Dave,
What do you mean the "slope" of a value? I'm thinking along the lines of a derivative, but I think I'm just not getting something.
My second question was more of a blending type question. Does the scene have to be sorted back to front for this method to work? Assume that there are two triangle located in approximately the same space in the scene graph. In any one 4 pixel rendering one triangle has 50% coverage, the other has 75%, how are they combined?
Lastly, if you are rendering at 2x the resolution for both color and z, how can you guarantee that you will get the same color value for all of the pixels related to any given triangle?
I think I'm just missing something conceptually, so I'll read the article again. If you could answer these questions it would be a big help though.