Geometry

• surface
• point
• face

The SLIDE surface, point, and face are special primitives that can not be directly instanced in groups.

surface

A surface statement defines the material properties of a surface.

A more detailed explanation of these values and their role in doing lighting calculations is provided in the Lighting Model section.

Examples:

# Red Matte Surface

surface sRed
  color        (1 0 0)
  reflectivity (0.95 0.95 0)
endsurface

# Red Matte Surface
tclinit {
  slide create surface sRed \
    -color        { 1 0 0 } \
    -reflectivity { 0.95 0.95 0 }
}

defines a fully saturated matte red material that reflects most light diffusely and none specularly.

# Shiny Metallic Violet Surface

surface sShinyViolet
  color        (0.8 0.3 1.0)
  reflectivity (0.1 0.1 0.9)
  exponent     2.0
  metallic     0.8
endsurface

# Shiny Metallic Violet Surface
tclinit {
  slide create surface sShinyViolet
    -color        {0.8 0.3 1.0}
    -reflectivity {0.1 0.1 0.9}
    -exponent     2.0
    -metallic     0.8
}

defines a metallic violet material that reflects some light diffusely and yields strong specular highlights.

Inheritance of Surface Properties

Surface properties are propogated down the scene hierarchy in the same manner as shading flags. A node in the scene hierarchy will pass down its local surface object unless that surface is SLF_INHERIT in which case it will pass along its parent's surface object. There is no mechanism that allows a parent node to override a descendant's locally defined surface property. The default surface object for a node is SLF_INHERIT.

The propogation of surface properties is further complicated by interactions with shading flags down at the face and point levels. The following example illustrates the difference in surface property inheritance between flat and Gouraud shading. The example contains a single triangle with a blue surface applied to it. Two of the points which define the triangle have their own red and green surface properties assigned to them while the third one has SLF_INHERIT which means that it will inherit the blue surface property from the triangular face.

With flat shading applied, the face ignores any surface properties defined on its points and uses the surface property of the face to do shading. The entire triangle will be a single color under flat shading. If lighting is disabled, the triangle will simply take on the color value of the surface. If lighting is enabled, one lighting calculation will be done for the triangle at its psuedo-centroid (the unweighted average of the positions of all of the vertices of the face) using the face normal and the color value from the face's surface property.

With Gouraud shading applied, every vertex of the face can have a separate surface property, although only a single texture map will be used per face. If a point has a surface property locally defined then it will use that surface. If the point uses the default SLF_INHERIT surface then it will inherit the surface property of the face. In the example, the first two points use their red and green surfaces, respectively, while the third point inherits the blue surface from the face. If lighting is disabled like in the image below, the color values assigned at the different vertices will be linearly interpolated across the area of the face. With lighting enabled, a separate lighting calculation will be preformed at each vertex using the point's location, surface property, and normal. Then the calculated color values will be linearly interpolated across the area of the face.

surface sRed color ( 1 0 0 ) endsurface surface sGreen color ( 0 1 0 ) endsurface surface sBlue color ( 0 0 1 ) endsurface point pRed ( 1 0 0 ) surface sRed endpoint point pGreen ( 0 1.7 0 ) surface sGreen endpoint point pInherit ( -1 0 0 ) surface SLF_INHERIT endpoint face fTriangle ( pRed pGreen pInherit ) surface sBlue endface object oFlat ( fTriangle ) shading SLF_FLAT endobject object oGouraud ( fTriangle ) shading SLF_GOURAUD endobject

oFlat oGouraud

point

A point statement defines a point in 3D space which can be reference by its id in a face to define the face's geometry.

Example:

point p1 ( 0 0 2.5 )
  surface sRed
endpoint

tclinit {
  slide create point p1 { 0 0 2.5 } \
    -surface sRed
}

defines a point named p1 which lies 2.5 units from the origin along the z-axis and has a vertex color defined by the surface sRed.

face

The face statement describes a planar convex polygon. The polygon of the face is defined by a list of point identifiers which enumerate the vertices of the face in counterclockwise order around the face contour.

The face is defined for planar convex polygons. The behavior for non-planar, concave, and self intersecting polygon descriptions is undefined.

A face has a front side and a back side which is important to the description of closed polyhedrons. The normal to a face points out of the face on the front side of the face. The ordering of the vertices around the contour of the face defines its orientation. This orientation is based on the right-hand rule for cross products. A counterclockwise ordering of vertices around the contour implies a normal which is pointing outward, so this is the front side of the polygon. A clockwise ordering of vertices around the contour implies a normal which is pointing inward, so this is the back side of the polygon.

Front Facing Polygon Counterclockwise Vertex Ordering	# Front Facing Square point p0 ( 1 0 0 ) endpoint point p1 ( 1 1 0 ) endpoint point p2 ( 0 1 0 ) endpoint point p3 ( 0 0 0 ) endpoint face fFront ( p0 p1 p2 p3 ) endface
Back Facing Polygon Clockwise Vertex Ordering	# Back Facing Square point p0 ( 1 0 0 ) endpoint point p1 ( 0 0 0 ) endpoint point p2 ( 0 1 0 ) endpoint point p3 ( 1 1 0 ) endpoint face fBack ( p0 p1 p2 p3 ) endface

Example:

# Square

point p0 ( 1 0 0 ) endpoint
point p1 ( 1 1 0 ) endpoint
point p2 ( 0 1 0 ) endpoint
point p3 ( 0 0 0 ) endpoint

face fSquare ( p0 p1 p2 p3 ) 
  surface sRed
endface

# Square
tclinit {
  slide create point p0 { 1 0 0 }
  slide create point p1 { 1 1 0 }
  slide create point p2 { 0 1 0 }
  slide create point p3 { 0 0 0 }

  slide create face fSquare { p0 p1 p2 p3 } \
    -surface sRed
}

defines a unit square face in the first quadrant named fSquare. The face has been assign a red surface material.