# Terrain gaps

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Hello, I have seperated my terrain into quads. Depending on the distance from the viewer to the quad, the quad is displayed in fitting level of detail. Between the quads, there might occur gaps. Currently I am connecting these points with a new triangle ("skirts"). But it looks very ugly: Picture 1(http://members.kabsi.at/devp/prob2.jpg): Gaps when texturing is used Picture 2(http://members.kabsi.at/devp/prob1.jpg): The mesh. How can I fix this problem, or are there better (same/better speed + not to difficult) ways to generate the terrain (btw: I'm using OpenGL)? Thanks,

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You need to avoid 'T' junctions, aiming for a smooth transition between detail levels. It's generally best to keep neighbouring tiles within one level of detail, and work out some sort of transition scheme, e.g something like this:

+---+---+---+---+-------+-------+|\  |\  |\  |  /|\      |\      || \ | \ | \ | / | \     | \     ||  \|  \|  \|/  |  \    |  \    |+---+---+---+   |   \   |   \   ||\  |\  |\  |\  |    \  |    \  || \ | \ | \ | \ |     \ |     \ ||  \|  \|  \|  \|      \|      \|+---+---+---+---+-------+-------+|\  |\  |\  |  /|\      |\      || \ | \ | \ | / | \     | \     ||  \|  \|  \|/  |  \    |  \    |+---+---+---+   |   \   |   \   ||\  |\  |\  |\  |    \  |    \  || \ | \ | \ | \ |     \ |     \ ||  \|  \|  \|  \|      \|      \|+---+---+---+---+-------+-------+

Of course, this approach gives rise to a new kind of visual artifact, known as 'popping' - caused by sudden changes in the mesh as it changes detail level. These can be avoided using a technique known as geomorphing, or can be ignored altogether if the effect is small.

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Whilst quads might be nice and easy to work with they aren't the most efficient. For large terrain triangle-strips are optimul. Using indexed gemoetry helps with vertex caching and can reduce small gaps appearing between polygons when floating-point numbers are rounded.

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I am using triangle strips within the quads.

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A simpler way to fix the problem is to draw a "skirt" around each quad.

Pros: The geometry is static, so no time is spent constructing it and sending it to the GPU.
Cons: It means drawing extra geometry (+13% for a 33x33 quad).

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I use skirts (=the wrong textured parts)

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ups. I got the problem: I assigned the wrong texture coordinates to the skirts. now it works.

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Edit: HURR I can't read, I didn't notice that you were using LOD... The following post doesn't apply to you, but I'll keep it here in case it helps someone in the future.

Correct me if I'm wrong, but you shouldn't need skirts, unless you're using LOD.

I had a very similar problem with my terrain engine last week.
The problem was that terrain chunks weren't sharing vertices at their edges, so in the example below...

    Chunk 1                 Chunk 2    =======                 =======0   1   2   3   4     0   1   2   3   4| \ | \ | \ | \ |     | \ | \ | \ | \ |5   6   7   8   9     5   6   7   8   9| \ | \ | \ | \ |     | \ | \ | \ | \ |10  11  12  13  14    10  11  12  13  14| \ | \ | \ | \ |     | \ | \ | \ | \ |15  16  17  18  19    15  16  17  18  19

Vertex 0 in chunk 1 wasn't the same vertex as vertex 4 in chunk 1. The solution was to change the terrain generation algorithm, so that adjecent terrain chunks shared vertices. So, vertex 4 in Chunk 1, is actually the same as vertex 0 in Chunk 2

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