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The SEDRIS Data Representation Model
APPENDIX A - Classes Primitive Colour |
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An instance of this DRM class specifies a single colour, consisting of individual ambient, diffuse, emissive, and specular components.
Generally speaking, to determine the colour of an object illuminated by a light source X, combine
Consider a <Geometry Model Instance> of a computer monitor, placed on top of a <Geometry Model Instance> of a desk. A <Positional Light> affecting the two objects is located so that the illumination is directed from above. (Assume that these instances are present within an environment simulating an ordinary room.)
Each <Polygon> within the desk <Model> has an <Inline Colour> component, which in turn has a <Primitive Colour> with both an <Ambient Colour> and a <Diffuse Colour> component. Due to the <Diffuse Colour> component, the area of the desk under the monitor that's visible to the observer appears to be in shadow. However, the shadowy area is not totally blacked out, because the <Ambient Colour> simulates the effect due to light reflected from the walls, ceiling, and so on that would reach that area.
Consider a <Line> used to simulate a line of runway lights. The <Line>'s <Colour> would resolve to a <Primitive Colour> consisting primarily (possibly completely) of an <Emissive Colour>, since the <Line> is pretending to emit light.
Consider a collection of <Polygon> instances used to represent the surface of a sunlit lake. These <Polygon> instances' <Colour> components resolve to <Primitive Colour> instances consisting of <Ambient Colour>, <Diffuse Colour>, and <Specular Colour> components.
The <Ambient Colour> prevents any <Polygon> instances in shadow from appearing black, while the <Diffuse Colour> provides most of the normal appearance of a lit object. However, since water is a reflective substance, its colour requires a <Specular Colour> component to simulate light reflected from the water.
No, although the terminology is similar. SEDRIS handles transparency somewhat differently than OpenGL does, among other things. For a description of the OpenGL lighting model, see [OPENGL], Chapter 5 "Lighting".
The question has eliminated any concerns about <Emissive Colour> and <Specular Colour> components -- the <Primitive Colour> instances won't have them. Having said that, the data provider is down to 2 choices -- <Ambient Colour> and <Diffuse Colour>. Here is a simplified description of the effects each will provide (i.e., considering only 1 light source).
<Ambient Colour> is independent of the positions of either the light source or the observer. That is, an object with only <Ambient Colour> appears uniformly lit across its surface, regardless of where the light source is or where the observer is. (This can create a very artificial-looking effect, since it distorts some of the visual cues that provide depth perception.)
<Diffuse Colour>, on the other hand, depends on the angle of the lit object to the light source (although not on the observer's position). An object with only <Diffuse Colour> will appear to be lighted on the side facing the light source, while the opposite side will be in shadow. The effect is consistent with the visual cues used to determine shape-from-shading in various image analysis methods.
Note that a <Primitive Colour> can have both an <Ambient Colour> and a <Diffuse Colour> component. This indicates that even if part of the object is in shadow, it is still somewhat visible. See example #1.
Each of the components of <Primitive Colour> has, in turn, a <Colour Data> component, which is either an <RGB Colour>, an <HSV Colour>, or a <CMY Colour> instance, depending on which colour model is being used.
<Colour Tables> used to be composed of <Inline Colour> instances, but problems arose since both <Colour Index> and <Inline Colour> instances can have <Translucency> components. When a <Colour Index> instance that has a <Translucency> component refers to an entry in a <Colour Table> instance, the interpretation is clearer if the <Colour Table> instance's entry cannot specify any additional <Translucency> instances. Consequently, <Primitive Colour> exists so that "just the colour" can be put into a <Colour Table> instance.
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