/* General-purpose geometric models. */
/* Last edited on 2020-10-03 20:23:36 by jstolfi */

#ifndef gmo_opto_H
#define gmo_opto_H

#include <quad.h>
#include <bool.h>
#include <frgb.h>
#include <hr3.h>

typedef enum 
  { gmo_opto_kind_EMIT     = 0,
    gmo_opto_kind_ABSORB   = 1,
    gmo_opto_kind_REFLECT  = 2,
    gmo_opto_kind_TRANSMIT = 3
  } gmo_opto_kind_t;
  
typedef struct gmo_opto_t
  { frgb_t color;          /* Fraction of coverage per band. */
    gmo_opto_kind_t kind;  /* Kind of optical effect. */
    float sigma;           /* Additional parameter, e.g. roughness. */
    int next;              /* Underlying opto, or -1 if none. */
  } gmo_opto_t;
  /* An `opto' (short for `optical operator') specifies how a surface 
    interacts with light at a point. It is assumed to be a linear combination
    of standard bi-directional radiation functions (BDRFs):

      * An EMIT function emits wite light (intensity 1) on all bands.
        If {sigma=0} the light is emitted perpendicularly to the surface;
        if {sigma=1} it is emitted like the Lambertian distribution
        (so that the apparent color is independent of viewing angle).

      * A REFLECT function reflects or diffuses light coming 
        from the same side as the observer. If {sigma=0} the
        surface is a smooth mirror; if {sigma=1} it is an opaque
        Lambertian diffuser.

      * A TRANSMIT function transmits or scatters light coming
        from the side of the surface opposite to the observer.
        If {sigma=0} the surface is clear glass, if {sigma=1}
        it is a translucent Lambertian diffuser. 

      * An ABSORB function emits nothing.

    An {opto} is actually a sequence of zero or more `stages', each
    described by an {gmo_opto_t} record. For each band {k}, the
    parameter {color.c[k]} specifies the percentage of the incident view
    rays that hit this stage of the opto. The remaining rays are assumed
    to interact with the {next} stage and following stages. An ABSORB
    stage with {color.c[k]=1} (i.e. a coal black layer) is assumed at
    the end of every stack, to catch any rays that are not caught by
    preceding stages.

    Note that rays that hit a TRANSMIT stage go through the surface 
    unchanged, while those that miss it may interact with its 
    subsequent stages. This behavior is not intuitive but 
    simplifies the math.

    Intermediate values of {sigma} give a BDRF that is intermediate between
    the cases {sigma=0} and {sigma=1}. Thus, for example, highlights
    are obtained by adding a REFLECT stage with small but nonzero {sigma}. */

/* INPUT/OUTPUT */

gmo_opto_kind_t gmo_opto_kind_from_char(char c);
char gmo_opto_kind_to_char(gmo_opto_kind_t oknd);

#endif