gleTextureMode(3GLE) GLE gleTextureMode(3GLE)NAMEgleTextureMode - set the type of GLE automatic texture coordinate gen‐
eration.
SYNTAX
void gleTextureMode (int mode);
ARGUMENTS
mode bitwise OR of GLE texture mode flags
DESCRIPTION
In addition to the default glTexGen modes that are supplied by OpenGL,
the tubing library also contains some of its own automatic texture
coordinate generation routines. In addition, user-defined texture coord
generation routines 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 texture
If using the routine below, then disable glTexgGen
gleTextureMode can be used to set the type of automatic texture coordi‐
nate 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 coordinate generation is dis‐
abled.
The way in which the automatic texture coordinate generation 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 texture "u" coor‐
dinate, and the accumulated segment length as the "v" coordi‐
nate.
GLE_TEXTURE_VERTEX_CYL
Uses u = phi/(2*pi) = arctan (vy/vx)/(2*pi) as the texture "u"
coordinate, and the accumulated segment length as the "v" coor‐
dinate. In the above equation, "vx" and "vy" stand for the ver‐
tex'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" coor‐
dinate. In the above equation, "nx" and "ny" stand for the nor‐
mal'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 tex‐
ture "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 tex‐
ture "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 coordinates. 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 under‐
stand 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 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 seg‐
ment number, and the segment length, the texture coordinates can be
adjusted. Knowing the length allows the length to be accumulated, so
that a texture is applied lengthwise along the extrusion. It is this
accumulated length that is used in the FLAT and CYL mapping 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 cor‐
responds 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 advantage! 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 the vertex x's
and y's, thus changing the texture coordinates.
o The contour "up" vector will NOT influence the texture coordi‐
nates.
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 unde‐
sired way, try using the "MODEL" version of the texture genera‐
tion flag.
o Typically, you will NOT want to use the "SPH" versions of the
texture generation engine unless you really, really have an
extrusion for which spherical 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@linas.org)
GLE 3.0 gleTextureMode(3GLE)