Hololens is not perfect! Interesting link here

"vizionvr" wrote:
P.S. Congrats on re-enlisting your happy avatar!

"vizionvr" wrote:
I would think black text/image could be achieved by incorporating a blocking-out effect, similar to how blocking allows your hand to appear to pass in front of the image. The only problem then might be a lack of ambient light in the room, which might be solved by haloing the black to compensate.

If they had a monochrome lcd layer behind the waveguide, they could black out areas of the real world from contributing to the view (like finer granularity HHGTTG peril sensitive sunglasses). But it's just clear glass, the waveguide is purely additive with the world.

Projectors work the same way. They can't shine black areas onto a white screen, instead they brighten everything else up so the unlit areas look dark by comparison. That would work with hololens (if at least one of those 5 cameras is colour, not all IR/depth), brighten the view in some regions to make others look dark, but because of it's tiny fov you'd notice a bright rectangle.
For example:

The words there are formed by brightening the surrounding area, not by darkening the letters. But the greater view fov than hologram fov would cause that rectangle.