Implemented IsoRegion leaping, as suggested by Fung and Heng (see posting of Feb 2, 2005). Speed increases significantly but is nowhere near interactive rates, for the size of volume and image that I target. A 700 x 650 image needs 6.3 secs to draw (compared to 14.4 without IsoRegion leaping). I am sure I could tweak some more speed out of this (e.g. by trying different brick sizes, optimizing some more) but the gain would probably be no more than 1 sec at the maximum. So the rest will have to come from reduced-quality rendering during interactive movement of the volume.
Meanwhile, during most of the past 2 weeks, I have been trying to implement a SVD (singular value decomposition) algorithm. I looked up the algorithm from Numerical Recipes in C, but the pdf files (c2-6.pdf) have a buggy old version. I finally figured that out (after many frustrated hours). The most recent version I could find on the net was the file svdcmp.cpp, which needed just two corrections, as mentioned in the Numerical Recipes bug reports. It is also written for zero-based arrays, in contrast to the one in the pdf file. I converted this to Delphi and it works perfectly (after correcting another two bugs that I introduced myself). Now I can use this to find least squares solutions. My aim is to calculate principal components of shape when some of the points are missing. This will allow me to find the most probable positions of the missing points, based on the other points and the shape variability of the sample.
And now for a different kind of recipe: chocolate prunes. I use prunes (dried) without the pits, place them for a few hours in brandy and red wine (half and half). Then I melt chocolate, pour some in small paper cups, immerse half of a prune in each cup and cover with some more chocolate. Very easy. Then leave in the fridge until set. 200 gr of chocolate make up approximately 30 small cups. Whatever brandy and wine is left over, you drink (but don't drive).