So, what is a structure and what is it for? Look out of the window. If there isn't a late night and not the ashes after A-bombardment, you'll see a lot of interesting and beautiful. For example, there are a lot of usual trunks of trees. They are freaked by grooves, their colour varies from light brown to dark and so on. At the opposite house the bricklaying isn't so simple either as it’s seems at first sight.
Each brick stands out forward, it's not homogeneous, it's decorated by cracks and shades from leaves of the next poplar. The cement gets the most freakish forms between bricks.
So how to simulate all this miracle? How many millions ranges will be spent for realistic display of all these beauty?
Imagine that we have made each brick a red parallelepiped and have laid the grey planes depicting a cement between them. It will look awfully. So what is to be done? The answer is simple - to apply structures!
The textureis a two-dimensional bit card (or a picture, bluntly speaking) which is imposed on range and represents the of its surface. The thing impossible to be stimulated by ranges can be simply drawn.
And if it's not on the surface modulated by a structure, and there are no strongly protruding or non-uniform details, it will look realistic. Structures are imposed onto house walls, grass, leaves, cars, clothes, faces of persons and many many other things.