Tessellation vs. High-Polygon Models?

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Deus Gladiorum

Deus Gladiorum

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Hey guys, so I'm trying to get an idea of how tessellation works and... well.. why we even have to use it. From my understanding, tessellation is a series of algorithms which break up a low-polygon model into a higher polygon model by adding extra points on a polygon where it can then be broken up into other points. Then a displacement map is applied and the given effect is that it's more detailed and more three-dimensional. But why is it that it needs to be there? Why can't the model already just be a high polygon model?

I'm guessing the answer must be practical and obvious -- that a higher polygon model would be more demanding -- but it seems like the extra work to tessellate and displace an object would be more demanding than just rendering a high polygon model from scratch. Perhaps I just don't understand what exactly happens when a polygon is rendered, but could someone explain that to me -- why tessellation is used instead of higher polygon models?
 
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ak47jar3d


Yes, it seems counter intuitive, but in particular purposes make a lot of sense. Its basically an algorithm (for highend cards only) of rapidly increases and decreases ploy count to depending on perspective. Texture mapping is the enemy here. Without tessellation, you have to map high ploy counts on...
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ak47jar3d

ak47jar3d

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The obvious answer is that performance won't be dragged down as bad as having more polygons. Tessellation is dynamic and higher polygon models are but require more complex code to work properly . If you looking at a particular surface, then tessellation creates more polygons and reverts back upon looking away from that particular objects. Otherwise, you would have to create you own algorithms to makes sure the objects revert back to low settings depending on player perspective. I becomes obvious when trying to optimize graphics on UDK (as it comes with dx 9 and dx 11).

tessellation-example-1.jpg
 
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Deus Gladiorum

Deus Gladiorum

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That's what I thought, but then here's what doesn't make sense to me: why is it considered a higher end feature? If tessellation required less work than just rendering something with higher polygons, why don't we just make every object incredibly low polygon and then tessellate it? Why would it be inadvisable to create models with a polygon count comparable to models from 2004 and then tessellate those models if it's less intensive?
 
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ak47jar3d

ak47jar3d

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Mar 3, 2014
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Yes, it seems counter intuitive, but in particular purposes make a lot of sense. Its basically an algorithm (for highend cards only) of rapidly increases and decreases ploy count to depending on perspective. Texture mapping is the enemy here. Without tessellation, you have to map high ploy counts on level load as oppose to just doing it in faster manner. Also, low end cards don't have the GPU power to perform real time tessellation in high graphic games. Tessellation on works on items in the game world that are repetitive (like grass and floor) with the same texture. Tessellation is impractical for most dynamic objects (character models, trees, guns etc..).
 
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