Extensions

Various extensions of the basic MLI format can be made to provide additional functionality, at the expense of slightly increased transmission and rendering costs:

• The Z value could be stored at each pixel (in addition to the intensity value) to provide 3D coordinates of pick points.
• If interaction with the lighting and/or more realistic specular coloring is desired, the intensity value for each pixel could instead be made to encode the surface normal. Material properties and light source positions could be used to compute for each structure a color lookup table with entries for each possible surface normal. By moving lighting computations to the client, we increase the rendering time but add some interactive functionality and realism.
• To eliminate the problem of invisible backfaces, a separate rendering of each structure with front-face culling could be recorded, and incorporated in the same way as any other structure. In semi-transparent renderings, the combination of these front and back shells would represent the whole structure more accurately.

Finally, it might be worth investigating the use of MLIs in combination with existing image-based techniques to permit more types of interaction than either method supports alone. If an MLI was substituted for each frame required by an existing image-based algorithm, then structure selection, transparency, and coloring could be accomplished in a pre-processing step by our rendering algorithm; image warping and interpolation could then be handled as before. This type of system could also be seen as an extension of the work in [9].