The little grey howling guy in this image is a tomograph — a 2d painted cutout — mounted a couple of centimetres above the background. You can see how he floats in the little detail to the right. The background was painted in plain old acrylic house paint with some impasto gel added to give it dimension; I found the gel tricky to work with because it shrinks on drying, and its milky whiteness changes the colour quite substantially until it cures so you pretty much have to guess what the final effect is going to be.