How to get rid of texture wrapping seam? - opengl

Question

Take a look at the following image - you will see the clouds in the background have a very annoying seam:
http://simoneschbach.com/seam.png
This seam is occurring when the wrap around occurs, as I am supplying texture coordinates programmatically with the following code:
gBackgroundPos += 0.0003f; // gBackgroundPos climbs indefinitely...
GLfloat bgCoords[] = { gBackgroundPos, 1.0,
gBackgroundPos + 0.5f, 1.0,
gBackgroundPos, 0.0,
gBackgroundPos + 0.5f, 0.0 };
I have enabled texture wrapping during texture init as follows:
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
What can I do here to get rid of the very visible seam?

Answer 1

The problem you have is exactly solved by this technique, which is extremely simple to implement:
http://vcg.isti.cnr.it/~tarini/no-seams/
There is an open-source demo at that link, which exposes the used fragment shader.
The trick is easy to adopt even without a complete understanding of why it works, but it is fully explained in the Journal of Graphic Tools article:
"Cylindrical and Toroidal Parameterizations Without Vertex Seams"
which can be found, for example, at
http://vcg.isti.cnr.it/Publications/2012/Tar12/.
Unfortunately, the other solutions listed here won't work:
GL_REPEAT (as GL_TEXTURE_WRAP), alone, does not do what you need. The problem, as noted, is that a triangle connecting point with S = 0.9 and S = 0.1 interpolates all the way back across the cylinder, not forward across the seam.
Replicating vertices on the "cut" (the texture seam) would work on static geometries, where texture coordinates are sent as attributes (but, even then, the drawbacks are many: introduces replication, and seams breaking the geometry: the two sides of the texture cut will be topologically disconnected). In this case specific case, texture coordinates are produced procedurally so that's not even an option.

Answer 2

This is an old question, but I recently had to deal with the same issue.
When a texture wraps around, say, a cylinder, a natural seam occurs where the edges of the map meet.
The strip of triangles that cross that boundary wind up having texture coordinates that cause the renderer to squeeze the entire texture, backwards, into those triangles.
Just looking at the U coordinate: a triangle that does not cross the texture edge will have have its texture coordinates in counter-clockwise order. However, when a triangle crosses the border (for example) you wind up with the opposite winding order, because the point that crosses the border gets mapped back to the opposite side of the texture. You'll typically see a triangle that has one or two texture coords in the 0.9-0.9999 U range, and the remaining coordinates in the 0-0.1 range.
When the renderer sees that, it does exactly what it's supposed to: it interpolates the face's texture coordinates from 0.9 down to 0.1, which includes most of the texture. Your seam is just what the texture looks like when it's squeezed backwards into a small space.
The solution is to split the edges that cross the border, so that each of the affected vertices appears twice in the list. The first one will have texture coordinates on the left end of the map, and the other will have all of its texture coordinates on the other end, so that no single edge spans the texture.
Note that you're not changing the XYZ values for the vertex: just UV.
Also note that this doesn't happen with surfaces that don't share vertices between disparate edges. Planes are immune. Your example image isn't online anymore so I can't verify that this is what's happening to you, but it seems likely based on your cursory description.

Answer 3

I'd vote for Shezan Baig if that comment was an answer.
GL_REPEAT is meant to do exactly what you want. If it does not, it's very likely because your texture itself has the seam in it, or alternatively, that the toolchain that loads the texture introduces the seam (say because the source texture is not a power of two size, e.g.).

Answer 4

You might be able to take advantage of texture borders (2^m+1 x 2^n+1 textures, rather than 2^m x 2^n) and copy data from the opposite side into the border pixels to make the texture cyclic.
You'll also want to change GL_TEXTURE_MAG_FILTER and GL_TEXTURE_MIN_FILTER to use linear interpolation or better (maybe GL_LINEAR_MIPMAP_LINEAR for best results).