Results

One of the key results of our approach is that non-artists can easily produce images such as those presented in this paper. We tested our interface on three novice users. We first demonstrated the software on the skin images and doll model, shown in Figure 4. We showed how one can select two triangles, duplicate them, and produce a hemisphere which darkens at the silhouettes and has soft highlights in the center. The users were also directed how to use the sliders on the spherical triangles to minimize the discontinuities between triangles. The tutorial for placing the skin on the doll model took 5 minutes from start to finish (see Figure 4). We observed that users are adept at determining the approximate light position in the 2D image and had no problem carrying out the creation of the lit sphere. In ten minutes, one user produced the image in Figure 8 by simply picking an approximately spherical region in the 2D image and forming a circle of triangles which mapped directly onto the hemisphere.

The ability to capture 3D shading models from 2D sources gives great flexibility to the user. With our system it is straightforward to capture shading from art work and reproject it onto geometry. For example, consider the Cezanne painting in Figure 9. We have created a coarse model of the scene, and captured a lit sphere environment map for each scene element from the original art work. Using this mapping, we can rotate the scene and achieve a plausible artistic rendering from the new viewpoint (Figure 10). Also, observe that from the original viewpoint the indirect lighting on the metal cup due to the adjacent lemon is clearly visible. The flexibility of the input used for creating shaders allows users to create their own palette. For example, consider Figure 11. We start with a palette of black and white. Following the work on non-photorealistic metal in technical illustration by Gooch et al. [5], it is straightforward to achieve the impression of brushed metal using a lit sphere map. An important characteristic of such metals is asymmetry of reflectance under rotation, termed anisotropy. In this example, we added two spherical triangles containing noise and an asymmetrical highlight to obtain the results in Figure 12.