Table 6.274—DRM_Spot_Light

Property

Description

Superclass

· <DRM Base Positional Light>

Class

· <DRM Spot Light>

Subclass

· none

Definition

An object of this DRM class specifies a <DRM Base Positional Light> that defines an elliptic cone of influence, whose tip is the specified <DRM Location 3D>. The cone is bounded where it intersects the sphere centred at the tip, whose radius 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. In addition, apply_to_children is true, then (as with all <DRM Light Source>s) only <DRM Primitive Geometry> which are below the <DRM Aggregate Geometry> to which the <DRM Spot Light> is attached will be affected.

The intensity drop-off 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 <DRM 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)

Example(s)

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

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

Inherited field elements

field_name	range	Field_Data_Type
apply_to_children^1		Boolean
override_positional_lights^2		Boolean
override_infinite_lights^3		Boolean
active_light_value^4		Boolean
radius^5		Float
constant_attenuation_factor^6		Long_Float
linear_attenuation_factor^7		Long_Float
quadratic_attenuation_factor^8		Long_Float

Field elements

field_name	range	Field_Data_Type
horizontal_drop_off_rate^9		Long_Float
vertical_drop_off_rate^10		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)

· one <DRM Ambient Colour>

· one <DRM Diffuse Colour>

· zero or one <DRM Light Source Control Link>

· one <DRM Location 3D>¹¹

· one <DRM Specular Colour

Composed of (two-way)

· one <DRM Lobe Data>¹²

Composed of (two-way metadata) (inherited)

· none

Composed of (two-way metadata)

· none

Component of (two-way) (inherited)

· one or more <DRM Aggregate Geometry>s

Component of (two-way)

· none

Constraints

· none

Notes

¹ Flag to allow lights to limit their scope to only affecting their children. If apply_to_children is FALSE, the light is assumed to apply globally.

² Flag to reset the current cumulative definition of local <DRM Light Source>s If override_positional_lights is TRUE, all positional light sources in the current scope are cleared.

³ Flag to reset the current cumulative definition of Infinite Light_Sources. If override_infinite_lights is TRUE, all infinite light sources in the current scope are cleared.

⁴ A value of TRUE indicates the light is on while a value of FALSE indicates the light is off. This is the default/active state of the light.

⁵ The radius (in metres) and position define the sphere of influence. This will be affected by hierarchical transformations.

⁶ Constant 'a' in the attenuation quadratic (a + bd + cd²).

⁷ Constant 'b' in the attenuation quadratic (a + bd + cd²).

⁸ Constant 'c' in the attenuation quadratic (a + bd + cd²).

⁹ Specifies, in terms of 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.

¹⁰ Specifies, in terms of 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.

¹¹ Position in three-space. This will be affected by hierarchical transformations.

¹² Specifies the lobe shape. The direction and axes will be affected by hierarchical transformations.

Class diagram

<DRM Spot Light>