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Wednesday, May 18, 2016

Terrain Synthesis

This is just a teaser. We are still working on this, but we got some results that are already good enough to show. It is not about where terrain types appear (that was covered here and here), but how a particular terrain type is generated.

We want to make procedural generation as accessible as possible. Just like a movie director who shows a portfolio of photos and concept art to the CGI team and just says "make it look like this", we wanted the creator to be entirely clueless about how everything works.

This is how it feels to create a new terrain type. You provide a few pictures of it and we take it from there:

This system builds a probabilistic model based on the samples you provide. That is enough to get an idea of the base elevation. On top of that, several natural filters are applied. It turns out we do know a bit more about this landscape. We know how dry it is, what is the average temperature among other things. The only fact we are missing and have to ask about is how old do you think this is. The time scales range from hundreds of millions of years to billions of years. (If you believe your terrain is 6000 years old we cannot accommodate you at the moment.)

You can provide one or more sample pictures. The more pictures you provide, the better, but just one picture is often enough. Ready to see some results? The following terrains were synthesized out of a single photo in every case (do not mind the faux coloring, this is only to indentify the different terrain layers for now):

Providing multiple samples creates some sort of mix, similar to how you find both mother and father features in their kids:

This works with any kind of image. It could be some fancy concept art as seen below:

The natural filters in this case added some realism to the concept, and eroded some of the original hill shape. This could be avoided if you are after a more stylized look. But if you are short on time, and want to prototype different realistic terrains, the ability to quickly sketch something and feed it to the generator is a big help.

Of course you can still look under the hood and tinker with generation frequencies, filter parameters, etc. You can still have terrain models imported from Digital Elevation Models, or from third party software like World Machine. The key here is you do not have to anymore.

I'd be glad to enter into details of how this works if you guys are interested. Just let me know. I still owe the Part 2 of the continent generation. That should come shortly.

Saturday, May 14, 2016

Turtle Mountain

If you have ten minutes or so to spare I encourage you to check out this video. The rest of this post will be about how it was done:

The shyamalanian twist here is the guy lives in the back of a giant turtle. (Maybe not so much of a twist since the video title and thumbnail pretty much give it away.)

What you are seeing here is a new Voxel Farm system in action. It gets a very low-resolution mesh as a base and enhances it by adding procedural detail.

I think this is an essential tool for world builders.. Very often procedural generation deprives the creator of control over the large scale features of the terrain. Or, when control is allowed, it comes in the form of 2D maps like heightmaps and masks. There is no way to drive the procedural generation into complicated shapes and topologies like intricate caves, floating islands, wide waterfalls, etc.

We chose a massive turtle mountain to drive the point anything you can imagine can be turned into a detailed terrain. This is how it works:

The first thing you need to do is create a low-resolution mesh for the base of the terrain feature. This project used three of these meshes, one for the turtle's body and shell, another for the terrain protuberance on the top of the shell and one last mesh for a series of caves. Here you can see them:

On their own they were rather simple to produce. The tortoise is a stock model from a third party site. The mountain was done by displacing a mesh using a heightmap that had a fluvial erosion filter applied to it. The cave system mesh is a simple mesh with additional subdivisions and 3D noise applied to it.

These meshes were imported into Voxel Studio (our creative world building tool) and properly positioned relative to each other.

In addition to triangles, the meshes were textured using traditional means. Here you can see the texture that was applied to the turtle body:

Here is how the textured top mountain looks like:

Note how the texture uses single flat colors. Each pixel in the texture represents a terrain type, not an actual color. You can think of these as instructions to be passed down to the procedural generators when the time comes to add detail.

The meshes may appear detailed at this distance, but if you stretched them to cover four kilometers (which is the size of the turtle base in the world), you would see a single triangle span a dozen of meters or more. A single texture pixel would cover several meters. This would make for a very boring and flat environment. Here is where the procedural aspect kicks in.

