A SpotLightGO.T is a geometric object that describes a light source which is emitting a cone of colored light.

The amount of light that falls onto another geometric object depends on the distance between this object and the light source. This phenomenon is referred to as {\em attenuation}. Refer to the description of the \interface{PointLightGO} interface for details.

The geometry of the light cone is described by three parameters: The location or {\em origin} of the light source (i.e.\ the tip of the cone), the direction vector (going out of the center of the cone), and $\alpha$, the angle between the center vector of the cone and its walls.

The concentration $\gamma$ describes how the light intensity is distributed within the cone. By definition, the intensity in the center of the cone is 1. Let $\beta$ be the angle between a particular ray of light within the cone and the center of the cone. Then the intensity of this ray is ${\rm cos}^\gamma(\beta)$. So, if $\gamma$ is 0, the intensity is uniform all over the cone (modulo attenuation).

INTERFACE SpotLightGO;

IMPORT Color, GO, LightGO, PointProp, Point3, RealProp;

TYPE
  T <: Public;
  Public = LightGO.T OBJECT
  METHODS
    init () : T;
  END;

l.init() initializes a new spot light source l and returns it.

PROCEDURE New (c : Color.T;  orig, dir : Point3.T;
               conc, spread, att0, att1 : REAL) : T;

l.init(c,orig,dir,conc,spread,att0,att1) initializes a new spot light source l and returns it. It also attaches the following properties to l: \begin{verbatim} (LightGO.Colour,ColourProp.NewConst(c)) (LightGO.Switch,BooleanProp.NewConst(TRUE)) (Origin,PointProp.NewConst(dir)) (Direction,PointProp.NewConst(dir)) (Concentration,RealProp.NewConst(conc)) (SpreadAngle,RealProp.NewConst(spread)) (Attenuation0,RealProp.NewConst(att0)) (Attenuation1,RealProp.NewConst(att1)) \end{verbatim}

VAR
  Origin        : PointProp.Name;
  Direction     : PointProp.Name;
  Concentration : RealProp.Name;
  SpreadAngle   : RealProp.Name;
  Attenuation0  : RealProp.Name;
  Attenuation1  : RealProp.Name;

In addition to the properties observed by all \type{GO}{T}'s and \type{LightGO}{T}'s, there are six additional properties that are observed by SpotLightGO.T's.

Origin is the name of a property that determines the origin of the light emitted by the light sources; it associates with a property value of type \type{PointProp}{Val}.

Direction is the name of a property that determines the direction of the light emitted by the light sources; it associates with a property value of type \type{PointProp}{Val}.

Concentration is a property that determines the intensity distribution within the light cone emitted by the spot light; it associates with a property value of type \type{RealProp}{Val}.

SpreadAngle is a property that determines the angle (in radians) between the center of the light cone and its walls; it associates with a property value of type \type{RealProp}{Val}.

Attenuation0 is the name of a property that determines the constant attenuation coefficient $C_0$; it associates with a property value of type \type{RealProp}{Val}.

Attenuation1 is the name of a property that determines the linear attenuation coefficient $C_1$; it associates with a property value of type \type{RealProp}{Val}.

PROCEDURE SetOrigin (o : GO.T; origin : Point3.T);

The expression SetOrigin(o,origin) is equivalent to o.setProp(Origin.bind(PointProp.NewConst(origin))).

PROCEDURE SetDirection (o : GO.T; dir : Point3.T);

SetDirection is a convenience procedure. The expression SetDirection(o,dir) is equivalent to o.setProp(Direction.bind(PointProp.NewConst(dir))).

PROCEDURE SetConcentration (o : GO.T; conc : REAL);

The expression SetConcentration(o,conc) is equivalent to o.setProp(Concentration.bind(RealProp.NewConst(conc))).

PROCEDURE SetSpreadAngle (o : GO.T; spread : REAL);

The expression SetSpreadAngle(o,spread) is equivalent to o.setProp(SpreadAngle.bind(RealProp.NewConst(spread))).

PROCEDURE SetAttenuation0 (o : GO.T; att : REAL);

The expression SetAttenuation1(o,att) is equivalent to o.setProp(Attenuation0.bind(RealProp.NewConst(att))).

PROCEDURE SetAttenuation1 (o : GO.T; att : REAL);

The expression SetAttenuation0(o,att) is equivalent to o.setProp(Attenuation1.bind(RealProp.NewConst(att))).

END SpotLightGO.