A 3D photo-montage is a photograph combined with 3D CGI elements that blend seamlessly together. Photo-montages are usually produced to visualize buildings or structures which are under construction or are still at the planning stage. To create an image like this you need a 3D package, for example 3DS Max or Cinema 4D, and also a photo editing package like ‘Photoshop’. A photograph of the scene, plans of the proposed structure, and a survey of the area for ground levels.
Before beginning you need to examine the photograph and consider which elements need to be modeled and which don’t, anything which isn’t directly visible in the scene can be left out (or modeled with minimum detail), but when considering this you need to also think about reflections in glass, etc. and whether the parts not visible to the camera will actually be mirrored on reflective surfaces and whether it is worth modeling detail into them or not.
When you begin the modeling stage it is important to work to a scale of 1:1 so the lighting reacts to the scene as realistically as possible. Import all views of the structure into the 3D package and set these to scale. The method I use is to create a parametric box and give it a length corresponding to a large distance in the scene for example the length of the whole building and then scale the corresponding elevation view to match.
During the modeling stage remember to crudely model surrounding elements like lampposts or house eves at the correct scale and level – these will make the camera matching stage much less frustrating.
When the scene is modeled, import the photograph into the 3D visualization package as a background and create a camera. You need to position the camera as closely as possible to match the scene. A good method is to draw a spline around the site at the correct level in the 3D package and hide all elements apart from the surrounding elements and the spline. If you know what lens was used to take the photograph put the same settings into the virtual camera settings(If not 35mm is a good starting point). Then adjust the camera until everything lines up as much as possible, you may need to also adjust the ‘focal length’ settings but only touch this if the camera is not lining up correctly. When you’re happy, un-hide all the geometry needed for the montage and hide everything else. Then light the scene as closely as possible to the photo – study the height and direction of the sun and match the virtual lights accordingly.
Once all these stages are complete you need to render the scene with an ‘alpha channel’ but don’t include the photograph as a background. Open the render and photograph in Photoshop and layer them up… blend the exposure and cut out any elements in the foreground eg trees, etc so the CGI element glues as much as possible to the photograph.