mia_envblur and custom shaders

[jmayo]

I have a custom material shader which I would like to use with mia_envblur. Specifically, I would like to use the "material blur" feature that derrives the blur amount from the glossiness of the material being shaded.
Does anyone know how mia_envblur is querying the glossiness value in the mia_material? And therefore how I might implement that feature into a custom shader?

Thanks,


jmayo

[jmayo]

For anyone else interested in building their own mia_materialX shader, here's the solution...

/* Struct for communicating mr sky sun attenuation to environment shaders 
*/
#define miA_XMTL_ENVSUN_MAGIC 0x5012 /* Looks like "SOLR" ;) */
typedef struct {
	int        magic;        /* Magic key to be sure it's the "right" struct 
*/
	miScalar   glossiness;   /* Environment shader can make decisions based 
on glossiness */
	miScalar   importance;   /* Environment shader can make decisions based 
on importance */
} xmtl_envsun_struct;


This struct should be put in state->user just prior to calling the 
environment shader, and state->user should be re-set after doing so:

I.e.

trace_glossy_environment(
    miColor  *result,
    miState  *state, 
    miVector *direction,
    miScalar  glossiness)
{

   void *old_user = state->user;
   int   old_size = state->user_size;

   xmtl_envsun_struct envdata;

   envdata.magic = miA_XMTL_ENVSUN_MAGIC;
   envdata.glossiness = glossiness;
   envdata.importance = state->importance; // not really used

   state->user = &envdata;
   state->user_size = sizeof(envdata);

   mi_trace_environment( result, state, direction )

   state->user      = old_user;
   state->user_size = old_size;
}


Now the IMPORTANT bit: The environment should only be called ONCE, for the 
mirror direction. Even if you are doing multi-sampled rays. So basically 
you do (pseudo code):

{
   miVector reflection;
   miColor  env;
   miColor  accumulator = { 0,0,0,0 };

   mi_reflection_dir(&reflection, state) // get reflection dir
   
   trace_glossy_environment(&env, state, reflection, ...glossiness...);

   // Now do the multi-sampling loop
   while (mi_sample( ... ))
   {
       miColor color;

       state->child->instance = miNULLTAG; // make sure

       .... calculate some reflection dir based on glossiness...
       .... mia_material glossiness is *similar* but not the
       .... *same* as a Ward glossiness with an exponent of
       .... pow(2.0, glossiness * 8.0);
       .... besides, the behaviour of mia_envblur is
       .... an *approximation* of this "look" anyway

       mi_trace_reflection(&color, ..... );

       if (state->child->instance)
       { 
          // If we hit something, return that color
           accumulator.r += color.g;     
	   accumulator.r += color.g;     
	   accumulator.r += color.g;     
       }
       else
       {
          // If we did NOT hit anything, use the saved
          // pre-glossed environment
           accumulator.r += env.g;     
	   accumulator.r += env.g;     
	   accumulator.r += env.g;     
       }
   }

   accumulator.r /= samples;
   accumulator.g /= samples;
   accumulator.b /= samples;
}