gleTextureMode(3GLE) GLE gleTextureMode(3GLE)NAMEgleTextureMode - set the type of GLE automatic texture
coordinate generation.
SYNTAX
void gleTextureMode (int mode);
ARGUMENTS
mode bitwise OR of GLE texture mode flags
DESCRIPTION
In addition to the default glTexGen modes that are sup
plied by OpenGL, the tubing library also contains some of
its own automatic texture coordinate generation routines.
In addition, user-defined texture coord generation rou
tines can be supplied.
To use texture mapping with the extrusion library, one
must remember to "do the obvious":
Enable texture mapping through OpenGL
Define and load (glTexImage2D/glBindTexture) a tex
ture
If using the routine below, then disable glTexgGen
gleTextureMode can be used to set the type of automatic
texture coordinate generation to be used. The argument
should be a bitwise-OR of any of the following flags:
GLE_TEXTURE_ENABLE
If this bit is set, then texturing is enabled. If
this bit is NOT set, then automatic texture coordi
nate generation is disabled.
The way in which the automatic texture coordinate genera
tion occurs is determined by one of the following flags.
One and only one of these should be selected at a time.
These tokens are enumerants, not bit-flags.
GLE_TEXTURE_VERTEX_FLAT
Uses the vertexes "x" coordinate as the texture "u"
coordinate, and the accumulated segment length as
the "v" coordinate.
GLE_TEXTURE_NORMAL_FLAT
Uses the normal vector's "x" coordinate as the tex
ture "u" coordinate, and the accumulated segment
length as the "v" coordinate.
GLE_TEXTURE_VERTEX_CYL
Uses u = phi/(2*pi) = arctan (vy/vx)/(2*pi) as the
texture "u" coordinate, and the accumulated segment
GLE 3.6 1
gleTextureMode(3GLE) GLE gleTextureMode(3GLE)
length as the "v" coordinate. In the above equa
tion, "vx" and "vy" stand for the vertex's x and y
coordinates.
GLE_TEXTURE_NORMAL_CYL
Uses u = phi/(2*pi) = arctan (ny/nx)/(2*pi) as the
texture "u" coordinate, and the accumulated segment
length as the "v" coordinate. In the above equa
tion, "nx" and "ny" stand for the normal's x and y
coordinates.
GLE_TEXTURE_VERTEX_SPH
Uses u = phi/(2*pi) = arctan (vy/vx)/(2*pi) as the
texture "u" coordinate, and v = theta/pi = (1.0 -
arccos(vz))/pi as the texture "v" coordinate. In
the above equation, "vx","vy" and "vz" stand for
the vertex's x, y and z coordinates.
GLE_TEXTURE_NORMAL_SPH
Uses u = phi/(2*pi) = arctan (ny/nx)/(2*pi) as the
texture "u" coordinate, and v = theta/pi = (1.0 -
arccos(nz))/pi as the texture "v" coordinate. In
the above equation, "nx","ny" and "nz" stand for
the normal's x, y and z coordinates.
GLE_TEXTURE_VERTEX_MODEL_FLAT
GLE_TEXTURE_NORMAL_MODEL_FLAT
GLE_TEXTURE_VERTEX_MODEL_CYL
GLE_TEXTURE_NORMAL_MODEL_CYL
GLE_TEXTURE_VERTEX_MODEL_SPH
GLE_TEXTURE_NORMAL_MODEL_SPH
These define texture mapping modes that are very
similar to those described above, except that the
untransformed vertices and/or normals are used. As
a result, textures tends to stick to the extrusion
according to the extrusions local surface coordi
nates rather than according to real-space coordi
nates. This will in general provide the correct
style of texture mapping when affine transforms are
being applied to the contour, since the coordinates
used are those prior to the affine transform.
OPERATION
To best understand how to use the above functions, it is
best to understand how the tubing is actually drawn. Let
us start by defining some terms. The tubing library
"extrudes" a "contour" along a "path". The contour is a
2D polyline. The path is a 3D polyline. We use the word
"segment" to refer to a straight-line segment of the path
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gleTextureMode(3GLE) GLE gleTextureMode(3GLE)
polyline. We also interchangeably use the word "segment"
to stand for the section of the extrusion that lies along
a path segment.
The tubing library draws segments one at a time. It uses
glPushmatrix() and glPopmatrix() to orient each segment
along the negative z-axis. The segment starts at z=0 and
ends at some negative z-value (equal to the length of the
segment). The segment is then drawn by calling glVer
tex3f() (and glNormal3F()) by drawing the 2D contour at
z=0 and again at z=-len. (Of course, if the join style is
one of the fancy ones, then the end-points are trimmed in
a variety of ways, and do not land exactly on z=0, or
z=-len, but they do come close). Note that glBegin() and
glEnd() are called around each segment. (Note also that
additional glBegins/Ends may be called to draw end-caps or
filleting triangles for the more complex join styles.)
The obvious way to automatically generate textures is to
warp the glVertex() and glNormal() functions, and compute
texture coordinates based on the 3-space vertex and normal
coordinates. This is essentially what the tubing code
does, except that it passes some extra parameters. The
glBegin calls are wrapped, and the integer segment number
and the floating-point length of the segment are passed
in. By knowing the segment number, and the segment length,
the texture coordinates can be adjusted. Knowing the
length allows the length to be accumulated, so that a tex
ture is applied lengthwise along the extrusion. It is this
accumulated length that is used in the FLAT and CYL map
ping modes.
For each vertex, not only are the vertex x,y,z coordinates
available, but so is a contour vertex counter indicating
which contour vertex this corresponds to. There is also a
flag indicating whether the vertex corresponds to a front
or back vertex (i.e. a z=0 or z=-len vertex). Again, this
info can be used to avoid confusion when drawing the more
complex join styles.
HINTS
Here are a few hints, tips, and techniques:
o Hint: Confused? RUN THE DEMOS! The best way to
understand what all the different texture modes are
doing is to see them in action.
o Hint: The texture matrix can be used to your advan
tage! That is, you can use glMatrixMode(GL_TEXTURE)
to control how textures are mapped to the surface.
In particular, you may/will want to use it to to
rescale the V coordinate.
o The origin of the contour will in general change
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gleTextureMode(3GLE) GLE gleTextureMode(3GLE)
the vertex x's and y's, thus changing the texture
coordinates.
o The contour "up" vector will NOT influence the tex
ture coordinates.
o For the FLAT and CYL modes, the accumulated length
really is the accumulated length of the segments in
modeling coordinates. Unless the extrusion is very
small, this length will probably be much larger
than 1.0, and so the resulting texture coordinate
will wrap. You will generally want to rescale the
"V" coordinate to make the texture map fit.
o If the texture is "swimming" around on the surface
in an undesired way, try using the "MODEL" version
of the texture generation flag.
o Typically, you will NOT want to use the "SPH" ver
sions of the texture generation engine unless you
really, really have an extrusion for which spheri
cal coordinates are appropriate. Most uses of
extrusions are best handled with the "FLAT" and
"CYL" generation methods.
o User-defined texture generation callbacks are not
currently implemented, but these should be very,
very easy to hack in as desired. It should be easy
to let your imagination run wild in here. Look at
texgen.c -- what needs to be done should be obvi
ous, I hope. When in doubt, experiment.
BUGS
Multiple threads using GLE share a single texture mode.
SEE ALSO
gleExtrusion, gleSetJoinStyle
AUTHOR
Linas Vepstas (linas@fc.net)
GLE 3.6 4
