This paper presents "Matisse", an interactive modeling system aimed at providing the public with a very easy way to design free-form 3D shapes. The user progressively creates a model by painting 2D regions of arbitrary topology while freely changing the view-point and zoom factor. Each region is converted into a 3D shape, using a variant of implicit modeling that fits convolution surfaces to regions with no need of any optimization step. We use intuitive, automatic ways of inferring the thickness and position in depth of each implicit primitive, enabling the user to concentrate only on shape design. When he or she paints partly on top of an existing primitive, the shapes are blended in a local region around the intersection, avoiding some of the well known unwanted blending artifacts of implicit surfaces. The locality of the blend depends on the size of smallest feature, enabling the user to enhance large, smooth primitives with smaller details without blurring the latter away. As the results show, our system enables any unprepared user to create 3D geometry in a very intuitive way.