This paper presents a non-photorealistic rendering technique for stylizing a photograph in the line drawing style. We first construct a smooth and direction-enhancing edge flow field from the eigenvectors of the smoothed structure tensor, and then we use the flow field to guide the line drawing process. In particular, we develop a new operator for detecting step edges, which outperforms the existing edge detectors in terms of feature preservation and edge localization. Our approach works by applying the proposed detector in the direction perpendicular to the edge flow tangent and then smoothing the intermediate results along the edge flow curve. Optionally, an anisotropic nonlinear filter with an elliptical kernel is incorporated into the algorithms to extract the line edges, which may extend our technique further for creating an image to convey a hand-painting style. The presented algorithms are all highly parallel, allowing a real-time performance with GPU implementation. Experimental results show that our approach can produce more attractive and impressive line illustrations with a variety of photographs.