Abstract: This paper presents a new method to control graceful scene degradation in complex ray-based rendering environments. It proposes to constrain the image sampling density with object features, which are known to support the comprehension of the three-dimensional shape. The presented method uses Non-Photorealistic Rendering (NPR) techniques to extract features such as silhouettes, suggestive contours, suggestive highlights, ridges and valleys. To map different feature types to sampling densities, we also present an evaluation of the features impact on the resulting image quality. To reconstruct the image from sparse sampling data, we use linear interpolation on an adaptively aligned fractal pattern. With this technique, we are able to present an algorithm that guarantees a desired minimal frame rate without much loss of image quality. Our scheduling algorithm maximizes the use of each given time slice by rendering features in order of their corresponding importance values until a time constraint is reached. We demonstrate how our method can be used to boost and guarantee the rendering time in complex ray-based environments consisting of geometric as well as volumetric data.