Abstract: Defining how 3D models respond to user actions is a crucial step in building an interactive 3D world. Unfortunately, existing tools make it difficult for interaction designers to assign responses to any part of a 3D model that is not a pre-defined group of polygons. This restriction is particularly problematic for image-based models and models where the texture map contains most of the detail. We overcome this restriction by allowing designers to specify a model's interaction surfaces (parts that can respond to events) by painting them onto the model. Designers capture the painted areas by saving them as a 2D interaction map. An interaction map is similar to a traditional texture, but its painted regions specify interaction surfaces instead of appearance. We allow designers to name interaction surfaces and assign them responses to events both statically and at run-time. In addition, designers can modify the size and shape of interaction surfaces at run-time and can pass parameters to surfaces' responses by encoding them in the model's interaction map.