Table 6.21 — DRM_Base_Positional_Light

Property

Description

Class
  • <DRM Base Positional Light>

Superclass

Subclass

Definition

An instance of one of the concrete subclasses of this abstract DRM class is a <DRM Light Source> that radiates in all directions from a specified point in 3D space with a sphere of influence centred at that point, the radius of which is specified by the value of the radius field.

Only objects that are within the sphere of influence of a <DRM Base Positional Light> instance (that is, an instance of one of its concrete subclasses) can be affected by that <DRM Base Positional Light>. Specific subclasses may further restrict the zone of influence to only a portion of the sphere.

The degree to which an object is affected in the zone of influence depends on the attenuation of the <DRM Base Positional Light>, which depends on the values of its attenuation factor fields. If attenuation is not specified - that is, when constant_attenuation_factor = 1,0, linear_attenuation_factor = 0,0, and quadratic_attenuation_factor = 0,0 - the light ends abruptly at the edge of the sphere of influence. Otherwise, the light intensity components (ambient, diffuse, and specular) are attenuated by the reciprocal of the attenuation quadratic (a + bd + cd**2), where a, b, c are the attenuation factor field values as noted in their footnotes, and d is the distance from the position of the <DRM Base Positional Light>.

Consequently, the formula for the final attenuation factor is: 

                                       1
       --------------------------------------------------------------
       [ constant_attenuation_factor +
            (distance * linear_attenuation_factor) +
                ((distance squared) * quadratic_attenuation_factor) ]

Class diagram

Figure 6.19 — DRM_Base_Positional_Light

Inherited field elements

Field name

Range

Field data type

apply_to_children1

 

Boolean

override_positional_lights2

 

Boolean

override_infinite_lights3

 

Boolean

active_light_value4

 

Boolean

Field elements

Field name

Range

Field data type

radius5

 

Float

constant_attenuation_factor6

 

Long_Float

linear_attenuation_factor7

 

Long_Float

quadratic_attenuation_factor8

 

Long_Float

Associated to (one-way) (inherited)
  • None.

Associated to (one-way)
  • None.

Associated by (one-way) (inherited)
  • None.

Associated by (one-way)
  • None.

Associated with (two-way) (inherited)
  • None.

Associated with (two-way)
  • None.

Composed of (two-way) (inherited)

Composed of (two-way)

Composed of (two-way metadata) (inherited)
  • None.

Composed of (two-way metadata)
  • None.

Component of (two-way) (inherited)

Component of (two-way)
  • None.

Constraints

Clarifications

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

2 For a <DRM Light Source> instance that is a component of some <DRM Aggregate Geometry>, override_positional_lights provides a mechanism to indicate with a value of TRUE that the effect of the given <DRM Light Source> overrides that of any <DRM Base Positional Light> instances that would have otherwise applied to that <DRM Aggregate Geometry> - for example, if the given <DRM Light Source> is so close to the affected <DRM Geometry> that the <DRM Base Positional Light> effects would be negligible. If override_positional_lights = FALSE, the effect of the given <DRM Light Source> is combined with that of any <DRM Base Positional Light> instances that are already in effect.

3 For a <DRM Light Source> instance (that is, an instance of one of the concrete classes descended from the class) that is a component of some <DRM Aggregate Geometry>, override_infinite_lights provides a mechanism to indicate with a value of TRUE that the effect of the given <DRM Light Source> overrides that of any <DRM Infinite Light> instances that would have otherwise applied to that <DRM Aggregate Geometry>. If override_infinite_lights = FALSE, the effect of the given <DRM Light Source> is combined with that of any <DRM Infinite Light> instances that are already in effect.

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

5 This radius, which is specified in metres, together with the <DRM 3D Location> component defines the zone of influence of the given <DRM Base Positional Light> instance.

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

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

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

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

10 This specifies the position of the given <DRM Base Positional Light> instance.

Example(s)

See subclasses for examples.