Adding a texture to a shader

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, which are often referred to as polygons; each vertex on the model can store data that shaders can access and use to determine what to draw.

One of the most important pieces of information that is stored in vertices is the UV data. This consists of two coordinates, U and V, ranging from 0 to 1 to get the full extent of the image. 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 diagram shows you how a 2D texture is mapped to a triangle...