1. A method for free-form sketching a renderable 3D model, comprising: acquiring an input stroke as a sequence of 2D points from an input device; resampling the 2D points; assigning depth values to the resampled 2D points to form a 3D contour; and fitting a variational implicit surface to the 3D contour to generate a 3D blob forming the 3D model to be rendered on an output device.