size and shape of the gearing mesh mark are the main parameters of testing the assembly quality of hypoid gear.Based on the analysis of the gray scale of the gearing mesh mark
the image is enhanced by the way of piecewise linear gray transformation.The edge of the gearing mesh mark and tooth profile is highlighted through enhancing its contrast.Then the image edge is extract by the Canny operator.The final results show that the method is available in the practical engineering.