Each color in a mesh texture represents what we call a "Meta-Material". I have posted about them before: here and here. In general a metamaterial is a set of rules that define how a coarse section of space can be refined. In this particular implementation for our engine, this is achieved by supplying two different pieces of information:
  1. A displacement map
  2. A sub-material map 
This is a very simple and effective way to refine space. The displacement map is used to change the geometry and add volumetric detail to an otherwise flat surface. The submaterial map registers closely to the displacement map so the artist can make sure materials appear at the right points in the displaced geometry. Once again the submaterial map does not contain final colors. Each pixel in this map represents a final voxel material that would be applied there.

Here you can see the displacement and submaterial map used for one of the metamaterials in the scene:

One particularly nice aspect of the system is that displacement properly follows the base mesh surface. It is possible to have nice looking cliffs and even apply displacement to bottom facing surfaces like the ceiling of a cave. For mesh-only displacement this is not usually difficult, but doing so in voxel space (so you can dig and destroy) can be quite complex. I'm happy to see we can have voxel cliffs that look right:

Metamaterials, beside displacement and submaterial maps, can be provided with "planting rules". This allows bringing in additional procedural detail in the form of larger instanced content. These can be voxel instances like the large rocks and boulders seen in the video or, they can be passed as instances to the rendering side so a mesh is displayed in that position. The trees in the video are an example of the later.

The previous image shows a mesh instance (a tree) at the left and a voxel instance (a boulder) at the right. Plants, grass, and small rocks are also instanced, but they are planted on top of materials, not meta-materials. One thing I did not mention before is this demo uses Unreal Engine 4. That is another key piece of tech that is coming along very nicely.

Already confused by these many levels of indirection? It is alright, once you start working with these features they begin to make perfect sense. More to that, it becomes apparent this is the only way you can get from a very coarse world definition into something detailed as seen in the video.

I hope you enjoyed this and that it gets your imagination started.

Monday, May 9, 2016

Applying textures to voxels

When I look back at the evolution of polygon-based content, I see three distinct ages. There was a time where we could only draw lines or basic colored triangles:

One or two decades later, when memory allowed it, we managed to add detail by applying 2D images along triangle surfaces:

This was much better, but still quite deficient. What is typical of this brief age is that textures were not closely fitted to meshes. This was a complex problem. Textures are 2D objects, while meshes live in 3D. Somehow the 3D space of the mesh had to be mapped into the 2D space of the texture. There was no simple, single analytical solution to this problem, so mapping had to be approximated to a preset number of cases: planar, cylindrical, spherical, etc.

With enough time, memory constraints relaxed again. This allowed us to write the 3D to 2D mapping as a set of additional coordinates for the mesh. This brought us into the last age: UV-mapped meshes. It is called UV because it is an additional set of 2D coordinates. Just like we have XYZ for the 3D coordinates in space, we use UV for coordinates in the texture space. This is how Lara Croft got her face.

We currently live in this age of polygon graphics. Enhancements like normal maps, or other maps used for physically based rendering, are extensions of this base principle. Even advanced techniques like virtual texturing or Megatextures still rely on this.

You may be wondering why is this relevant to voxel content. I believe voxel content is no different than polygon content when it comes to memory restrictions, hence it should go through similar stages as restrictions relax.

The first question is whether it is necessary to texture voxels at all. Without texturing, each voxel needs to store color and other surface properties individually. Is this feasible?

We can look again to the polygon world for an answer. The equivalent question for polygon content would be, can we have all the detail we need from just geometry, can we go Reyes-style and rely on microgeometry? For some highly stylized games maybe, but if you want richer realistic environments this is out of the question. In the polygon realm this also touches the question about use of unique texturing and megatextures, like in idTech5 and the game Rage. This is a more efficient approach on having a unique color per scene element, but still was not efficient enough to compete with traditional texturing. The main reason is that storing unique colors for entire scenes was simply too much. It led to huge game sizes while the perceived resolution remained low. Traditional texturing on the other hand allows to reuse the same texture pixel many times over the scene. This redundancy decreases the required information by an order of magnitude at often no perceivable cost.

