void achangBirchMedium(bool printBB) {
    // The following variable must be declared static, so that it doesn't
    // get reset to NULL every time the function is called, since this is
    // how we avoid reloading the OBJ file every single time.
    static GLMmodel* modelObject = NULL; // the model object
    const char* modelFilename = "birchMedium.obj"; // lives in $TWHOMEDIR/objects

    if(NULL==modelObject) {
        modelObject = glmReadOBJ(twPathname(modelFilename,true));
        if (!modelObject) {
            fprintf(stderr,"error reading OBJ file %s\n",modelFilename);
            return;
        }
        glmUnitize(modelObject);
        // It makes sense to delay printing the BB until after we've
        // rescaled the object.
        if(printBB) {
            GLfloat BoundingBox[6]; // [minx,maxx,miny,maxy,minz,maxz]

            glmBoundingBox(modelObject,BoundingBox);
            printf("Bounding Box is (%f,%f,%f,%f,%f,%f)\n",
                   BoundingBox[0], BoundingBox[1],
                   BoundingBox[2], BoundingBox[3],
                   BoundingBox[4], BoundingBox[5]);
        }
        // Finish initialization
        glmFacetNormals(modelObject);
        glmVertexNormals(modelObject, 90.0);
    }
    // Now the actual drawing code, which is always executed
    glPushMatrix();
    glTranslatef(0,1,0);        // up to the middle of the object
    glmDraw(modelObject, GLM_SMOOTH | GLM_MATERIAL);
    glPopMatrix();
}