Hi Sebastian,
Hi Robert,
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...
The question was purely rhetorical. I just wanted to prove my point regarding the
uniform's content management vs already provided setDefines("LIGHT0",
osg::StateAttribute::OFF). I didn't expect all that. Sorry for the fuzz :) BTW, I'm not
planing to toggle all possible on/off combinations just to see all the permutations, the
normal graph usage is what I'm after.
Normal graph usage might exactly trigger a lot of combinations. My point is not
that theoretical if you take into account that basically everyone could have
used the feature if it was part of the core, thus the example is a practical
one :-)
I didn't want to prove anything here, I simply wanted to point out some
practical problems.
I know and I'm grateful for that. Anyway, based on feedback, I already ditched
the idea and I'm now looking into alternative possibilities, mainly if shader
composition can be extended in a nonintrusive way.
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.
That would defeat the purpose of the #pragma(tic) shader composition. You may
then as well pre-parse #pragma import_defines and #pragma requires and
pre-generate all the shader permutations. The purpose of #pragma(tic) shader
composition is shader generation based on current define set (see the
osg::State).
Yes and? I'm parsing include hierarchies and therefore need to add the defines
found there to the #pragma import_defines. The bulk work done by the osg::State
is to determine if a new program needs to be compiled based on the set of
defines.
My composition is purely to keep the shader code maintainable and reusable. Currently
some of the "Ubershaders" have 20-30 defines, which seriously don't want to
write in the the import_defines by hand.
Again I simply wanted to point out some possibilities to get you your
loop-mechanism.
I see. Thank you for that, too.
Cheers,
Robert Milharcic
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