width=300
height=150
pixels={}

local positions={{1,1,30,10}, {100,10,50,20}, {50,100,100,15}}
local color = -1

for _, p in ipairs(positions) do
  local x1, y1, w, h=unpack(p)
  for x=0, w-1 do
    pixels[(x1+x)+y1*width] = color
    pixels[(x1+x)+(y1+h-1)*width] = color
  end
  for y=1, h-2 do
    pixels[x1+(y1+y)*width] = color
    pixels[x1+w-1+(y1+y)*width] = color
  end
end