Adding a texture to a shader

Textures can bring our shaders to life very quickly in terms of achieving very realistic effects. In order to effectively use textures, we need to understand how a 2D image is mapped to a 3D model. This process is called texture mapping, and it requires some work to be done on the shader and 3D model that we want to use. Models, in fact, are made out of triangles; each vertex can store data that shaders can access. One of the most important information stored in vertices is the UV data. It consists of two coordinates, U and V, ranging from 0 to 1. They represent the XY position of the pixel in the 2D image that will be mapped to the vertices. UV data is present only for vertices; when the inner points of a triangle have to be texture-mapped, the GPU interpolates the closest UV values to find the right pixel in the texture to be used. The following image shows you how a 2D texture is mapped to a triangle from a 3D model:

The UV data is stored in the 3D model...