Unique geometry and surface properties per voxel are no different than megatextures. They are slightly worse as the geometry is also unique, and polygons are able to compress surfaces much more efficiently than voxels. With that in mind, I think memory and size constrains are still too high for untextured voxels to be competitive. So there you have the first voxel content age, where you still see large primitives and flat colors, and size constraints won't allow them to become subpixel:

(Image donated by Doug Binks @dougbinks from his voxel engine)

The second age is basic texturing. Here we enhance the surface detail by applying one ore more textures. The mapping approach of choice is tri-planar mapping. This is how Voxel Farm has worked until now. This is sufficient for natural environments, but still not there for architectural builds. You can get fairly good looking results, but requires attention to detail and often additional geometry:

In this scene (from Landmark, using Voxel Farm) the pattern in the floor tiles is made out of voxels. The same applies to table surfaces. These are quite intricate and require significant data overhead compared to a texture you could just fit to each table top for instance, as you would do for a normal game asset.

We saw it was time for voxels to enter the third age. We wanted voxel content that benefited from carefully created and applied textures, but also from the typical advantages you get from voxels: five-year-olds can edit them and they allow realistic realtime destruction.

The thing about voxels is, they are just a description of a volume of space. We tend to think about them as a place to store a color, but this is a narrow conception. We saw that it was possible to encode UV coordinates in voxels as well.

What came next is not for the faint of heart. The levels of trickery and hackery required to get this working into a production ready pipeline were serious. We had to write voxelization routines that captured the UV data with no ambiguities. We had to make sure our dual contouring methods could output the UV data back into triangle form. The realtime compression had to be now aware of the UV space, and remain fast enough for realtime use. And last but not least we knew voxel content would be edited and modified in many sorts of cruel ways. We had to understand how the UV data would survive (or not) all these transformations.

After more than a year working on this, we are pleased to announce this feature will make it into Voxel Farm's next major release. Depending on the questions I get here, I may get more into detail about how all this works. Meanwhile enjoy a first dev video of how the feature works:

Tuesday, April 26, 2016

Geometry is Destiny

In the previous post, I introduced our new land mass generation system. Let's take a look at how it works.

For such a large thing like a continent, I knew we would need some kind of global generation method. Global methods involve more than just the point of space you are generating. The properties for a given point are influenced by points potentially very far away. Global methods, like simulations, may require you to perform multiple iterations over the entire dataset. I favor global methods for anything larger than a coffee stain in your procedural table cloth. The reason is they can produce information whereas local methods cannot: information is limited to the seeds used in the local functions.

The problem in using a global simulation is speed. Picking the right evaluation structure is paramount. I wanted to produce maps of approximately 2000x2000 pixels, where each pixel would cover around 2 km. I wanted this process to run in less than five seconds for a single CPU thread. Running the generation algorithm over pixels would not get me there.

The alternative to simulating on a discrete grid (pixels) is to use a graph of interconnected points. A good approach here is to scatter points over the map, compute the Voronoi cells for them, and use the cells and their dual triangulation as the scaffolding for the simulation.

I had tried this in the past with fairly good results, but there was something about it that did not sit well with me. In order to have pleasant results, the Voronoi cells must be relaxed so they become similarly shaped and the dual triangulation is made of regular triangles.

If the goal was to produce a fairly uneven but still regular triangle mesh, why not just start there and avoid the expensive Voronoi generaion phase? We would still have implicit Voronoi cells because they are dual to the mesh.

We started from the most regular mesh possible, an evenly tessellated plane. While doing so we made sure all diagonal edges would not go in the same direction by making their orientation flip randomly:

Getting the organic feel of the Voronoi driven meshes from here was simple. Each triangle is assigned a weight and all vertices are pulled or pushed into triangles depending on these weights. After repeating the process a few times you get something that looks like this:

This is already very close to what you would get from the relaxed Voronoi phase. The rest of the generation process operates over the vertices in this mesh and transfers information from one point to another using the edges connecting vertices.

