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I'm trying to create geometry to represent the Earth in OpenGL. I have what's more or less a sphere (closer to the elliptical geoid that Earth is though). I map a texture of the Earth's surface (that's probably a mercator projection or something similar). The texture's UV coordinates correspond to the geometry's latitude and longitude. I have two issues that I'm unable to solve. I am using OpenSceneGraph but I think this is a general OpenGL / 3D programming question.
There's a texture seam that's very apparent. I'm sure this occurs because I don't know how to map the UV coordinates to XYZ where the seam occurs. I only map UV coords up to the last vertex before wrapping around... You'd need to map two different UV coordinates to the same XYZ vertex to eliminate the seam. Is there a commonly used trick to get around this, or am I just doing it wrong?
There's crazy swirly distortion going on at the poles. I'm guessing this because I map a single UV point at the poles (for Earth, I use [0.5,1] for the North Pole, and [0.5,0] for the South Pole). What else would you do though? I can sort of live with this... but its extremely noticeable at lower resolution meshes.
I've attached an image to show what I'm talking about.
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There are 2 ways to texture a sphere. Either by applying a cubemap or by applying a 2D texture. For best result, use a cubemap. The problem with applying a 2D texture is that when you wrap a 2D texture onto a sphere, the top and bottom area of the sphere, the texture looks squeezed.
A texture is an OpenGL Object that contains one or more images that all have the same image format. A texture can be used in two ways: it can be the source of a texture access from a Shader, or it can be used as a render target.
Texture coordinates, also called UVs, are pairs of numbers stored in the vertices of a mesh. These numbers are often used to stretch a 2D texture onto a 3D mesh, but they can be used for other things like coloring the mesh (see Vertex color), controlling the flow across the surface (see Flow map), etc.
The general way this is handled is by using a cube map, not a 2D texture.
However, if you insist on using a 2D texture, you have to create a break in your mesh's topology. The reason you get that longitudinal line is because you have one vertex with a texture coordinate of something like 0.9 or so, and its neighboring vertex has a texture coordinate of 0.0. What you really want is that the 0.9 one neighbors a 1.0 texture coordinate.
Doing this means replicating the position down one line of the sphere. So you have the same position used twice in your data. One is attached to a texture coordinate of 1.0 and neighbors a texture coordinate of 0.9. The other has a texture coordinate of 0.0, and neighbors a vertex with 0.1.
Topologically, you need to take a longitudinal slice down your sphere.
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