Be that as it may, I've read two different versions of how that image is achieved. One report was that it was a Pepper's Ghost, another said it was a projection on a scrim.
I disagree with both. From the videos I've seen, it looks more like a large HDTV screen, just like the talking portraits. Set dressing hides the edges of the TV screen. Dumbledore appears solid, and had it been Pepper's Ghost or a projection on a scrim, he would appear ghostly and see-through.