+/* Put the labels on the atoms.
+ This can't be a part of the display list because of the games
+ we play with the translation matrix.
+ */
+void
+draw_labels (ModeInfo *mi)
+{
+ molecule_configuration *mc = &mcs[MI_SCREEN(mi)];
+ int wire = MI_IS_WIREFRAME(mi);
+ molecule *m = &mc->molecules[mc->which];
+ int i, j;
+
+ if (!do_labels)
+ return;
+
+ if (!wire)
+ glDisable (GL_LIGHTING); /* don't light fonts */
+
+ for (i = 0; i < m->natoms; i++)
+ {
+ molecule_atom *a = &m->atoms[i];
+ GLfloat size = atom_size (a);
+ GLfloat m[4][4];
+
+ glPushMatrix();
+
+ if (!wire)
+ set_atom_color (mi, a, True);
+
+ /* First, we translate the origin to the center of the atom.
+
+ Then we retrieve the prevailing modelview matrix (which
+ includes any rotation, wandering, and user-trackball-rolling
+ of the scene.
+
+ We set the top 3x3 cells of that matrix to be the identity
+ matrix. This removes all rotation from the matrix, while
+ leaving the translation alone. This has the effect of
+ leaving the prevailing coordinate system perpendicular to
+ the camera view: were we to draw a square face, it would
+ be in the plane of the screen.
+
+ Now we translate by `size' toward the viewer -- so that the
+ origin is *just in front* of the ball.
+
+ Then we draw the label text, allowing the depth buffer to
+ do its work: that way, labels on atoms will be occluded
+ properly when other atoms move in front of them.
+
+ This technique (of neutralizing rotation relative to the
+ observer, after both rotations and translations have been
+ applied) is known as "billboarding".
+ */
+
+ glTranslatef(a->x, a->y, a->z); /* get matrix */
+ glGetFloatv (GL_MODELVIEW_MATRIX, &m[0][0]); /* load rot. identity */
+ m[0][0] = 1; m[1][0] = 0; m[2][0] = 0;
+ m[0][1] = 0; m[1][1] = 1; m[2][1] = 0;
+ m[0][2] = 0; m[1][2] = 0; m[2][2] = 1;
+ glLoadIdentity(); /* reset modelview */
+ glMultMatrixf (&m[0][0]); /* replace with ours */
+
+ glTranslatef (0, 0, (size * 1.1)); /* move toward camera */
+
+ glRasterPos3f (0, 0, 0); /* draw text here */
+
+ /* Before drawing the string, shift the origin to center
+ the text over the origin of the sphere. */
+ glBitmap (0, 0, 0, 0,
+ -string_width (mc->xfont1, a->label) / 2,
+ -mc->xfont1->descent,
+ NULL);
+
+ for (j = 0; j < strlen(a->label); j++)
+ glCallList (mc->font1_dlist + (int)(a->label[j]));
+
+ glPopMatrix();
+ }
+
+ /* More efficient to always call glEnable() with correct values
+ than to call glPushAttrib()/glPopAttrib(), since reading
+ attributes from GL does a round-trip and stalls the pipeline.
+ */
+ if (!wire)
+ glEnable (GL_LIGHTING);
+}
+
+