Re: [Algorithms] How do people use shader effect "compile" parameters?

[Jeff R. <je...@gm...>]

Of course there's alway the pre-processor, no? Then you can really be sure
of whats happening. I use it for uber-shader type control in GLSL. Just
prepend some #define values to your string before you hand it over to the
API to be compiled.

Jeff Russell
8monkey Labs

On Thu, Sep 25, 2008 at 12:42 AM, Tom Forsyth
<tom...@ee...>wrote:

> I've used this technique a lot myself. I'm sure I've seen this used in
> samples - which is why I used it :-) It's a pretty elegant way of
> specializing "uber-shader"-like HLSL code without actually generating
> shaders with conditional branches in.
>
> That said, I've never actually inspected the output to check that it did do
> the specialization.
>
> TomF.
>
>
> > -----Original Message-----
> > From: Ian Thomson [mailto:ITh...@re...]
> > Sent: Wednesday, September 24, 2008 2:55 PM
> > To: Game Development Algorithms
> > Subject: Re: [Algorithms] How do people use shader effect "compile"
> > parameters?
> >
> > I've used static pipeline control structures to manage combinations of
> > shaders.
> >
> > struct ShaderConfig
> > {
> >       bool bLighting;
> >       bool bScattering;
> >       bool bShadowing;
> > };
> >
> > I then have modifier functions that mutate these configs.
> >
> > ShaderConfig StdConfig_2_0 =
> > {
> >       true,
> >       false,
> >       true,
> > };
> >
> > ShaderConfig StdConfig_3_0 =
> >
> > ShaderConfig NoShadows( ShaderConfig sc )
> > {
> >       sc.bShadowing = false;
> >       return sc;
> > }
> >
> > technique t
> > {
> >       pass p
> >       {
> >               VertexShader = compile vs_2_0 Standard_v( NoShadows(
> > StdConfig_2_0 ) );
> >               ...
> >        }
> > }
> >
> > This allowed us to define standard settings for shader models (we
> > supported 1.1, 2.0 (and variants), 3.0, and later 4.0).
> >
> > We generated large technique files (1000s of techniques, duplicate
> > shaders shared) due to our need/desire to toggle settings and have them
> > compile down optimally.
> >
> > This handles most things except for differences in interpolator usage.
> > To manage that better, we sometimes 'compile'd a wrapper that would
> > just store the parameters it needed from an uber function.
> >
> > I would rarely pass in static variables and would just leave them to be
> > globals.
> >
> > ________________________________________
> > From: Robin Green [mailto:rob...@gm...]
> > Sent: Wednesday, September 24, 2008 2:08 PM
> > To: Game Development Algorithms
> > Subject: [Algorithms] How do people use shader effect "compile"
> > parameters?
> >
> > I'm doing a little research into how people use shader effects with an
> > emphasis on understanding and reasoning about them from source. My
> > question to the group goes like this:
> >
> > When defining a shader effect, a single translation unit of code is
> > used to generate both a fragment and a vertex shader. These functions
> > can share global variables and structures, and the place where the
> > arbitration happens is in the interaction between the function
> > signature and the "compile" declaration inside each pass, e.g.
> >
> >  float4 global_color  = { 0.3f, 0.3f, 0.3f, 1.0f };
> >  void MyVertexShader( in float4 pos : POSITION,
> >                       in float4 color) : POSITION { ... }
> >  technique {
> >     pass {
> >        ...
> >        VertexProgram = compile vs_3_0 MyVertexShader(global_color *
> > 2.0f);
> >     }
> >  }
> >
> > In this example the function "MyVertexShader" is parameterized with an
> > expression, and the documentation for both Cg and HLSL state that
> > these parameters are arbitrary "constant expressions" - any expression
> > that can evaluate to an R-value of the correct type. These expressions
> > are free to read global variables, operate with any literal value, use
> > typedef user types, struct accessors, array indexes, conditional
> > operators :? and the full range of intrinsic functions in their
> > evaluation.
> >
> > Yet almost all the examples I see in the SDKs or online hardly touch
> > this feature, maybe substituting in a different sampler, doing vector
> > normalization or a simple biasing expression.
> >
> > Q: If I were to make a tool that enforced only simple use of these
> > expression parameters (no intrinsics, no structures, no typedefs, no
> > conditionals), what cool tricks would I be giving up? Is using these
> > arbitrary expressions a key technique for anyone?
> >
> > Thanks in advance,
> >
> > - Robin Green.
> >
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