spent a long time finding the optimal values of texture xscale, yscale, xoffset, and yoffset to use with each of the 12 building textures. This helped, but didn't really work for buildings with nonuniform window size or placement. Some textures have large gaps between every few floors. Some have different window size/spacing/placement on some floors. Others have groups of 2-4 windows closely spaced together with larger gaps between the groups. It's just too difficult and time consuming to manually align all of the light rectangles with the actual windows for each texture. I did the best I could in one night of work.

Street view showing city lights. Nearby lit windows don't look as good here.

Street view again. These buildings have better alignment of window lights because windows are spaced out in a regular grid.

One hack to help with this problem is to fade nearby window lights to transparent by adjusting their alpha values based on distance to the camera. It's one line of shader code. This improves the screenshots, but the fade looks a bit odd when the player is moving.

Street view with nearby window lights faded out to transparent. See the mostly faded lighting on the left side building.

Here is a screenshot showing buildings placed on the hilly landscape. There are no roads, cars, streetlights, or traffic lights here. The foreground buildings are office towers, and there are some brick/stone buildings in the background. This is what the placement algorithm produces when roads are disabled.

Buildings lit up at night. These buildings are scattered over the scene at various elevations, with no roads.

I added an experimental bloom effect for city building windows. It works well enough for mid distance windows. Bloom applies to headlights, streetlights, and traffic lights as well as windows. I tried using a two pass x/y separable Gaussian blur, but the x and y blur directions were obvious. This technique works better with round or spherical light sources than it does with square light sources. I had to switch to using a more expensive single pass 2D Gaussian blur for the bloom effect. A 17x17 kernel size produces pretty good results with little change in frame rate. I suspect this is because the GPU is spending so much time in the vertex shader drawing car models. The bloom shader can be found here. The following two screenshots show city lights with bloom. Note how bright it makes the car headlights appear. Nearby buildings have lights that are fading out and therefore have less bloom.

Street view of city night time lights with bloom effect applied to make lights appear to be brighter.

City lights with bloom effect, shown from above.

Enabling all of these effects drops the frame rate to 58 FPS for a street level view facing into the heart of the city. This includes terrain, grass, 3700 buildings, roads, 4000 cars, traffic lights, streetlights, 1024 dynamic spot light sources, 40 dynamic shadow maps, a city lights pass, and a large kernel radius bloom filter. Oh, and also water with reflections, even though it's occluded behind the buildings. I'm sure I can optimize this if needed.

That's it for now. I think the results are good, a huge improvement to the look of night time cities. I still need to work on some lighting effects. I would like to add a better bloom effect to make windows look brighter, especially in the distance. It would be great if I could find a way to improve window lights for nearby buildings. I would also like to come up with some system for adding detail to the windows: shadows, furniture, etc. I'm not sure if I can generate these details in the shader using the window ID. Maybe, I'll have to experiment with this.

Dynamic City Shadows

This post is a short update on my procedural city project within 3DWorld. Last time I added car headlights, streetlights, and parking lots. This time I've added shadows to go with nearby headlights and streetlights. This was challenging to do efficiently due to the dynamic nature of the cars, the large number of nearby light sources, and the high complexity of the car models.

All lights used are directional spotlights, so their shadows can each be stored in a single shadow map. Both the light sources and the shadow casters can include dynamic objects, which means that shadow maps usually can't be reused across frames. I didn't attempt to detect and optimize for the case where everything was static (streetlights with no moving cars in their light volumes). No, every shadow map is recreated each frame by rendering all objects within their view frustums. Only the texture slots are shared across frames.

In the case of cars, I used a lower detail model where the materials that had little effect on the shadow were disabled. In most cases, I only enabled the car body and underside. These materials represent maybe 20-25% of the total triangles. This means that there were some light leaks through holes in various places where objects/triangles were missing such as wheels, headlights, etc. This made such a huge difference in frame rate (14 FPS => 48 FPS for 64 lights) that I decided it was worth the loss in quality. For most of the car models, the visual difference wasn't really that noticeable.

