A method of automatically computing tool properties of a virtual paint brush includes obtaining an edge map from a digital image, wherein boundaries of objects in the image are detected, computing contours for the edge map, receiving a brush stroke of the virtual paint brush from a user at a point on a contour, and finding an object boundary that corresponds to the contour that received the brush stroke, computing a tangential slope of the object boundary at the point of the brush stroke, adjusting tool properties of the virtual paint brush based on a change in the tangential slope of the object boundary, and visually displaying the adjusted tool properties dynamically while the user is moving brush strokes around the object boundaries.