When working with or adding meshes, I've seen UV Spheres and Icospheres being presented to me. What are the differences between them, when should either be used and is there anything I should take into consideration if I want to have them render cleanly?
The major fundamental difference is how each sphere is modeled. The UV sphere, somewhat like latitude and longitude lines of the earth, uses rings and segments. Near the poles(both on the Z-axis with the default orientation) the vertical segments converge on the poles. This sphere is useful for projecting terrain onto planets and/or complex modeling as it is readily subdivided even after being created, and maps onto equirectangular projections (plate carrée) readily.
The icosphere uses a different approach. A polyhedron is made with triangles which are placed(at various subdivision levels) as an icosahedron (thus the name) and more finely-subdivided solids. All faces have the same area, which may be useful for certain types of UV mapping containing non-organic textures. An example that comes to mind is an isocahedral die or billiard balls where stretch must be minimized near the point where the number is printed onto the ball.
A conparison of UV and icospheres. UV spheres are on the left. Going downward, the subdivision levels are increased. 10 rings, 3 segments; 16/8, and 32/32 for UV spheres, and subdivision levels 1,2, and 3 for the icosphere. Default BI material is used, and light is directly behind camera.
Source: manual page and experimentation with UV mapping.
Since you are asking what the difference is I thought I'd point out one of the most important differences between the two kinds of spheres in relation to modelling.
Comparison using many subdivisions
UV spheres are best used in situations where you require a very smooth, symmetrical surface (the more subdivisions the better). However, they do relatively poorly when realistic topology is your key goal. UV spheres are not the best for organic natural shapes, because of their symmetry and the use of quads.
ICO spheres are best used for objects such as:
- Geodesic Domes
- Planets with realistic terrain
- Rough surfaces, such as golf balls.
If symmetry is important, use UV spheres because ICO spheres are asymmetrical.
A displacement modifier on the ICO sphere yields more satisfying results. With a higher number of subdivisions the difference between ICO and UV sphere won't be as noticeable. Generally, though, the ICO sphere looks more realistic and much smoother.
Displace modifier settings
ICO Spheres do have symmetry:
Icospheres are not asymmetrical. From Wikipedia: "A regular icosahedron has 60 rotational (or orientation-preserving) symmetries, and a symmetry order of 120 including transformations that combine a reflection and a rotation." Each of these two approximations of a sphere has many symmetries - they just have different types, with different biases and (by some standards) imperfections. – mjwach
At the risk of beating a dead horse, another practical difference between the two is that a UV sphere has a distinct bias - it has poles.
When I'm making things like spaceships, they are usually elongated cigar-shapes with a symmetrical cross-section, it's far easier to shape it when the sphere has poles at each end, and all 'longitudes' run from nose to tail, while all 'latitudes' are cross-sections. That would be a big pain if I were to try to use an icososphere.
Contrarily, if I wanted to create something like a billiard ball, which should be uniform in any orientation, I'll use an icososphere.
A UV sphere has faces with 4 edges, while an Icosphere has triangular faces. This results in the UV sphere having a smoother appearance and the Icosphere having a jagged appearance. However, the more faces you have on them, the smoother the Icosphere looks compared to the UV sphere.
As for which version shape better for planets, if you give the polyhedrons a very high resolution, either would work unless you knew which shape works for certain formats. When models were exported to 3DS, UV "spheres" were useful. For that purpose, I made a planet model with a 256x128 resolution after realizing that there didn't seem to be any higher resolution that could be read for 3DS. A displacement map was added to render the geology without the aid of normal/bump mapping. Some apps like Celestia use the 3DS format for meshes.