An ideal bump map is one that is MADE as a bump map. In other words, one that have little white dots where a pimple is and a little darker one where a pockmark goes. One of the things that makes a negative image wrong is places like nipples, they will typically be reversed and also pores in skin that can easily be little spikes on a bumpmap. SOMETIMES a negative image works ok, and it's usually better than NO bumpmap, but not really good enough. A .bum is not really a negative image, it's more like an embossed version of it. And with a sick green look to it too, if you open the .bum in something like PSP (as you probably know, .bum files are essentially .bmp's, which also is why they're so immensely HUGE compared to jpg bump maps.) The renderer handles bump mapping quite good, and if you need "bigger bumps" there's always displacement mapping, one of the REALLY REALLY NIFTY features in Poser 5.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.