get("#348") -- rgb functions

brightnessThreshold = 0.9

function getcrop2(img)
  local w, h, g = img.width, img.height, img.getInt
  
  local x1, y1, x2, y2 = 0, 0, w, h
  
  -- left side
  
  while x1 < x2 do
    for y=y1, y2-1 do
      if bright(rgb(g(x1, y))) <= brightnessThreshold then goto out0 end
    end
    x1=x1+1
  end
  ::out0::
    
  -- top side
  
  while y1 < y2 do
    for x=x1, x2-1 do
      if bright(rgb(g(x, y1))) <= brightnessThreshold then goto out1 end
    end
    y1=y1+1
  end
  ::out1::
    
  -- right side
  
  while x2 > x1 do
    for y=y1, y2-1 do
      if bright(rgb(g(x2-1, y))) <= brightnessThreshold then goto out2 end
    end
    x2=x2-1
  end
  ::out2::
    
  -- bottom side
  
  while y2 > y1 do
    for x=x1, x2-1 do
      if bright(rgb(g(x, y2-1))) <= brightnessThreshold then goto out3 end
    end
    y2=y2-1
  end
  ::out3::
  
  if x2 <= x1 or y2 <= y1 then -- weird result? return full image
    x1, y1, x2, y2 = 0, 0, w, h
  end
  
  return x1, y1, x2, y2
end

Began life as a copy of #346

Assumes light background