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Geometric objects can have associated "appearance" information,
specifying shading, lighting, color, wireframe vs. shaded-surface
display, and so on. Appearances are inherited through object
hierarchies, e.g. attaching an appearance to a LIST means that the
appearance is applied to all the LIST's members.
Some appearance-related properties are relegated to "material", "lighting" and "texture" substructures. Take care to note which properties belong to which structure. Any geometric object can be preceded by an appearance definintion like in the following example:
{
appearance { +edge }
LIST { < "file1" } { QUAD 1 0 0 0 0 1 0 1 0 1 0 0 }
}
Appearances are also OOGL objects in their own right and can be given symbolic names and referenced by them (see References). See Appearance objects.
Here's an example appearance structure including values for all attributes. Order of attributes is unimportant. As usual, white space is irrelevant. Boolean attributes may be preceded by "+" or "-" to turn them on or off; "+" is assumed if only the attribute name appears. Other attributes expect values.
A "*" prefix on any attribute, e.g. "*+edge" or "*linewidth 2" or "material { *diffuse 1 1 .25 }", selects "override" status for that attribute.
appearance {
+face # (Do) draw faces of polygons. On by default.
-edge # (Don't) draw edges of polygons
+vect # (Do) draw VECTs. On by default.
-transparent # (Disable) transparency. Enabling transparency
# does not (necessarily) result in a correct Geomview
# pictures, but alpha values are used in RenderMan
# snapshots.
-normal # (Do) draw surface-normal vectors
normscale 1 # ... with length 1.0 in object coordinates
+evert # do evert polygon normals where needed so as
# to always face the camera
+texturing # (Enable) texture mapping
+linear # (Enable) linear average of closest texture elements
+mipmap # (Enable) texture mip-mapping
+mipinterp # (Enable) linear mip-mapping
-backcull # (Don't) discard clockwise-oriented faces
-concave # (Don't) expect and handle concave polygons
-shadelines # (Don't) shade lines as if they were lighted cylinders
# These four are only effective where the graphics system
# supports them, namely on GL and Open GL.
-keepcolor # Normally, when N-D positional coloring is enabled as
# with geomview's (ND-color ...) command, all
# objects' colors are affected. But, objects with the
# "+keepcolor" attribute are immune to N-D coloring.
shading smooth # or ``shading constant'' or ``shading flat'' or
# or ``shading csmooth''.
# smooth = Gouraud shading, flat = faceted,
# csmooth = smoothly interpolated but unlighted.
linewidth 1 # lines, points, and edges are 1 pixel wide.
patchdice 10 10 # subdivide Bezier patches this finely in u and v
material { # Here's a material definition;
# it could also be read from a file as in
# ``material < file.mat''
ka 1.0 # ambient reflection coefficient.
ambient .3 .5 .3 # ambient color (red, green, blue components)
# The ambient contribution to the shading is
# the product of ka, the ambient color,
# and the color of the ambient light.
kd 0.8 # diffuse-reflection coefficient.
diffuse .9 1 .4 # diffuse color.
# (In ``shading constant'' mode, the surface
# is colored with the diffuse color.)
ks 1.0 # specular reflection coefficient.
specular 1 1 1 # specular (highlight) color.
shininess 25 # specular exponent; larger values give
# sharper highlights.
backdiffuse .7 .5 0 # back-face color for two-sided surfaces
# If defined, this field determines the diffuse
# color for the back side of a surface.
# It's implemented by the software shader, and
# by hardware shading on GL systems which support
# two-sided lighting, and under Open GL.
alpha 1.0 # opacity; 0 = transparent (invisible), 1 = opaque.
# Ignored when transparency is disabled.
edgecolor 1 1 0 # line & edge color
normalcolor 0 0 0 # color for surface-normal vectors
}
lighting { # Lighting model
ambient .3 .3 .3 # ambient light
replacelights # ``Use only the following lights to
# illuminate the objects under this
# appearance.''
# Without "replacelights", any lights listed
# are added to those already in the scene.
# Now a collection of sample lights:
light {
color 1 .7 .6 # light color
position 1 0 .5 0 # light position [distant light]
# given in homogeneous coordinates.
# With fourth component = 0,
# this means a light coming from
# direction (1,0,.5).
}
light { # Another light.
color 1 1 1
position 0 0 .5 1 # light at finite position ...
location camera # specified in camera coordinates.
# (Since the camera looks toward -Z,
# this example places the light
# .5 unit behind the eye.)
# Possible "location" keywords:
# global light position is in world (well, universe) coordinates
# This is the default if no location specified.
# camera position is in the camera's coordinate system
# local position is in the coordinate system where
# the appearance was defined
}
} # end lighting model
texture {
clamp st # or ``s'' or ``t'' or ``none''
file lump.tiff # file supplying texture-map image
alphafile mask.pgm.Z # file supplying transparency-mask image
apply blend # or ``modulate'' or ``decal''
transform 1 0 0 0 # surface (s,t,0,1) * tfm -> texture coords
0 1 0 0
0 0 1 0
.5 0 0 1
background 1 0 0 1 # relevant for ``apply blend''
}
} # end appearance
There are rules for inheritance of appearance attributes when several are imposed at different levels in the hierarchy.
For example, Geomview installs a backstop appearance which provides default values for most parameters; its control panels install other appearances which supply new values for a few attributes; user-supplied geometry may also contain appearances.
The general rule is that the child's appearance (the one closest to the
geometric primitives) wins. Further, appearance controls with
"override" status (e.g. *+face or material { *diffuse 1 1 0 })
win over those without it.
Geomview's appearance controls use the "override" feature so as to be effective even if user-supplied objects contain their own appearance settings. However, if a user-supplied object contains an appearance field with override status set, that property will be immune to Geomview's controls.