Motion Control Cameras
3D Integration FAQ |
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CG vs Real WorldIn Computer Graphics anything is possible; unfortunately the same cannot be said about the real world. This presents some considerations that need to be planned for when developing camera moves that are to be used on motion control rigs or visa versa. Firstly, the world co-ordinate system must be accounted for such that forward is forward, up is up, etc. 3D CG applications often have different ways to describe the "world space", some have the Z axis up, some the Y, some have the X axis to the left, some to the right. It is necessary to match (or convert) between the 3D application and the motion control system. Flair software, used to operate the motion control rigs, has the following "world space" co-ordinate system:
All this is NEARLY the same a what can be called a Right Handed X forward Z up co-ordinate system, EXCEPT for the Tilt and Roll which rotate in the opposite directions. Flair "world space" is relative to the camera track with the origin being directly in line with the centre of rotation of the swing turret of the rigs when the rig is resting at "zero". The Z height (the camera height above the ground) is variable and needs to be measured on-set to accommodate for the camera and mounting head. A value can be entered into Flair to adjust for this. The scale needs also to match between 3D and the motion control system. CG applications can be set to many different units such as feet and inches, or centimetres, yards etc. We work in millimetres but can convert to and from anything.
LensesCG applications represent perfect lenses, however real lenses are not so perfect. The main differences know as "aberrations" are:
Rather than going through these all here, we'll just say that image distortion occurs with real lenses, and you'll find plenty of information about these aberrations if you type them into an internet search engine. One of the main things of importance in motion control is the Nodal Point of the lens. This can be described as the pivot point of the lens, the point at which no arcing of the image occurs under rotation, and it is this point that is measured accurately in relation to the motion control rig. This Nodal Point is comparable to the exact position of the 3D camera in the CG application. It is the Nodal Point and not the Image Plane that is the equivalent of the 3D camera in a CG application. The Field Of View (FOV) is a measurement either horizontally or vertically describing what the camera sees. Although these are marked on the real lenses, they are not accurate enough to rely on and we measure the lenses ourselves. Calibration between the 3D and real value needs to be done. Zoom Lenses can cause problems since there is more than one Nodal Point, and they move as you change zoom. We create lookup tables to assist when using zoom lenses.
Real Mechanical RigsMotion Control Camera rigs have physical limitations including:
No system can be 100% rigid and free from these issues, however the manufacturers have done an excellent job over the years in minimising them. There are an infinite number of ways that you could pose a Motion Control Rig to have the camera in a certain position, and we use tools such as PECS MoCo to find the best possible solution in conjunction with our years of experience. For example,in a simple scenario such as a camera moving in a straight line facing forward, it is possible to do that motion with either the Track, the Extend, or a combination of the two. Each shot requires examination and experience to optimise the mechanical properties of the rig and avoid the issues mentioned above.
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