Useful sprite draw function for PICO-8 (and maybe Lua in general)

outline_sprite.p8.lua

--
-- draws a sprite to the screen with an outline of the specified colour
-- transc is color to use as transparent; default is black (0)
function outline_sprite(n, col_outline,transc x,y, w,h, flip_x,flip_y)
  transc=transc or 0
  
  -- reset palette to black
  palt(transc,true)
  for c=1,15 do
    pal(c,col_outline)
  end
  -- draw outline
  for xx=-1,1 do
    for yy=-1,1 do
      spr(n,x+xx,y+yy,w,h,flip_x,flip_y)
    end
  end
  -- reset palette
  pal()
  -- draw final sprite
  palt(transc,true)
  spr(n,x,y,w,h,flip_x,flip_y)	
  pal()
end