How do i overwrite an variable inside an if loop (1 Viewer)

[bluedxca93]

Am thinking of a script to autocrop small pngs using lua 5.1 and imlib2 on ubuntu.
while doing this i did get the idea of writing the following code:

  for y=0, xhalf do
     c1 = im:get_pixel(xmid, y)
          c1a = (c1.alpha)
          if c1a < 20 then temp_top=y  io.stdout:write( "top_margin=",y+1, "\n")  end
  end

What i want there is that temp_top is a variable in lua and that it only is defined for the biggest temp_top possible inside that for loop and exported to a variable called top_margin that i can use later.
inside scripts,zip i did the following hackish workaround:
lua script.lua > offset-analyz; cat offset-analyz | sort -r | grep -m 1 top;.
This does export top_margin in a way i can use it in bash. That works but there schould be a way to do it entirely in lua.

 

[bluedxca93]

To explain it in pictures i want that:

becomes

 

[GavinW]

What about this?

function (xmid, im)
 local top_margin = 1
 for y = 0, xhalf do
     local c1 = im.get_pixel (xmid, y)
     if c1.alpha < 20 then top_margin = y + 1 end -- if
 end -- for 
 return top_margin
end -- function