When rendering a geometry, we usually provide multiple attributes for each vertex--positions to draw the model, texture coordinates for texturing, and normal vectors for lighting calculation. Checking if all this data is correct may not be easy, but sometimes, when our rendering technique doesn't work as expected, it may be necessary.
In graphics programming, there are some debugging methods that are commonly used. Texture coordinates, which are usually two-dimensional, are displayed instead of the usual color. We can do the same with the normal vectors, but as they are three-dimensional, we can also display them in a form of lines. For this purpose, a geometry shader may be used.
