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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> 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)
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> 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 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.
This radius, which is specified in metres, together with the <Location 3D> component defines the zone of influence of the given <Base Positional Light> instance.
This is the constant 'a' in the attenuation quadratic (a + bd + cd**2).
This is the constant 'b' in the attenuation quadratic (a + bd + cd**2).
This is the constant 'c' in the attenuation quadratic (a + bd + cd**2).
This 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.
This 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.
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