An image processing system. An input/output device receives information for pixels in an image corresponding to an object, wherein the information specifies optical properties. A storage device stores the information. A processor determines an image of preliminary contour of the object based on the information. For pixels located on the preliminary contour are assigned as primary pixels, wherein anchor points determined by the location of the primary pixels, and the reference pixels determine modification vectors according to the information corresponding to the primary and reference pixels, and adjusts the positions of the anchor points according to the modification vectors. These processes are applied on every pixels or selected pixels located on the preliminary contour repeatedly to determine the final modified contour with sub-pixel accuracy.