1.6. 渲染管线 Processing Pipeline

For specifying the behavior of OpenGL,the varIoUs operations are defined to be applied in a

particular order,so we can also think of OpenGL as a GRAPHICS PROCESSING PIPELINE.

Let's start by looking at a block diagram of how OpenGL was defined up through OpenGL 1.5.

Figure 1.1 is a diagram of the so-called FIXED FUNCTIONALITY of OpenGL. This diagram shows the

fundamentals of how OpenGL has worked since its inception and is a simplified representation

of how OpenGL still works. It shows the main features of the OpenGL pipeline for the purposes

of this overview. Some new features were added to OpenGL in versions 1.1 through 1.5,but

the basic architecture of OpenGL remained unchanged until OpenGL 2.0. We use the term fixed

functionality because every OpenGL implementation is required to have the same functionality

and a result that is consistent with the OpenGL specification for a given set of inputs. Both the

set of operations and the order in which they occur are defined (fixed) by the OpenGL

specification.

Figure 1.1. Overview of OpenGL operation

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It is important to note that OpenGL implementations are not required to match precisely the

order of operations shown in Figure 1.1. Implementations are free to modify the order of

operations as long as the rendering results are consistent with the OpenGL specification. Many

innovative software and hardware architectures have been designed to implement OpenGL,and

most block diagrams of those implementations look nothing like Figure 1.1. However,the

diagram does ground our discussion of the way the rendering process appears to work in

OpenGL,even if the underlying implementation does things a bit differently.