As I explained yesterday (seperate characters of an image
]) The whole idea of Captcha is that it is as difficult as possible for machines to read. There are no guaranteed blank lines vertically. Or horizontally. Remember, captcha work in different ways, some are rotated, others have the characters mashed together, others invert the colours in an oval area, and s forth an so forth, all or which methods will break your "algorithm".
You are on a loser, trying to break captchas - give up before you waste too much of your time.