Staring with a real physical object, in this case a sandstone brick wall, I create an inexpensive but high quality digital 3d scan using a stills camera and the program Agisoft Photoscan. I then model the lego brick shapes over this scan using 3d software, I make this model fit perfectly onto the scanned wall by using boolean subtraction, I add colours clone stamped from the photos, correct the scale, and finally I do a colour 3d print, which fits seamlessly onto the sandstone wall, producing a novel illusion that would be difficult to achieve by other methods.
Step 1: 3d Scanning
I took around 29 photos from different angles, and the software automatically derived a 3d mesh from those photos. I chose a sandstone block with a chipped off corner, so that my 3d print could fit into empty space left by that missing corner. The process of using agisoft photoscan is very user friendly, but poor quality photos will tend to result in a less detailed mesh with more artifacts. Try to avoid taking photos that are poorly exposed, or out of focus. The object should remain completely still while you take the photos, if it changes shape or moves it's position then the reconstruction probably won't work. The photos should not be taken from dramatically different angles, there must be some level of overlap and similarity among the photos, or a mesh cannot be derived.
I generated the mesh with Target quality set to Medium, and Geometry type set to Smooth. I generated a texture at 2048*2048 using Orthphoto Mapping mode, and Mosaic Blending mode. Then exported the mesh as an .obj file, and the texture as a .bmp.
These programs tend to work best with very textured objects. The texture gives the software features to lock on to. To get the Best results with smooth untextured objects you may have to apply a random blotchy paint pattern. Blotchy facepaint works really well when scanning people, though it may look rather silly! Shiny objects can also cause problems, because they look different from every angle. Shine may be reduced in a number of ways, talcum powder, dulling spray, or it may be filtered out using polarizing filters, one on the lens, and one on the light source. The filter should be rotated until the shine disappears. Large polarizing filters can be bought online, and it's possible to get very cheap small polarizing filters by cannibalizing the lens from a pair of 3d glasses. Some light sources such as the sky, or LDC screens, are already polarized, so they won't need a filter.
In many cases it is possible to approximate tiny micro details from the scanned object. By applying a "high pass" filter (in PhotoShop, or an equivalent image editing program) to the colour texture map one can create an image that may be used as a bump or displacement map. The results may not be perfectly accurate, but in many cases this will result in a visually convincing level of very fine detail.