The insight into a physical process is always improved if a pattern produced by or related to this process can be observed by visual inspection. Flow visualization techniques have always played an important role in the understanding of fluid-mechanical problems. Besides such instructive applications, the greater importance of many flow visualization techniques is that one can derive quantitative data from the obtained flow picture. The recent advent of fast, large-core digital computers, analogue-to-digital converters and sophisticated video systems has enabled us to carry out rational automatic analyses of flow visualization pictures by menas of digital image processing. In this paper, the tendencies in flow visualization technique and computer aided flow visualization are reviewed, and a specifical technique based on the digital image processing approach is introduced for analyzing the two-dimensional flow field from pathline photographs.