std::pair<etc1_bits, error> = ETC1Encode(pixels, options).
Which actually gives me a really fast way of accurately computing this:
error = ETC1Distance(pixelsA, pixelsB, options).
I'm seriously considering a SIMD implementation next. I wrote one for DXT1 just for fun last week.
I need this distance function to be fast in order to justify another series of bottom->up clusterization experiments, and on improving the clusterization process itself.