[osg-users] Pragmatic shader - a new #pragma directive proposition

[Sebastian Messerschmidt]

Am 14.01.2016 um 15:01 schrieb Robert Milharcic:
> Hi Sebastian,
>
> First of all, the lightning shader only illustrates one of the cases 
> where (optionally) repeating a code block containing substitution 
> parameter can be useful. I used the multi-light lightning as an 
> illustration only.
>
>>> First, thank you for your input. Yes, that is more or less the same 
>>> approach I'm currently using. The downside of this approach is that 
>>> it requires additional nontrivial code logic for the uniform array 
>>> management (u_LightColor) and that is why I started to look at the 
>>> alternatives.
>> What could be more complicated there than to setup individual 
>> uniforms? Sorry this doesn't pass as a valid argument. If you have to 
>> hold the number of used lights somewhere you can hold a reference to 
>> the uniform as well.
>>
>
> Sorry, but that is not what I had in mind.  The problem here is the 
> management of the array uniform's content and not the reference to it 
> or the light count etc. Let say I only want to disable one of the 
> lights, say LIGHT0 eg. stateset->setDefines("LIGHT0", 
> osg::StateAttribute::OFF). What will you do?
Let's say you have a fixed maximum on N, than you create a UBO/Array of 
this size and provide a count-uniform for the maximum valid entries.
If a light gets disabled, you simply remove it, copy it to the back and 
set your count to N-1.
In the shader you simply loop from 0 to count instead of N. That might 
break compiler optimization, but it won't hurt too much I guess.
When using the setDefine, OSG will have to issue a recompile of this new 
variant, which will degrade performance if all combinations are needed.

Assume 16 Lightsource, that will produce 65536 possible shader programs 
... and that is only 16, not 500, or 1000 ...
Having 1024 individually switchable Lightsources would totally wreck 
your idea of having an alternative to my solution.
2^1024 might exceed the numbers of atoms in the known universe, so there 
is not a remote chance to solve this with shader combinations...


>
>>> There is also an upper limit for the size of the array that needs to 
>>> be taken into account. 
>> At least 512. If this is not enough you can use Uniform buffer 
>> objects (UBO)[1] or Shader Storage Blocks[2] which support
>> If this is not enough for your light-count you will probably hit 
>> performance problems first.
>>
>
> Yes, that is true. The upper limit is not a problem for this specific 
> multi-light problem.
Even so, UBOs support in the order of ten-tousands. If this is still 
isn't enough, a texture will provide maximum space.
Might be your example, but the number of individual uniforms is less 
than that. And putting a heap of uniforms into the stateset will fasten 
the handbrake on the CPU side.
>
>>> Also, the loop represents unnecessary overhead (though, this is not 
>>> a problem on a never hardware). 
>> That's an assumption of yours. Usually constant folded loops with 
>> single return and without break, continue-statements are unrolled by 
>> the compiler.
>> I'll accept performance comparisons however ;)
>>
>
> Yes, you are right about that. The constant loop will most probably be 
> unrolled by the compiler. Of courses, that is another assumption on 
> top of the previous assumption :)
>
>>> On the other hand, my suggestion fits well into existing pragmatic 
>>> shader composition logic and probably has less downsides.
>>>
>> Downside is that you're trying to invent a meta-language here out of 
>> reasons that I commented on. The downside of your approach is a 
>> preprocessor language with no clear advantages over the tools already 
>> at your disposal.
>> So to say, the current language is already turing-complete and you're 
>> trying to put some syntactic sugar on top, which adds some high 
>> degree of complexity to the parser and to the shader-code.
>>
> Exactly, that was my intention, to add a higher degree of complexity 
> and hopefully solve some problems that "turing-complete" language can't.
Turing complete solves all computable problems ;-)
I do understand your motivation, but I still don't see the gain really.
I've presented some alternative ways to solve your problem, as I came 
across some of those challenges too.
The idea Robert mentioned might be a good one, basically we could use 
some kind of callback to let the user-code preprocess the shader, wait 
actually we can already do this, when it is loaded via a LoadCallback :-)
Actually I'm preprocessing my shader code when it's loaded to perform 
"include" and automated shader-define setup, so this might work for your 
"loop" too.


Cheers
Sebastian
>
>> An alternative for you is to manage this part yourself by simply 
>> overriding the parts managing the define-states. Maybe Robert O. can 
>> fill in on the details here.
>>
>> Cheers
>> Sebastian
>>
>
> Cheers,
> Robert Milharcic
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