I'm using 512x512 texel shadow maps for dynamic objects, which is a compromise between performance and quality. For reference, the global sun and moon shadows use multiple 4096x4096 shadow maps - about one per city. Shadow maps use a 9-tap PCF (Percentage Closer Filter) with a Poisson distribution of sample points. This makes the shadow edges softer and hides aliasing.

Now that I have the 3DWorld source code up on GitHub, I can actually point to the source files:

• C++ code for city generation, road networks, and cars can be found here.
• C++ code for building generation and rendering can be found here.
• GLSL Shader code for dynamic light sources with shadow maps can be found here. Note that this is just part of a shader.

Here are some night time city screenshots with shadows. They're not the highest quality, but I was able to get 48 FPS with 64 shadowed lights and 58 FPS with 40 shadowed lights. There aren't too many detail objects in the city to cast interesting shadows. I'll probably add in some more objects later.

The lamp post casts a shadow on the cars and building behind it.

Dynamic lights + dynamic shadows with car headlights. The area under and in front of the cars is dark due to shadowing.

The orange car is caught in the police car's headlights and projects a shadow on the building behind it.

The traffic light casts a shadow on the blue car to the left.

Soft shadows from cars parked under a streetlight.

Shadows cast by various parked cars in a parking lot with streetlights.

Streetlight shadows produced by the lamp post and the building.

I also added car shadows for the sun and moon, but only for static parked cars. These were relatively easy and inexpensive to add, once I had the framework in place. If you look closely you can see some of the new car models I'm using. The number of unique models has increased from 6 to10. I also tuned some of the level of detail parameters to improve frame rates.

Here is a screenshot of the current state of parking lots. In addition to shadows, I randomized car parking positions some more.

Soft sun shadows from parked cars. Moving cars still use the cheap method of a dark textured quad under each car.

That's it for this post. I still need to figure out how to add better sun and moon shadows for dynamic cars. Currently, sun and moon shadow maps are generated once and cached, and only updated when the sun/moon moves. There are many high resolution shadow maps involved, and objects are drawn to them in full detail. I'm not sure how to do this efficiently for moving cars. I think it may require a new system.

I'm probably done with city lighting and shadows for now. There are still quite a few other tasks that I can work on. For example, I need to add more objects to fill in the empty spaces between buildings. I also need to add windows to the smaller brick and concrete block buildings somehow. I've removed them from the city scenes because they appear too plain and somewhat out of place. Ideally, I want to find a way to combine different window textures/types with the various brick and block exterior building material textures.

Procedural City Update: Lights and Parking Lots

I'm continuing to work on 3DWorld's procedural city. In the last post, I added cars. In this post, I'll describe some of the other features I've added to improve the look and realism of cities. Here is a summary of the main accomplishments, listed in the order in which I implemented them:

• Ambient Occlusion under cars (added as an update in the previous post)
• Dynamic lighting for car headlights on roads, buildings, and other cars
• Parking lots full of parked cars
• Occlusion culling of cars using buildings
• City streetlights

That's three (two and two halves) of my future work list from the last post. Pretty good progress so far.

Ambient occlusion was already shown in the previous post. It's a pretty simple trick using a partially transparent gray texture under each car, so there's not much to say about it. I would love to add car shadows, but unfortunately I still haven't figured out how to do that efficiently. I'll keep it on the todo list for now.

There's nothing to show for occlusion culling. If done correctly, the player will never see it. It's an optimization where cars that are behind nearby buildings don't need to be drawn. I do this by selecting all visible buildings (in the view frustum) within two city blocks of the player. Then, for each car, I generate rays from the top 4 corners of its bounding cube to the player camera. If all rays intersect any one building, the car is occluded by the building and doesn't need to be drawn. This is only needed for nearby, high detail car models. Since buildings are mostly convex, and rarely have overhangs, this tends to work correctly. Occlusion culling improves frame rates, especially now that I've added parked cars. This optimization increases in-city framerate from 50-60FPS to 80-90 FPS. Yes, most of the time is spent drawing car models. Everything else can be drawn at 200 FPS.

