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awickedmind

OpenGL Shader pipeline differences OGL DX

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I've been wanting to write an engine with both an OpenGL and an Directx path (primarily DX10 and OGL 2/3). Having programmed OpenGL for a long time I'm having some problems creating my "Graphics Abstraction Layer". The biggest problem for me currently are shaders.

The biggest difference I've noticed is the lack of an program a la glCreateProgram in DX10. That in it self is not an issue but I'm starting to worry about costs for different operations. I'm wondring when DX10 does its shader linkage and how costly it is. For example if swap shaders for different passes I would change using glUseProgram. This does not make sense in a DX10 context (at least not to my knowledge). So my question:

If I wish to swap shaders ~5 times a frame is linkage so cheap on DX10 hardware that it's just to attach my new vertex/pixel shader setup and in OpenGL do the same and then call glLinkProgram (which I avoid during my main loop otherwise). Is there a similar DX10 call? Is an ubershader the only practically viable way? How does the DX9 pipeline differ (for future branches)?

A link for a read up or just a straight answer would be nice. I haven't found anything that truly clarifies this for me on Google...

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I don't have a clear vision of this performance as well, but I think you're overrating GL's linking stage.
There's chance GL's linking stage is yet another relic of an already conceptually distant past, you might read about ARB_separate_shader_objects.
I am sure I'll get bashed about performance by saying this, but considering that there's an official extension to do that, looks like nobody really cares about the old extra perf from binding stages together once.

To deal with the mix-and-match approach you probably have to "meta-link" the shaders yourself and expand each combination in a single program. Let me say I never liked this approach since day 1.

I sincerely wish you'll never have to go back to D3D9, but in the unfortunate case you'll need to, you'll find out its shading pipe works more or less like a very stupid D3D10 pipe.

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Quote:

The biggest difference I've noticed is the lack of an program a la glCreateProgram in DX10.


HLSL's Effect is equivalent to GLSL's Program. They can be created with D3DX10CreateEffectFromFile.

With that said, you can use different shaders for different passes with Effects :

technique10 ShaderModel4_Technique
{
pass P0
{
SetVertexShader( CompileShader( vs_4_0, Main_VS1() ) );
SetGeometryShader( NULL );
SetPixelShader( CompileShader( ps_4_0, Main_PS1() ) );
}
pass P1
{
SetVertexShader( CompileShader( vs_4_0, Main_VS2() ) );
SetGeometryShader( NULL );
SetPixelShader( CompileShader( ps_4_0, Main_PS2() ) );
}
}




since the above code sample is compiled through one API interface (D3DX10CreateEffectFromFile) as opposed to the multiple calls with glAttachShader, I would assume that D3D10 does it shader linkage slighty faster than OpenGL.

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Quote:
Original post by 16bit_port
HLSL's Effect is equivalent to GLSL's Program. They can be created with D3DX10CreateEffectFromFile.
Except that effects are purely a convenience layer built on top of the underlying shaders - they don't actually add any functionality of their own.

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Effect files are a decent way to prototype complex shaders, but for in-game use they're just a pain in the ass.

If your engine just services requests from effects files then you will see pathetic performance and be spending half your dev time adding lighting options and stuff.

Better to just write the core pixel generating code in a file and reference functions to do lighting and shadows and stuff. And by the time you get that far, the power of effects files becomes lost anyway. They're just the wrong tool for the job.

Playing in rendermonkey has a place, but that's not how your game should run imo.

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Quote:

but for in-game use they're just a pain in the ass.


They weren't that bad in D3D9, but they did changed stuff around in D3D10 where it did become kind of annoying.

Quote:

If your engine just services requests from effects files then you will see pathetic performance and be spending half your dev time adding lighting options and stuff.

How bad of a performance are we talking about here?

Quote:

Better to just write the core pixel generating code in a file and reference functions to do lighting and shadows and stuff.

What do you mean? Do you mean write the shaders without Effect, and use these :

ID3D10Device::CreateVertexShader
ID3D10Device::CreatePixelShader

instead?

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Effects are mostly designed for convenience. They totally abstract away shader parameter management, which allows you to freely set parameters without having to worry at all about pipeline state or data management. Naturally this doesn't scale for setting lots of effects and parameters, since you'll start to spend significant amounts of time touch data from disparate memory locations (or on PC you can start to incur significant API overhead). If you want less overhead, you need to design your shader constant (constant buffer) layout and the layout of CPU-side data structures to match the actual usage patterns. This means clearly separating parameters by how often they change, and storing the CPU data in memory in the same layout used by the shaders so that you can quickly transfer it.

Any in regards to the OP's question...from what I know linkage between the different shader stages is not a big deal. I don't think most GPU's do anything fancy in that regard. There will probably be some shader-side work for unpacking vertex attributes, but that should be handled by input layouts. Either way...5 changes a frame is nothing at all to worry about.

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Quote:
Original post by Hodgman
I know a 360 game (that I can't name) that spends 30% of it's CPU time inside the effect API... ;(
We gave up on effects after that projcet.


Can you disclose what the main sinks were for this lost CPU time? With my hobbyist tinkering I found that the effects framework can be made to run quite a bit smoother by hanging on to the native handles of constants instead of addressing them by their names, but otherwise I haven't seen such performance drains. Just curious mind you, not arguing effects are super efficient when you hang on to the handles [smile]

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Quote:
Original post by remigius
Can you disclose what the main sinks were for this lost CPU time?
IIRC it was mostly setting uniform data, like matrices, model colours, etc... Without effects we put this data into specific registers, so they don't have to be re-uploaded with each shader switch.

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Thanks for all the responses! Things are becoming a bit clearer for me.

Effects don't appeal to me since the break any kind of GAL pipeline which branches off into more API-specific code later down the line. But for prototyping they look neat. Also, based on the all the replies I won't reconsider it.

Yes, I wish to use the ID3D10Device::CreateVertexShader and the ID3D10Device::CreatePixelShader. Even though I've been using OpenGL for a long time the DX10 approach makes a lot of sense to me. Coming from OpenGL though I felt a bit uneasy about when linkage occurs (on assignment per shader I presume). My issue is more of how to not break my OpenGL code when designing for both systems (forcing me to make a more loose abstraction which will give me some overhead).

Further I'm guessing that the cost is also based on the HW generation, with DX9/10 cards having cheap linkage?

I was worried the driver would do expensive stuff on the CPU side when linking using glLinkProgram but if it's a relic then all is well. There is still an issue of relinking and again relocating all my attributes and constants (something I suppose DX10 need not worry about because of the IA stage?). But I haven't fully decided yet on how I wish for my pipeline to look.

@ Krohm: Thanks a lot for the link, this would solve a lot of issues for me if implemented on my target platforms (OSX is still a bit slow here). I was actually designing to "meta-link" keeping track of different combinations. Let's just say the solution turned out to be less elegant...

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Quote:
Original post by Hodgman
IIRC it was mostly setting uniform data, like matrices, model colours, etc... Without effects we put this data into specific registers, so they don't have to be re-uploaded with each shader switch.


Thanks for the response, I just realized keeping data around across shader switches isn't something I've considered doing before [smile]

I have to ask however, isn't this what EffectPool sharing should fix, or is the performance of the EffectPool part of the problem?

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