The invention relates to a system (100) for extracting an object (Ob) from a source image, said object being delineated by a contour (C), the system (100) comprising a gradient unit (110) for computing the source image gradient field, based on the source image, a smoothing unit (120) for smoothing the source image gradient field, and an integration unit (130) for calculating an object image by integrating the smoothed source image gradient field, thereby extracting the object (Ob) from the source image. At each point of the source image, the smoothing is defined by a 2-dimensional convolution kernel which is a product of a first 1-dimensional convolution kernel in the first direction substantially parallel to the contour (C), and a second 1-dimensional convolution kernel in the second direction substantially normal to the contour (C). The first 1-dimensional convolution kernel defines smoothing within each region separated by the contour, while the second 1-dimensional convolution kernel defines smoothing across the contour separating two regions, independently of the orientation of the object and the contour curvature.