demo_viewport.f90

     program demo_viewport
     use :: M_pixel
     use :: M_pixel__writegif, only : writegif
     implicit none
        call prefsize(400, 400) ! set up drawing surface
        call vinit()
        call color(7)
        call linewidth(40)
        call clear()
        call ortho2(-88.0, 88.0, -88.0, 88.0)
        ! draw the same circle, just changing viewport

        call viewport(   0.0, 200.0,   0.0, 200.0 ); call draw_circle(1)
        call viewport( 200.0, 400.0,   0.0, 200.0 ); call draw_circle(2)
        call viewport(   0.0, 200.0, 200.0, 400.0 ); call draw_circle(3)
        call viewport( 200.0, 400.0, 200.0, 400.0 ); call draw_circle(4)
        call viewport( 250.0, 350.0, 150.0, 300.0 ); call draw_circle(5)

        call writegif('viewport.3M_pixel.gif',P_pixel,P_colormap)
        !call execute_command_line('display viewport.3M_pixel.gif')
        call vexit()
     contains
     subroutine draw_circle(icolor)
     integer,intent(in) :: icolor
        call color(0)
        call rect(-88.0,-88.0,88.0,88.0)
        call color(icolor)
        call makepoly()
        call circle(0.0,0.0,88.0)
        call closepoly()
     end subroutine draw_circle
     end program demo_viewport