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The SEDRIS Data Representation Model
APPENDIX A - Classes Spot Light |
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An instance of this DRM class is a <Base Positional Light> instance that specifies an elliptic cone of influence, the tip of which is the specified <Location 3D> component. 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:
0.0 <
horizontal_drop_off_rate,
0.0 <
vertical_drop_off_rate
I = I0 * (1.0 - |Ah| *
horizontal_drop_off_rate)
*
(1.0 - |Av| *
vertical_drop_off_rate)
case:
0.0 =
horizontal_drop_off_rate,
0.0 <
vertical_drop_off_rate
I = I0 * (1.0 - |Av| *
vertical_drop_off_rate)
case:
0.0 <
horizontal_drop_off_rate,
0.0 =
vertical_drop_off_rate
I = I0 * (1.0 - |Ah| *
horizontal_drop_off_rate)
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.
A headlight that actually emits light could have a <Positional Light> source with a direction located at the light, directed out from the headlight.
<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.
| SE_Long_Float | horizontal_drop_off_rate; | (notes) |
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| SE_Long_Float | vertical_drop_off_rate; | (notes) |
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.
This specifies the position of the given <Base Positional Light> instance.
This specifies the lobe shape.
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.
For a <Light Source> instance 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 Representation> that the <Base Positional Light> effects would be negligible. If override_positional_lights = SE_FALSE, the effect of the given <Light Source> is combined with that of any <Base Positional Light> instances that are already in effect.
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>. If override_infinite_lights = SE_FALSE, the effect of the given <Light Source> is combined with that of any <Infinite Light> instances that are already in effect.
A value of SE_TRUE for the active_light_value field 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.
The radius field, which is specified in metres, together with the <Location 3D> component specifies the zone of influence of the given <Base Positional Light> instance.
The constant_attenuation_factor field is the constant "a" in the attenuation quadratic (a + bd + cd**2).
The linear_attenuation_factor field is the constant "b" in the attenuation quadratic (a + bd + cd**2).
The quadratic_attenuation_factor field is the constant "c" in the attenuation quadratic (a + bd + cd**2).
The horizontal_drop_off_rate field specifies, in radians, 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.
The vertical_drop_off_rate field specifies, in radians, 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.
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