UV mapping

【UV mapping】

　　UV mapping is the 3D modeling process of making a 2D image representation of a 3D model‘s surface.

　　This process projects a texture map onto a 3D object. The letters "U" and "V" denote the axes of the 2D texture^{[note 1]} because "X", "Y" and "Z" are already used to denote the axes of the 3D object in model space.

　　UV texturing permits polygons that make up a 3D object to be painted with color from an image. The image is called a UV texture map,^[1] but it‘s just an ordinary image. The UV mapping process involves assigning pixels in the image to surface mappings on the polygon, usually done by "programmatically" copying a triangle shaped piece of the image map and pasting it onto a triangle on the object.^[2] UV is the alternative to XY, it only maps into a texture space rather than into the geometric space of the object. But the rendering computation uses the UV texture coordinates to determine how to paint the three-dimensional surface.

　　When a model is created as a polygon mesh using a 3D modeler, UV coordinates can be generated for each vertexin the mesh. One way is for the 3D modeler to unfold the triangle mesh at the seams, automatically laying out the triangles on a flat page. If the mesh is a UV sphere, for example, the modeler might transform it into anequirectangular projection. Once the model is unwrapped, the artist can paint a texture on each triangle individually, using the unwrapped mesh as a template. When the scene is rendered, each triangle will map to the appropriate texture from the "decal sheet".

参考：http://en.wikipedia.org/wiki/UV_mapping

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