## anim3D/src/PointLightGO.i3

Copyright (C) 1994, Digital Equipment Corporation
Digital Internal Use Only

Created on Fri Feb  4 15:25:53 PST 1994 by najork

A PointLightGO.T is a geometric object that describes a point-shaped light source which is emitting light uniformly into all directions. 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}.

The underlying graphics system (PEX or OpenGL) uses the following formula to compute the attenuation: Let\\ $L_p$ : Position of the light source \\ $O_p$ : position of (some part of) an object \\ $C_0$ : Constant attenuation coefficieny \\ $C_1$ : Linear attenuation coefficieny \\ Then the overall color attenuation factor $L_a$ for the light hitting the object at point $O_p$ is: $\frac{1}{C_0 + C_1 | O_p - L_p | }$

This attenuation model is supported by both PEX and OpenGL (in hardware). Note, however, that it does not capure real-world attenuation, which behaves like this: $L_a = \frac{1}{| O_p - L_p |^2 }$

OpenGL supports a third, quadratic, attenuation coefficient; PEX, however, does not. As lighting computation is (and, for efficiency reasons, must be) supported by the graphics hardware, and as right now PEX is the only API to our hardware, we are stuck with its simplistic model.

INTERFACE PointLightGO;

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

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

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

PROCEDURE New (c : Color.T; origin : Point3.T; att0, att1 : REAL) : T;

New(c,origin,att0,att1) initializes a new point 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)) (Attenuation0,RealProp.NewConst(att0)) (Attenuation1,RealProp.NewConst(att1)) \end{verbatim}

VAR
Origin       : PointProp.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 three additional properties that are observed by PointLightGO.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}. Attenuation0 is the name of a property that determines the 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 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 SetAttenuation0 (o : GO.T; att : REAL);

The expression SetAttenuation0(o,att) is equivalent to o.setProp(Attenuation0.bind(RealProp.NewConst(att))).

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

The expression SetAttenuation1(o,att) is equivalent to o.setProp(Attenuation1.bind(RealProp.NewConst(att))).

END PointLightGO.