## Friday, May 29, 2009

### Horizontal displacement

As always I do not keep strictly to the plan, and decided to try one of things I wanted to do someday - horizontal displacement of terrain.

The fractal map computed for quadtree node already contains 3 independent fractal noise channels. The first one computes elevation and is seeded from heightmap data. Other two are used for detail material mixing and other things. There is also a fourth channel containing global slope.
I modified the shader that computes vertex positions to displace also in horizontal directions, using one of the two independent fractal channels. Amount of displacement also varies with global slope - areas in flat regions are shifted minimally, but sloped parts that are also treated as rock are displaced a lot. This makes rocky parts much more interesting. For the record, the actual equation used for displacing point on a sphere in tangent space is this:

Next thing that had to be done was to compute the normals of such deformed surface. Article in GPU Gems about deformers provides nice info about Jacobian matrix that can be used for the job. After some pounding to my math circuits I managed to produce the following Jacobian of the above equation (in tangent space):

Normal is then computed as cross product between the second and third column, since the tangent and binormal are {0,1,0} and {0,0,1} respectively.

Finally, here is the result - left side original with only the vertical displacement, on the right side vertical&horizontal displacement:

There are still some issues but the overall effect is quite nice. Of course, collisions with sloped parts are no longer accurate and I'll have to do something with it later.

Outerra planetary engine

## Friday, May 22, 2009

Complete dataset for Earth with 76m sampling is 192GB, after wavelet compression it is roughly 14GB. That is still too much for direct download, and it will be even slightly larger after more detailed data are used for some parts at higher latitudes that are currently only coarsely defined. Possible solution could be to use a coarser sampling, but this rapidly washes out nice terrain features that the fractal cannot supersede easily.
Fortunately it seems we will be able to use the 76m dataset since progressive download works quite nicely.
Wavelet compression of elevation data helps here because it not only nicely compresses the data but perhaps more significantly it arranges the data in layers by level of detail. Particular level of detail for quad-tree nodes can be then downloaded when needed.

Results are that landing on the ground in a mountainous area (where the data are largest) requires around 30-40MB of compressed data to be downloaded progressively and cached. Further data (3-8MB) are needed again only after traveling some 300km from the spot. So it seems that even camera following a cruise missile can be handled too, though possibly it could skip the most detailed data at that speed anyway.

Using finer sampling manifests itself mainly in mountainous areas where high frequency features occur most.
Here's comparison of High Tatras as rendered by the engine and by the nature.

Currently we are working on material mixer that will allow us to assign different materials to different climatic bands, so the peaks here will not be covered by grass and mountain pine will grow at higher elevations too.

Outerra planetary engine