With the simulation scafolding ready, the first actual step into creating the land mass is to define its boundaries. The system allows a user to input a shape, in case you were looking for that heart-shaped continent, but if no shape is provided a simple multiresolution fractal is used. This is a fairly simple stage, where vertices are classified as "in" or "out". The result is the continent shoreline:

Once we have this, we can compute a very important bit of information that will be used over and over later during the generation: the distance to shoreline. This is fairly quick to to compute thanks to the fact we operate in mesh space. For those triangle edges that cross the shoreline we set the distance to zero, for edges connected into these the distance is +1 and so on. It is trivial to produce a signed distance if you add for edges in mainland and subtract for edges in the ocean.

It is time to add some mountain ranges. A naive approach would be to use distance to shore to raise the ground, but this would be very unrealistic. If you look at some of the most spectacular mountain ranges on Earth, they happen pretty close to coast lines. What is going on there?

It is the interaction of plate tectonics what has produced most of the mountain ranges that have captured our imagination. This process is called orogeny, and there are basically two flavors of it, accounting for most mountains on Earth. The first is when two plates collide and raise the ground. This is what gave us the Himalayas. The second is when the oceanic crust (which is a thinner New-York-pizza-style crust) sinks below the thicker continental crust. This raises the continental crust producing mountains like the Rockies and the Andes. The two processes are necessary if you look for a desirable distribution of mountains in your synthetic world.

Since we already have the shape of the continental land, it is safe to assume this is part of a plate that originated some time before. More-so, we can assume we are looking at more than one continental plate. This is what you see when you look at northern India, even if it is all a single land mass, three plates meet at this point: the Arabian, Indian and Eurasian plates.

Picking points fairly inland, we can create fault lines going from these points into the map edge. Again this works in mesh space, so it is fairly quick and the results have the rugged nature we initially imprinted into the mesh:

Contrary to what you may think, this is not a pathfinding algorithm. This is your good-old midpoint displacement in action. We start with a single segment spanning from the fault source to the edge of the map. This segment, and each subsequent segment, is refined by adding a point in the middle. This point is shifted along a vector perpendicular to the segment by a random amount. It is fairly quick to know which triangles are crossed by the segments so the fault can be incorporated into the simulation mesh.

In this particular case the operation has created three main plates, but we are still missing the oceanic plates. These occur a bit randomly, as not every shoreline corresponds to an oceanic plate. We simulated their occurrence by doing vertex flood fills on selective corners of the map. Here you can see the final set of plates for the continent:

The mere existence of plates is not enough to create mountain ranges. They have to move and collide. To each plate we assign a movement vector. This encodes not only the direction, but also the speed at which the plate is moving:

Based on these vectors we can compute the pressure on each vertex and decide how much it should be raised or lowered, resulting in the base elevation map for the continent:

All the mountains happened to occur in the South side of the continent. You can see why this was determined by the blue plate drifting away from the mainland, otherwise we would have had a very different outcome. This will be an interesting place anyway. While the gray-scale image does not show it, the ground where the blue plate begins sinks considerably, creating a massive continent-wide ravine.

Getting the continent shape and mountain ranges is only half the story. Next comes how temperature, humidity and finally biomes are computed. Stay close for part two!

Tuesday, April 19, 2016

Geography is Destiny

Here are some images from the output of a new land mass generator we wrote for the Voxel Farm suite:

While the images are symbolic, they contain very detailed information about biome placement. Each pixel is approximately 2Km wide. You can imagine each biome type/pixel in this map replaced by a rich biome manifestation, which will provide elevation, erosion and other layers of detail. Rivers and lakes do not appear at this point because they need the final elevation. What you see here is more like a blueprint for where the next generation phase starts.

The challenge in this case was to make biomes appear in the right location. The method behind the images uses tectonic plate simulation for mountain ranges and a pretty cool humidity transfer system. I believe there is no other way if you want plausible maps. Just in case you want to see if the deserts and jungles make sense, note the wind in the three maps above comes from the south-west corner.

I will be covering how this works in my next post.

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