Class Name: Spot Light

Superclass - <Base Positional Light>

Subclasses

Definition

An instance of this DRM class is a <Base Positional Light> that specifies an elliptic cone of influence, the tip of which is the specified <Location 3D>. The cone is bounded where it intersects the sphere centred at the tip, the radius of which is given by the radius field. This results in a a cone-shaped volume of influence, possibly smashed in one direction. Only objects located within this cone can be affected.

The intensity dropoff is computed as follows:
Given a point on a ray in the cone at fixed distance D from the cone apex, if
I0 = intensity on the <Lobe Data> direction axis at distance D
Ah = the horizontal angle of the ray in degrees from direction vector
Av = the vertical angle of the ray in degrees from direction vector
then the intensity I at the point is:

case: 0< horizontal_drop_off_rate 0< vertical_drop_off_rate I= I0*(1-|Ah|*horizontal_drop_off_rate)*(1-|Av|*vertical_drop_off_rate)

case: 0= horizontal_drop_off_rate 0< vertical_drop_off_rate
I= I0*(1-|Av|*vertical_drop_off_rate)

case: 0< horizontal_drop_off_rate 0= vertical_drop_off_rate
I= I0*(1-|Ah|*horizontal_drop_off_rate)

Primary Page in DRM Diagram:

• 21

Secondary Pages in DRM Diagram:

Example

1. A <Model> of a lighthouse has a <Geometry Model> with a <Union Of Geometry Hierarchy>, to which is attached an <Spot Light> representing the lighthouse's light. The <Spot Light> has a <Light Source Control Link> that turns the light on or off, depending on the time of day and the weather.

2. A headlight that actually emits light could have a <Positional Light> source with a direction located at the light, directed out from the headlight.

FAQs

How does <Spot Light> differ from the OpenGL concept of a spot light?

<Spot Light> can losslessly represent an OpenGL spot light.

<Spot Light>, however, has the additional capability of specifying that the light is to spread horizontally and vertically, while in an OpenGL <Spot Light>, the horizontal and vertical lobe angles shall be identical, as shall the fall off angles.

Constraints

Composed of (two-way)(inherited)

• one <Ambient Colour> instance
• one <Diffuse Colour> instance
• zero or one <Light Source Control Link> instance (notes)
• one <Specular Colour> instance
• one <Location 3D> instance (notes)

Composed of (two-way)

• one <Lobe Data> instance (notes)

Component of (two-way)(inherited)

• one or more <Aggregate Geometry> instances

Inherited Field Elements

Field Elements

Notes

Composed of Notes

Light_Source_Control_Link

 If a <Light Source> instance is subject to dynamic control -
 that is, if it can be turned on and off - it will have a
 <Light Source Control Link> component to express that
 control mechanism.

Location_3D

 3D position in 3 space.

 This will be affected by hierarchical transformations.

Lobe_Data

 Specifies the lobe shape.
 The direction and axes will be affected by hierarchical transformations.

Fields Notes

apply_to_children

 For a <Light Source> instance (that is, an instance of one of the
 concrete classes descended from the class) that is a component of
 some <Aggregate Geometry>, apply_to_children provides a mechanism
 for limiting the scope of the <Light Source>. If apply_to_children
 is SE_TRUE, only <Primitive Geometry> in the component tree of those
 <Aggregate Geometry> instances are affected by the given
 <Light Source>. If apply_to_children is SE_FALSE, the <Light Source>
 is not limited to the scope of those <Aggregate Geometry> instances
 and thus applies globally.

override_positional_lights

 For a <Light Source> instance (that is, an instance of one of the
 concrete classes descended from the class) that is a component of
 some <Aggregate Geometry>, override_positional_lights provides a
 mechanism to indicate with a value of SE_TRUE that the effect of the
 given <Light Source> overrides that of any <Base Positional Light>
 instances that would have otherwise applied to that
 <Aggregate Geometry> - for example, if the given <Light Source> is
 so close to the affected <Geometry> that the <Base Positional Light>
 effects would be negligible. If override_positional_lights is
 SE_FALSE, the effect of the given <Light Source> is combined with that
 of any <Base Positional Light> instances that are already in effect.

override_infinite_lights

 Flag to reset the current cumulative definition of Infinite Light_Sources.
 For a <Light Source> instance (that is, an instance of one of the
 concrete classes descended from the class) that is a component of
 some <Aggregate Geometry>, override_infinite_lights provides a
 mechanism to indicate with a value of SE_TRUE that the effect of the
 given <Light Source> overrides that of any <Infinite Light>
 instances that would have otherwise applied to that
 <Aggregate Geometry> - for example, if the given <Light Source> is
 so close to the affected <Geometry> that the <Infinite Light>
 effects would be negligible. If override_infinite_lights is
 SE_FALSE, the effect of the given <Light Source> is combined with that
 of any <Infinite Light> instances that are already in effect.

active_light_value

 A value of SE_TRUE indicates that the light is on, while a value of
 SE_FALSE indicates that the light is off. For a <Light Source>
 with a <Light Source Control Link> instance as a component,
 this is the default state of the light.

radius

 The radius, which is specified in metres, and which, with
 the <Location 3D> component, defines the zone of influence
 of this <Light Source>.

constant_attenuation_factor

 This is the constant 'a' in the attenuation quadratic (a + bd + cd**2).

linear_attenuation_factor

 This is the constant 'b' in the attenuation quadratic (a + bd + cd**2).

quadratic_attenuation_factor

 This is the constant 'c' in the attenuation quadratic (a + bd + cd**2).

horizontal_drop_off_rate

 Specifies, in degrees of arc, the horizontal angular intensity
 distribution of the light. The higher this value, the more focused the
 light.

 A value of 0.0 specifies a light that equally illuminates all
 objects within the cone of influence, and instantly falls to an
 intensity of 0.0 at the edge of the cone of light.

vertical_drop_off_rate

 Specifies, in degrees of arc, the vertical angular intensity
 distribution of the light. The higher this value, the more focused the
 light.

 A value of 0.0 specifies a light that equally illuminates all
 objects within the cone of influence, and instantly falls to an
 intensity of 0.0 at the edge of the cone of light.