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The OpenGL Graphics Pipeline is changing every year. So the programmable Pipelines are growing. At the end, as an opengl Programmer we create many little programms (Vertex, Fragment, Geometry, Tessellation, ..)
Why is there such a high specialization between the stages? Are they all running on a different part of the hardware? Why not just writing one code-block to describe what should be come out at the end instead of juggling between the stages?
http://www.g-truc.net/doc/OpenGL%204.3%20Pipeline%20Map.pdf
In this Pipeline PDF we see the beast.
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Within a graphics processor, all stages are working in parallel. Because of this pipeline architecture, today's graphics processing units (GPUs) perform billions of geometry calculations per second. They are increasingly designed with more memory and more stages, so that more data can be worked on at the same time.
The OpenGL pipeline performs conditional or selection operations at a number of stages in the pipeline. One or more of these stages can be used together to implement simple conditional operations in a multipass algorithm. Examples include the depth and alpha tests.
During the application step, changes are made to the scene as required, for example, by user interaction by means of input devices or during an animation. The new scene with all its primitives, usually triangles, lines and points, is then passed on to the next step in the pipeline.
A graphics pipeline can be divided into three major steps: application, geometry and rasterization.
In the days of "Quake" (the game), developers had the freedom to do anything with their CPU rendering implementations, they were in control of everything in the "pipeline".
With the introduction of fixed pipeline and GPUs, you get "better" performance, but lose a lot of the freedom. Graphics developers are pushing to get that freedom back. Hence, more customization pipeline everyday. GPUs are even "fully" programmable now using tech such as CUDA/OpenCL, even if it's not strictly about graphics.
On the other hand, GPU vendors cannot replace the whole pipeline with fully programmable one overnight. In my opinion, this boils down to multiple reasons;
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Historically, there were actually different processing units for the different programmable parts - there were Vertex Shader processors and Fragment Shader processors, for example. Nowadays, GPUs employ a "unified shader architecture" where all types of shaders are executed on the same processing units. That's why non-graphic use of GPUs such as CUDA or OpenCL is possible (or at least easy).
Notice that the different shaders have different inputs/outputs - a vertex shader is executed for each vertex, a geometry shader for each primitive, a fragment shader for each fragment. I don't think this could be easily captured in one big block of code.
And last but definitely far from least, performance. There are still fixed-function stages between the programmable parts (such as rasterisation). And for some of these, it's simply impossible to make them programmable (or callable outside of their specific time in the pipeline) without reducing performance to a crawl.
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Because each stage has a different purpose
Vertex is to transform the points to where they should be on the screen
Fragment is for each fragment (read: pixel of the triangles) and applying lighting and color
Geometry and Tessellation both do things the classic vertex and fragment shaders cannot (replacing the drawn primitives with other primitives) and are both optional.
If you look carefully at that PDF you'll see different inputs and outputs for each shader/
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