## Write out a triangle mesh scene as an OFF format file.  This only
## outputs the geometry; color and transparency are ignored for now.
## The format supports adding three or four rgb values to each face
## line, but MeshLab seems to ignore these.

saveTrianglesAsOFF <- function(scene, filename = "scene.OFF") {
    scene <- misc3d:::colorScene(scene)
    triangles <- misc3d:::canonicalizeAndMergeScene(scene, "color", "color2",
                                                    "alpha", "col.mesh",
                                                    "fill", "smooth")
    ve <- misc3d:::t2ve(triangles)
    f <- file(filename, open = "w")
    on.exit(close(f))
    write("OFF", f)
    write(c(ncol(ve$vb), ncol(ve$ib), 3 * ncol(ve$ib)), f, 3)
    write(ve$vb, f, 3)
    write(rbind(3, ve$ib - 1), f, 4)
    invisible(NULL)
}