We have immersive HMDs and those are the ones that do not show anything directly from the scene. So the immersive MHDs are showing strictly computer sent images, whether they are real or synthetic is a different issue, but people are only seeing the image on the displays not the images in the real world, so you are not getting direct images from the real world.
The second category is a video see-through images, where the computer is merging electronically two things: one is real world images and the other is computer-generated images. So, video see-through HMDs allows the combination of the two.
Then optical see-through image MHDs have an interesting property, that they allow that images coming straight from the real world and this is combined with other information. So these allow direct light from the scene and combine that image with another source of images and that combination is done through various kinds of combiners, optical combiners.
And finally there are the fourth kind of HMDs, where there is no hardware sticking in front of your eye in the sense that you are not seeing a display or a foreign thing, indeed there is a laser which paints the image directly on your retina through you pupil. So here the display devices are eliminated and the virtual images are actually scanned while a laser on the back your eye on the retina.
