The additive pixels are taking a separate optical path, combined in at the last second by either a beamsplitter or a waveguide.
In current commercial HMDs, the additive pixels are statically lensed to a fixed focus distance (not infinity, as a sibling comment asks, but rather a best guess of where the user will be focusing most often, e.g. 1.25 meters).
Using dynamic lenses to put the additive pixels at any focus distance is an exciting topic of current research:
Dynamic lenses could also be put on the forward-facing / physical-reality optical path both before and after the subtractive pixel layer, to put the subtractive pixels at any distance as well, solving the problem. (This would incidentally also allow lensing physical reality itself to any distance too, creating perfect / hyper-real vision correction.)
In current commercial HMDs, the additive pixels are statically lensed to a fixed focus distance (not infinity, as a sibling comment asks, but rather a best guess of where the user will be focusing most often, e.g. 1.25 meters).
Using dynamic lenses to put the additive pixels at any focus distance is an exciting topic of current research:
http://www.quora.com/Volumetric-3D
Dynamic lenses could also be put on the forward-facing / physical-reality optical path both before and after the subtractive pixel layer, to put the subtractive pixels at any distance as well, solving the problem. (This would incidentally also allow lensing physical reality itself to any distance too, creating perfect / hyper-real vision correction.)