-- needs rgb functions and rectangle functions

function shrinkRectangle(r, img)

  local w, h = img.width, img.height

  local function g(x, y)

    return bright(rgb(img.getInt(x, y)))

  end

  local x1, y1, x2, y2 = r.x, r.y, r.x+r.width, r.y+r.height

  -- left side

  while x1 < x2 do

    for y=y1, y2-1 do

      if g(x1, y) <= 0.5 then goto end1 end

    end

    x1=x1+1

  end

  ::end1::

  -- top side

  while y1 < y2 do

    for x=x1, x2-1 do

      if g(x, y1) <= 0.5 then goto end2 end

    end

    y1=y1+1

  end

  ::end2::

  -- right side

  while x2 > x1 do

    for y=y1, y2-1 do

      if g(x2-1, y) <= 0.5 then goto end3 end

    end

    x2=x2-1

  end

  ::end3::

  -- bottom side

  while y2 > y1 do

    for x=x1, x2-1 do

      if g(x, y2-1) <= 0.5 then goto end4 end

    end

    y2=y2-1

  end

  ::end4::

  if x2 <= x1 or y2 <= y1 then

    return r

  else

    return newRectangle(x1, y1, x2-x1, y2-y1)

  end

end