\setupoutput[pdftex]
\setupcolors[state=start]

\starttext

\startMPinclusions
  % default unit
  u:= .5 ; % u:=1 works, u:=2 doesn't

  pen upen ;
  upen = makepen(unitsquare scaled u shifted -(.5u,.5u)) ;

  % width, height, shift-x, shift-y, color
  def fill_square(expr w,h,a,b,c)=
    fill unitsquare xyscaled(w*u,h*u)
    shifted (a*u,b*u)
    withcolor c
  enddef ;

  def fill_hline(expr w,a,b,c)=
    fill_square(w,u,a,b,c)
  enddef ;

  def fill_vline(expr h,a,b,c)=
    fill_square(u,h,a,b,c)
  enddef ;

  % the drawing macros always use a pen of 1pt ;
  % even with an extra defined pen
  def draw_hline(expr w,a,b,c)=
    draw (.5u,.5u)--(w*u-.5u,.5u) withpen upen
    shifted (a*u,b*u)
    withcolor c
  enddef ;

  def draw_vline(expr h,a,b,c)=
    draw (.5u,.5u)--(.5u,h*u-.5u) withpen pensquare scaled u
    shifted (a*u,b*u)
    withcolor c
  enddef ;

  def draw_frame(expr w,h,a,b,c)=
    draw (.5u,.5u)--(w*u-.5u,.5u)--(w*u-.5u,h*u-.5u)--(.5u,h*u-.5u)--cycle
    withpen pensquare scaled u
    shifted (a*u,b*u)
    withcolor c
  enddef ;

\stopMPinclusions

\startMPpage

  fill_hline(20,5,5,red) ;
  fill_vline(20,5,5,red) ;

  draw_vline(20,5,25,blue) ;
  draw_hline(20,25,5,blue) ;

  draw_frame(50,50,0,0,black) ;

\stopMPpage

\stoptext