CODEWITHSUNDEEP
openglshader
Jagoly
Jagoly

Reputation: 1009

Correct usage / purpose of OpenGL Program Pipeline Objects

With OpenGL 4.1 and ARB_separate_shader_objects, we are able to store different stages of the shading pipeline in shader programs. As we know, to use these, we need attach them to a Program Pipeline Object, which is then bound.

My question is, why do we need the program pipeline objects at all? In my renderer, I have only one of these, and I change it's attachments to change shaders. I can't think of any case where you'd actually want more than one of these. If you store many pipeline objects, each containing different combinations of shader programs, then things end up even messier than not using separate shaders at all.

So, what is the purpose of the pipeline object? Is changing attachments (much) more expensive than binding a different pipeline object? What's the reason that the spec has this, rather than, say, having glUseProgramStages operate in the same way as glUseProgram?

Upvotes: 5

Views: 4737

Answers (2)

Nicol Bolas
Nicol Bolas

Reputation: 473272

The principle reason pipeline objects exist is that linking stages together in a program object did have certain advantages. For example, there are a lot of inter-shader-stage validation rules. And if the sequence of separate programs aren't valid, then people need to know.

With a program that links all stages together, you can detect these validation failures at link time. All of these tests are done precisely once and no more.

If you made "glUseProgramStages operate in the same way as glUseProgram", then every single time you render with a new set of shaders, the system will have to do inter-stage validation tests. Pipelines represent a convenient way to cache such tests. If you set their programs once and never modify them afterwards, then the result of validation for a pipeline will never change. Thus validation happens exactly once, just as it did for multi-stage programs.

Another issue is that implementations may need to do some minor shader fixup work when associating certain programs with each other. Pipeline objects represent a convenient place to cache such fix-up work. Without them, they'd have to be done every single time you change shaders.

Upvotes: 7

dari
dari

Reputation: 2455

Why do we need the program pipeline objects?

We don't need program pipeline objects they are purely optional. Using one Program Object for every shader combination that is in use is the easiest and most common way to do it.

So, what is the purpose of the pipeline object?

From https://www.opengl.org/registry/specs/ARB/separate_shader_objects.txt:

[...] Many developers build their shader content around the mix-and-match approach where they can use a single vertex shader with multiple fragment shaders (or vice versa). This extension adopts a "mix-and-match" shader stage model for GLSL allowing multiple different GLSL program objects to be bound at once each to an individual rendering pipeline stage independently of other stage bindings. This allows program objects to contain only the shader stages that best suit the applications needs. [...]

Upvotes: 1

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