The vecorization of dot matrix image is a classical question of computer graphics. The least-square algorithm

which distinguishes the result of vectorization right or not by defining whether the fitting errors is in the range of threshold or not

is a main method of vectorization of dot matrix image. So how to define the threshold is a key. These errors are analyzed while dot matrix image is recognized as vector diagram by the least-square algorithm

the threshold is defined scientifically

and the judgement formulas to evaluate recognition resul are put forward to in this thesis. In order to complete vectorization automatically

a new method of the vectorization of dot matrix image--the double direction roll least-square algorithm is presented

and some recurence formulas are deduced. The judgement formulas are scientific and accurate

the mothod of the vectorization is rapid and efficient. The formulas and the method are all quite well and universal.