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Monday, November 28, 2011

Streaming LOD

If you remember an earlier post, I had built a little program called Sputnik to test mesh streaming over HTTP. Over the weekend I had some time and made it stream multiple levels. Here you can see some results. Please be forgiving, I did not spend any time on the texturing, lighting or anything but streaming.

LOD transitions are smoothed now whenever possible. I do it by gradually blending the changes into the scene. There are a couple of bugs in the normals which make popping more noticeable than what it should.

I capped download speed at 200 KBytes per second. Often the downloading lags behind the camera position. The scene shows with less resolution until the download catches up. I think 200 KBytes per second may not do it once I add large trees and buildings. I will also reduce the amount of mesh simplification, the rock profiles are too straight on this one.

Friday, November 25, 2011

Material World

I often get questions about the texturing in the screenshots and videos I have posted so far. There is nothing really new in what I do, still it may help to discuss it.

The material system I devised shares the same core philosophy as the voxel system. It is all based on my hope that if you put enough layers into something it will eventually look good. I know this is an awfully technocratic approach to art and design, it is just the way I found to compensate my lacking skills. I also hope this system is flexible enough for true artists when the time comes.

The volumetric world definition is a set of layers. Each new layer adds or subtracts to the layers before. If you look at a cliff formation, there is one base layer for the main volume of the cliff, then other layers on top of it, each one adding a different type of rock.

Each volumetric layer has a different material assigned to it. The material system defines the look of each layer, and very important, how transitions between different materials are rendered.

Let's look first at how material boundaries work.

Once voxels are converted into a triangle mesh, each triangle ends up having a material assigned to it. A change of material happens because the predominant voxel layer changes too. Very often the material boundaries align perfectly with sharp features in the geometry.

This is what you want in some cases. Imagine where the foundation of a building goes under the rock. You don't want the rock and the wall materials to blend, it would look weird. In other cases you do want different materials to blend. For instance where snow ends and bare rock starts. A clean line would not be right.

This behavior had to be controlled by a material setting. The engine then looks at all the triangles contributing to each vertex and compiles a list of materials for the vertex. Material boundaries form based on these blend settings.

At some point I was alpha-blending materials at the boundaries and baking the resulting image into a texture. I would later use this texture for rendering. This alpha-blending turned to be a problem when I moved all material rendering to realtime shaders. As you will see next, each material is a also collection of layers. Rendering a single material means all these layers must be evaluated and blended together. Now, imagine a point where three different materials overlap: I would need to evaluate all the layers for all materials there. It was often worse than that, in some spots I could find up to seven or eight different materials.

I knew rendering a single set of layers was fast enough, so what I did was pick one of the materials and render just that one. Materials already have an alpha value. Picking the one with the highest alpha does the trick. This still creates sharp transition lines, but they can be fixed by offsetting the alpha by a noise function. The transitions become more natural that way.

The following screenshot shows several transition areas as produced by the shader:

Then there is the other half of the material system: how each material is defined. As I mentioned before, it is just a set of layers. Each layer has defines several properties that control how it looks and how and where it is applied. Here are some of them:
  • Diffuse map: A texture with the color information for the layer
  • Normal map: A texture that describes additional orientation changes in the surface of the layer
  • Start and Bottom angles: These angles determine where the texture appears based on the orientation of the geometry. They are based on the world's up vector. This is what you would use to put moss on the top of rocks for instance
  • Mask noise: A noise definition that determines the intensity of the layer at any point in world space.
I have some other properties, but the ones listed above do most of the work.

I leave you with a screenshot of Buddha, where you can see the angle properties used to place some grass in the horizontal areas. It does not make sense in this case, this is just so you can see it in action:

Thursday, November 17, 2011

Streaming Meshes

Here is a quick video I did over the weekend. It shows a new program which connects to a web server and downloads chunks of terrain as you move.

I had posted similar videos in the past, but there is a radical difference. In earlier videos the meshes were loaded from the hard-drive. This baby you could run from your home. I actually plan to put it out for everyone to try it early next year. Since it will be the fist foray of this technology out there, I called it Sputnik.

It is only the first level of the clipmap, but I have expanded it so the load it generates is similar to a full scene spanning several kilometers. I capped the download speed to one megabyte per second to see if the downloading can cope with the camera movement.

The meshes are not textured in this video, but the all the information required for the final rendering is being downloaded. So this is really it. The chunks average 20K. They contain the mesh, normal maps and material information. I tried hard to make them smaller than that, but the loss in quality was not acceptable.

Using HTTP for transfer is a gamble, I'm still not sure about it. So far the results are encouraging, but I'm still weary of the worst case scenario happening too often. With HTTP it is not really about the transfer speed, it is the overhead of starting requests what worries me.

If you have any experience on this field, I would really appreciate your insight.