When you apply a two dimensional image (like a photo of marble) to a three dimensional model, the process is called “mapping”. The easiest way to understand the concept is to think of putting a sticker on your lunch box or anything else with rounded edges you wish to decorate. The sticker is flat when you peel off the backing but it will bunch up if you try to stick it over any complex curvature… like the corner of your lunch box. This same problem exists in 3D rendering but can be overcome by means of what is called “custom UV mapping”. Think of U as one direction on a piece of graph paper (side to side) and V as the other (up and down). These directions are used to map or stick your image on the 3D model. Anytime you apply an image in a material and then apply that material to your model, you are using UV texture mapping.
So if you don’t specify how a texture in a material will map to your model, how does it know how to stick to the surface and where? This depends on the default settings in the rendering program you are using to add materials to your model.
In the image below, the model is a NURBS polysurface. With this type of geometry, the seams shown indicate separate surfaces and each surface will by default have its own UV texture space. The image or sticker in the material will not pass seamlessly across these edges.
This break in the marble texture across seams in the model is due to each surface having its own unique structure. As if they each came from a uniquely sized pad of graph paper and were then taped together.
In order to make the marble texture blend smoothly across the seams, the UV space of the model can be changed from the default of “Surface” to another style. You can for instance apply six copies of a texture simultaneously from different directions. This method is called “Box” mapping and is one of the common primitive options.
Notice how the texture changes along the interior curvature of the basin in this model. This is caused by the separate faces of the box mapping widget (highlighted in yellow) intersecting as they project inward and wrap over the model.
There are other primitive projection options as well such as planar, cylindrical and spherical which is shown below.
Each primitive option has its own appropriate use but on a complex form such as this sink basin, a different style of mapping is required for the texture to be believable.
In advanced texture mapping tools, there is often an option to “Unwrap” the model. This is like flattening out a bunched up piece of cloth before you iron it. Most custom UV unwrapping algorithms allow the user to select seams in the model that can rip apart in the process of flattening. In the case of polygonal modelers, these seams will be “edge loops” in the mesh and marked as seams prior to unwrapping the UVs.
The unwrapping process will smooth out the texture space for the model without overlapping the individual surfaces or polygons. The texture in the material can then be stuck on the 3D model using this custom mapping information for position and scale.