Moving image backplates

If anything at all moves in the backplate you're intending to shoot, then the backplate should be shot as a moving image, not a digital still.  So whether it's a lion racing across the African savannah or a tree gently blowing in the breeze, you're going to need a television camera of some description.  (You can add some movement to digital stills, but it's a costly and sometimes disappointing post production process).

Camera moves

Backplates for CGI-based sequences tell the same story as shooting the footage for real.  Just because dinosaurs no longer exist does not mean you film your backplate sequence as a stilted set of still photos.  The story you're telling should be as exciting and pacey as if the dinosaurs you will eventually composite into the backplates are actually there on location.  It's the same for any sequence that relies on CGI - it's a device to tell a story you can't actually film, but it's still a story.

So, it's perfectly acceptable to move the camera on a crane, track & dolly or even hand held just as if shooting you were shooting real live action. Remember that CGI is expensive to produce and composite, and therefore storyboarding your sequence first is nearly always essential, and to do that should involve your CGI team from the very outset.  They'll be able to tell you what camera moves they can cope with on your budget and schedule; and they'll be able to advise you how to get the most "action" out of your backplates.

Here are a few things CGI designers need you to bear in mind when moving the camera during a shot:

• Make sure that any camera move is reasonably slow to minimize motion blur – the shot can always be sped up in post and motion blur can be added if needed.

• Make sure there are adequate tracking points in the scene. These can be naturally occurring features or your own tracking markers.  See Using tracking markers

• If you are using a zoom lens, keep it at a fixed focal length during the shot.

• Never zoom as the tracking software will not be able to emulate the zoom because of the lens distortion. The exception of course is where the camera zooms before the cgi object enters frame.

• If you need to move the camera  then use a crane or dolly or even go hand held to achieve the moving shot

• Be careful of large foreground objects as perspective may be an issue.

Tracking Software

Tracking software is used in complex CGI and backplate sequences.  It works by picking out reference points within the scene which could either be tracking markers you have deliberately placed in the scene or other naturally occurring features. The software uses these reference points to calculate the change in perspective and angle as the camera moves and uses this data to replicate the move in the computer. This move is then applied to the CGI elements so that the whole scene appears to have been shot at the same time with the same camera move.