HD is High Definition. Instead of pushing the base mesh, the SubD cage, around as regular morph do HD morphs move the SubD mesh itself - it's a bit like displacement in that it adds finer detail, but with several advantages: doesn't depend on the UV set used, can be combined with a separate displacement map or other HD mrophs and its contribution adjusted; shows in preview; isn't limited to moving the surface in and out along the normal.
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HD is High Definition. Instead of pushing the base mesh, the SubD cage, around as regular morph do HD morphs move the SubD mesh itself - it's a bit like displacement in that it adds finer detail, but with several advantages: doesn't depend on the UV set used, can be combined with a separate displacement map or other HD mrophs and its contribution adjusted; shows in preview; isn't limited to moving the surface in and out along the normal.