Part II: Principles of Motion Control

This is an unfinished draft for a book I wrote back in 1999 on the topic of motion control. Over the next couple of days I will publish the next two parts, just as it was written and with lots of gaps in the text (I told you it was unfinished).

Table of Contents

  1. Programming
  2. Virtual axis
  3. Leveling the track
  4. Zero position
  5. Cartesian check
  6. Bloop Light / X-frame
  7. Relative motion
  8. Multipass photography
  9. Scaling moves
  10. Roto & Motion Tracking
  11. Stop-Motion, Go-Motion & Live Action
  12. Motion Control and CGI

Programming

Of course the camera doesn’t move by itself. Someone have to tell the computer how to move the camera in the shot. That is called “programming the move” and there are two basic approaches: encoding or jogging.

Jogging are the normal way to create a move. The operator control the rig with a jogbox, a remote control on which all axes do have two push buttons each. One button to move the axes forward and one to move it backwards. When pushing a button the rig will move in a constant speed. The operator move the rig between the positions the rig has to be in during the move and set a key frame for every position. Just like you would do with a virtual camera in any 3D software. The operator then let the computer calculate the in-between positions of the camera. Most systems let you to edit the move graph after the calculation, also like in a 3D package.

The second way is to use encoders. Encoders are sensors that let the computer record motion. Encoders looks just like electric motors, but instead of creating motion by turning the shaft they sense motion by how much the shaft is turned. You can put encoders on a regular camera and camerahead to records its motion that can later be repeated with a motion control rig. Or you can put the gearhead wheel handles on them and control the motion control rig camerahead just like any other head. This helps to create more natural and human camera motion, and the cinematographer get more hands-on aproach to the programming. The disadvantage of using encoders are that the move data often get very “jittery” or “shaky” and need to be smoothed out in the graph editor to be usable.

In the Nuts & Bolts part of this book, you will learn more about how to program the different types motion control systems.

Virtual axis

[To be written]

Leveling the track

When doing motion control, it is of importance that the track is normally leveled. When doing a normal dolly move there is no need for the precision because the interference of many human parameters: the dolly grip pushing the dolly, the camera operator moving the camera in a uneven and shaky motion. But when doing motion control and especially you have to repeat the exact move over and over again, the smallest bump will…

[To be written: The instrument used to level track. / Different methods of leveling, wedges and so on. / The procedure to check, level, check… ]

Zero position

A rigs zero position is often its resting position and the position for an axis that is considered to be a numerical value of 0.000. This position is marked so the operator can check it with his own eyes. Because the resting position always are the same physically it will also be the same in the move data. He can then always put the rig in its resting position and shut down the system. And next time the system are turned on, and zeroes checked, he will know that the computer can repeat exactly the same move. Putting the camera in the same physical place. But if the resting position were moved the whole move will be offset.

The zeroes are often marked by two marks, one fine and one curse mark. The fine mark are often put on the steper motors inching knob, and the curse are put on the axis itself (for example the track and the dolly). The curse mark tell you if the axis are roughly in the right spot, and the fine tells you exactly in which position the motor should be when the curse mark is close. Checking zeroes do have the same importance for the motion control operator that it’s for a flight pilot to check his airplane before take-off.

Cartesian check

[To be written]

Bloop Light / X-frame

Bloop light and X-frames are used to mark the beginning of the move. This is very useful when different passes need to be lined up in post.

The blooplight itself consist of a black box with bright LEDs that the camera assistant will hold up in front of the camera when rolling. The blooplight are usually set to fire off a couple of frames before the actual camera move begins. The camera assistant will make a note for the post-production how many frames that is. When using time-code slates, e.g. for music videos, it is common practice to do a so called “smart slate” where the bloop light are taped down on to the slate. The blooplight are often fired of during preroll.

When shooting models, dry for wet and other low frame rate moves without need for sync-slates, it is often more practical to use a X-frame instead of blooplight. After the shot is slated with some sort of effect slate (slated without the use of timecode or clapstick that just contain info), a slate containing a taped X are hold in front of the lens and the camera are rolled one frame. And all the frame after the X frame belongs to the move.

Relative motion

If the camera can move, so can the object if it shot against bluescreen or matted out in any other way. By moving the camera an impression of that the object moves are created, often used because it is easier to move the camera than the object. If you move the camera towards the object, it will look like the object are moving towards the camera. If you pan right the object will appear of moving to the left. If you tilt down, the object will move upwards in the frame. These simple rules can be combined in infinity to create very complex moves.

Usually it is the camera that provides the linear motion, East/West (left and right), North/South (up and down), track (back and forth) and additionally pan and tilt. But if the object has to rotate, it is then usually mounted on a “model mover”. Which then provide the objects bank, pitch and heading movement.

[More to be written]

Multipass photography

The first problem you realize, when you try to shoot small miniature models to look realistic, is the depth of field. When an object are shown on film, the human mind use focus to detect the scale.

Say, for example, that you take a picture of your car. Most likely the whole car will be in focus. But then try to take a picture of a small miniature model of your car and get the same result. Most likely you would not succeed. Part of the miniature car would be in focus while others would be blurry and the model would not look more than just a miniature. One way out of this dilemma is to add considerably more light to the model scene and stop down the lens to get more depth of field. But that is mostly not practical, the model could in the really bad cases melt of the heat the added lights is emitting.

Another way is to increase the exposure time for every frame. But if there have to be a motion in the scene, long exposure times that means low frame rates make it hard to create a impression of smooth motion. This can be solved with motion control. You can shot models during long exposures but still have them appearing being in a smooth motion. Or you can shoot two ore more focus passes, with different parts of the model in focus and composite the passes later in post process so the whole model will appear to be in focus.

Models of different scales. By the time the Star Wars films was made, a single X-wing fighter could be placed in position and photographed. Then, with the camera returned to its original starting point, the X-wing would be placed in a new position and the same set of exposures would be made. When this was repeated, that single fighter had created an entire formation of spacecraft banking through space. But with this ability you can not just use one model to create armadas you can also use models in different scales. For example, the Star Destroyer model that are hunting the small spacecraft in the beginning of Star Wars, are of the same size that the small spacecraft model. But by shooting the two models in different passes and with different distance from camera, they appear to be smaller or larger than each other. More about models of different sizes in the Scaling moves part.

[To be written: Multiplicity where Keaton acted against him self in multiple passes. / Animals that have to be recorded separate from each other. / Different light passes.]

All this can also be applied on full size real objects like cars. I have concentrate on miniature models because it is a concept many people are familiar with. Remember it is only your imagination that put boundaries on what the technique can be used for.

Scaling moves

[To be written]

Roto & Motion Tracking

[To be written]

Stop-Motion, Go-Motion & Live Action

[To be written]

Motion Control and CGI

[To be written]

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