1. A method for single-view hair modeling and portrait editing, comprising the steps: (1) image preprocessing: segmenting an image into four layers based upon sparse user strokes by adopting a Lazy-Snapping image segmentation method: human hair, human face, human body and background, calculating opacity values in a segmented human hair region by an alpha-matting method, setting apart the foreground color of the segmented human hair region from its background color, and completing occluded portions in the human body region and the background region; (2) 3D head model reconstruction: automatically locating facial feature points by an ASM method, estimating head transformation parameters and fitting a 3D model by least squares optimization based on the feature points and a 3D head model database after PCA dimension reduction, and carrying out inverse texture mapping in accordance with the segmented human face region obtained by step (1), so as to obtain a texture image of a head model; (3) 2D strands extraction: computing a 2D orientation field and its corresponding confidence map by using a Gabor kernel function within the hair region obtained in step (1), and obtaining a set of sparse 2D strand curves by tracing based on the 2D orientation field and the confidence map; (4) 3D hairstyle reconstruction: carrying out energy optimization in conjunction with the fitted head model according to step (2), depth coherence among nearby strands and occlusion correlations among strands, solving and obtaining depth information of all 2D strand generated from step (3), volumizing according to the 3D strands so as to obtain an adequately dense 3D strand set around the head as a recovered hairstyle model, and sampling all strand vertices from an original image to obtain hair color values; (5) synthesis and application: carrying out portrait editing and manipulations such as portrait pop-ups, hairstyle replacements and hairstyle editing by adopting reconstructed 3D information such as the human hairstyle model, the human head model and so on according to the aforementioned steps (1)-(4).