From LightWiki

Many thanks to David Ridlen for allowing me to repost a thread he started on NewTek's forum

Here's what I consider my cutest LW trick that allows you to zoom into and follow tiny detail for tracking reference within an image sequence. Have used this to achieve extremely accurate 2D (nodal) tracking in some unusual shots where even Boujou or Syntheyes would not work. It can even work for 3D tracking but that requires an experienced eye. This may be kinda hard to wrap your brain around at first, but hopefully you will see how it works even if the setup doesnt quite make sense.

• In your Open GL prefs, select a resolution to accomodate your plate (film is usually 2048x).

• You'll need a screen on which to project your plate sequence. Create a vertical, rectangular 'card/screen' in Modeler centered at 0, that approximates the aspect of your camera. Parent the screen to your camera (oriented perpendicular to camera). Move the screen far enough out on the Z from camera (X & Y remain 0) until it is positioned behind any relevant foreground objects for the duration of the shot. The screen should be perfectly centered and scaled to fill frame.

• Front projection map your plate sequence to the screen at lum 100%, dif 0%. The first camera in your scene will automatically be referenced for the projection. A planar mapped background card does not work well as parallax will throw off the perspective.

• To speed up frame scrubbing, use a QT movie of your plate. For realtime scrubbing, make a preview in Layout with only your background plate visible from frame 0 and with Layout window maximized. Then, in Display options set Camera View Background to Preview. But you cant switch between cameras when using the preview.

• Add a new camera, name it something like "Camera_zoomable," and parent it to the first camera. Disable the X,Y,Z motion. This second camera can now be rotated (panned & tilted) and zoomed way into any specific point within the background sequence when you need to see detail up close and track to a visual mark.

• To track a camera motion or item within a plate you need to be able to pan and tilt the camera in very fine increments. For instance, you may use say a tiny pebble on the ground as a tracking reference that barely moves a pixel or less per frame, or maybe a button or seam on the shirt of a character walking thru frame. Normal amera rotation does not allow fine enough movements. Instead, use a target null.

• Add a null, name it something like "+Cam_Target+" (I use"+" to identifiy a null object in the list) and parent it to the screen. Enable Parent in Place, unparent the null, then disable Parent in Place. This should leave the Target null in the center of frame, at the same position as the screen. Disable the Z motion of the null. Then under the MotCion Options of your first camera, select this null as the 'Target.'

This is the part that is sometimes difficult for people to get their synapses around. While looking thru your render camera, you can now pan & tilt by moving the tracking null and your background plate remains perfectly framed in the screen that is parented to camera. You may track nulls and objects against visual markers within the plate with very fine camera movements, and you may switch to the second camera to zoom in on detail or even individual pixels.

So, let's say you have a plate of the camera panning left to right over an exterior ground. And you have to place a CG tree in the ground. Somewhere within the plate there may be a distinct pebble or pattern in the grass or ground that you can use as a visual tracking marker. You could place a null over this marker, use your zoom camera to zoom very close in on the marker, and use the target null to 'pan' your camera and keep the null lined up with the visual marker thruout the shot. You may have to key every frame, but this process allows you track a tiny marker very accurately and can go quickly as you arrow forward a few frames... move target...forward... move... forward...

We used this technique on dozens of shots in the movie, Battle of Red Cliff to track arrows and spears striking soldiers and horses. May view a couple of the spear shots in the Pixel Magic demo- http://www.pixelmagicfx.com/Gallery.htm I can track even a non-nodal camera dolly or crane move but is too tricky to explain. Even when I show a person in the same room it is difficult for them to pick up on. But as you play with this setup you may come up with your own better tracking tricks.

NOTE: A front projection mapped card will only appear accurately in GL if it is perpendicular to camera. If it is angled away from camera, the front projection map will appear distorted in GL so it is not accurate to track off of, but it will still render correctly. The more vertices in your object, the more accurate a front projection map appears in GL.

An animated example can be seen here: courtesy of Pixel Magic