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Tessellation

Tessellation can be treated as subdivision (like the one in Blender) automatically executed by GPU. In OpenGL, it is achieved by tessellation shaders.

Tessellation can be used to implement the dynamic level of detail (LOD) of a terrain. Assume that a terrain is composed of several quads, based on the distance between the viewer and a certain quad, tessellation shaders decide to either subdivide that quad or not. The closer the quad is to the viewer, the more subdivisions happen. For a large terrain, this technique can significantly improve performance.

You can refer to [1, 2] for more details.

License

The MIT License (MIT)

Copyright (c) 2021, Jiang Ye

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Note

The shared edge problem

Within a terrain, quads always have shared edges. For a shared edge of two quads with different resolutions, if their gl_TessLevelOuter parameters are different, gaps appear between the two quads. This problem has been reported by this thread.

The vertex order of a quad patch

The official document [1] provides the corresponding edges of gl_TessLevelOuter and gl_TessLevelInner parameters of a quad patch. But it does not mention the vertex order. After some tests, I have found the correct order on my computer (NVIDIA GeForce GT 750M).

vertex order

Since the implementation of OpenGL may be different between graphics cards, the vertex order may also be different.

Result

output

Reference

[1] OpenGL: Tessellation

[2] OGL Tutorial 30: Basic Tessellation

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Tessellation and dynamic LOD

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