void pdraw(Model mdl){
  int i,j;
  i=1;
  float mdl_color[4]={0.8,0.8,0.8,1.0};
  float mdl_diffuse[4]={0.2,0.2,0.2,1.0};
  float mdl_specular[4]={1.0,1.0,1.0,1.0};
  float mdl_ambient[4]={1.0,1.0,1.0,1.0};
  double dx0, dy0,  dz0,  dx1,  dy1,  dz1, nx, ny, nz, r;
  for(i=0;i<mdl.num_s;i++){
    dx0 = mdl.surf[i].vertex[1].x - mdl.surf[i].vertex[0].x;
    dy0 = mdl.surf[i].vertex[1].y - mdl.surf[i].vertex[0].y;
    dz0 = mdl.surf[i].vertex[1].z - mdl.surf[i].vertex[0].z;
    dx1 = mdl.surf[i].vertex[2].x - mdl.surf[i].vertex[1].x;
    dy1 = mdl.surf[i].vertex[2].y - mdl.surf[i].vertex[1].y;
    dz1 = mdl.surf[i].vertex[2].z - mdl.surf[i].vertex[1].z;
    nx = dy0*dz1 - dz0*dy1;
    ny = dz0*dx1 - dx0*dz1;
    nz = dx0*dy1 - dy0*dx1;
    r=sqrt(nx*nx+ny*ny+nz*nz);
    nx=nx/r;
    ny=ny/r; 
    nz=nz/r;
    glBegin(GL_POLYGON);
    glNormal3d(nx,ny,nz);
    glMaterialfv(GL_FRONT,GL_DIFFUSE,mdl_color);
    glMaterialfv(GL_FRONT,GL_SPECULAR,mdl_specular);
    glMaterialf(GL_FRONT,GL_SHININESS,8.0);
    for(j=0;j<mdl.surf[i].num_v;j++){
    glVertex3d(mdl.surf[i].vertex[j].x,
               mdl.surf[i].vertex[j].y,
               mdl.surf[i].vertex[j].z);}
    glEnd();
  glFlush();
  }
}