1. A method for single-view hair modeling and portrait editing, comprising the steps: (1) image preprocessing: segmenting, by a processor, 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, by the processor, opacity values in a segmented human hair region by an alpha-matting method, setting, by the processor, apart the foreground color of the segmented human hair region from its background color, and completing, by the processor, occluded portions in the human body region and the background region; (2) 3D head model reconstruction: automatically locating, by the processor, facial feature points by an active shape model (ASM) method, estimating, by the processor, head transformation parameters and fitting a 3D model by least squares optimization based on the feature points and a 3D head model database after principle component analysis (PCA) dimension reduction, and carrying, by the processor, 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, by the processor, 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, by the processor, a set of sparse 2D strand curves by tracing based on the 2D orientation field and the confidence map; (4) 3D hairstyle reconstruction: carrying, by the processor, 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, by the processor, depth information of all 2D strand generated from step (3), volumizing, by the processor and 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, by the processor, all strand vertices from an original image to obtain hair color values; (5) synthesis and application: carrying, by the processor, out portrait editing and manipulations including portrait pop-ups, hairstyle replacements and hairstyle editing by adopting reconstructed 3D information including the human hairstyle model and the human head model according to the aforementioned steps (1)-(4).