I'll describe what I did for parking lots, headlights, and streetlights below.


Parking Lots

I found something to fill in the empty space between buildings - parking lots! What's a big city without parking lots? Each parking lot is between 12 and 28 spaces wide and between 1 and 3 rows long, as constrained by config file parameters. They're placed wherever there's enough space between a road and a set of buildings. Each parking lot is populated by cars using a random density between 0% and 75%. This results in about the same number of parked cars as there are moving cars on the roads. The parking lots are randomly oriented with randomly selected car directions. I left the cars all pointing in the same way so that the spatial sorting algorithm would magically draw them back to front to make alpha blending of transparent windows work. Here is what a parking lot with 12x3 parking spaces looks like.

Parking lots partially filled with cars. These cars are inactive/static and their headlights are always off.

These parked cars don't take much extra CPU time and memory because they're static instances with no update logic. I haven't implemented car parking behaviors, and I'm not really planning to add that. However, parked cars do take extra GPU resources to draw. This is why I had to add occlusion culling using nearby buildings. Parking lots are only generated when buildings are enabled. They're guaranteed to have padding between streets and buildings so that cars have space to enter and exit.


Car Headlights

It was actually pretty easy to add dynamic lighting support to the city scene. I used the same lighting system as I did for gameplay mode as explained in a previous blog post. Each nearby, visible car has two headlights that automatically come on in the dark. Car headlights make a huge difference in night time scenes and dramatically improve realism. Headlights illuminate the road surface, buildings, and other cars. There are currently no headlight shadows. Here are two screenshots showing car headlights with no buildings.

A row of cars stopped at a light, shining headlights on each other. Buildings have been disabled.

Cars with headlights traveling on roads. Buildings have been disabled.

I've modeled headlights as spotlights (cones) pointing slightly downward, with smooth lighting falloff at the edges of the beam. This is very similar to the "flashlight" in gameplay mode. Colors vary randomly from yellow to blue-white to simulate different lighting technologies (incandescent vs. halogen vs. LEDs). Here are three more images, this time showing buildings.

Cars with headlights driving in a city at night.

Cars with high beams in the city, shown from the point of view of a car. Maybe headlights aim too high?

Cars with headlights driving through an intersection.

The headlights are probably too bright and pointed too much upward so that they shine into the windows of the office buildings. This would probably drive people crazy! I left the settings that way just to make the lighting easier to see; I've switched to "low beams" after taking these screenshots.


City Streetlights

Parking lots and headlights are great, but there are still too many dark, open areas. I decided that I can fill some of the space with streetlights along the roads that come on at night. Each city block has four streetlights, one along each side. They're offset from each other and spaced out evenly along roads to produce somewhat uniform light coverage. I used the same dynamic lighting system as car headlights. Streetlights are also spotlights, but with a 180 degree field of view and pointing down.

Technically, streetlights aren't dynamic because they don't move. However, there are thousands of them, and I can't have them all on at the same time. This would hurt the frame rate too much. Instead, I select and enable only a few hundred of the most influential lights sorted by (radius/distance) that are within the camera's view frustum. The upper limit is 1024 lights combined across car headlights (two per car) and streetlights. Here are some screenshots.

Closeup of a streetlight shining on an empty road in the city.

Streetlights and cars illuminate the roads and buildings at night. Yes, these buildings have no windows or doors.

I haven't implemented shadows for streetlights yet.

These next two images show just how many lights there are in the city. Currently, only nearby cities have lights enabled. Maybe I should add a slider to control streetlight color?

Streetlights and headlights in the city, shown from above.

Streetlights light up the entire city at night. There are about a thousand lights in this scene, half of them dynamic.

All that's left is to add lights in the office buildings. I'm not sure how difficult that is. Would it be possible to have the fragment shader somehow detect windows and add emissive light inside their bounds? I currently don't really know how to do this efficiently, but I guess we'll see in the next post. Oh, and I probably need to add windows to those all-brick buildings, or use different textures.

One more thing: 3DWorld is now on GitHub! Here is the project page. Now you can actually go look at the code, though it may not be easy to read.