TRACKING OVERVIEW

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Like 2D trackers, 3D trackers rarely if ever use colour information. "Chroma is relatively unimportant to boujou's feature tracker," says 2d3's Steve Hill. "Pre-processing the images rarely improves the tracking results and we generally discourage pre-processing of any kind because of the many adverse effects that it cause. The exceptions are reducing the image size to reduce the effects of motion blur, and tweaking the linearisation settings of very dark cineon images to bring out more detail in the shadows."

Unlike the 2D experts we spoke with, the 3D experts have very strong opinions on what will track best and provide a large number of innovative tips on how to improve obtaining 3D camera solutions. Perhaps the most definitve comments on trackers comes from 2d3's Hill who strongly advocates triangular trackers. "The advantage to the triangle in the circle is that it gives you three well-spaced, high contrast corners for boujou's automatic tracker to pick up. The crash test marker has a high contrast area in the centre, and lower contrast areas around the edge. When this marker gets small and pixelated the central region starts to give four noisy tracks instead of one stable one (the detected features get pushed away from the corners by the blurring of the image at lower scales). The triangle in the circle works much better throughout a wider range of scales. Boujou's target tracker works well on both sorts of marker, but for best results place the crosshairs of your keyframes in the centre of an area of black or white rather than at a corner."

Digital Domain's Doug Noble points out that their approach is very much that of modelling the environment from a highly accurate recording of on-set measurements. They aim to not so much give the 3D artist a camera track set of data points, but rather to "track the room to the camera" so the artist has a set with the camera moving through it. Noble's TRACK software now usese optical flow and a range of solutions including Laser scans or "poor man's LIDAR" (LIght Detection And Ranging).

3D tracking falls into two classes, automatic trackers and manual trackers. Many programs do both. Sci-d-vis' Rolf Schneider, notes that "3D Equalizer has 3 different (manual) tracking modes: pattern, marker and edge/corner. The marker tracking mode is specialized on unicolored, flat discs placed on an unicolored background. 3D Equalizer computes the 'center of mass' of the disc which becomes the tracking point, on a frame by frame basis. That means that no previous frames are needed during the tracking process so no error accumulation happens. Marker tracking is very precise, but specialized. The cross/cheque patterns can be tracked quite well by regular 'pattern' trackers, that's the reason why people like to use them."


3D trackers normally require a set of 8 to 11 points to be valid at any one time, but the same points do not need to remain valid for the entire clip. Unlike 2D tracking, automatic point generation is common in 3D tracking. This allows 3D camera tracks to solve seemingly impossible tracks. Perhaps the one of the most impressive and difficult tracks is to track helicopter moves over vast featureless oceans. This is extremely tricky as Hill explains, "Tracking water is always a gamble - if the sea is too rough then boujou won't be able to find enough consistent tracks." That being said, results can be outstanding as evidenced by the fact that Boujou has been used to track water movies such as 'Enemy at the Gates' (Double Negative) and 'Troy' (Framestore-CFC and MPC).


Tom King, 3D artist at Digital Pictures in Sydney comments that he finds that Maya� Live will still track some scenes other automatic trackers can't. He also feels it can also give more accurate Z depth than other packages at times. Maya� Live, like some 2D trackers such as Digital Fusion, show percentage confidence levels for the trackers in the UI. King suggests the following tips for Maya� Live:

• Mmake sure tracking confidence is 70 to 80 percent green for all points. Bi-directional tracking can be repeated from a lower confidence area that looks right in the point centred window. Repeating this will result in longer high confidence sections.


• Delete keyframes for sections that deteriorate into yellow and red confidence levels.


• Using smart update behaviour in the options section can improve the chances of good tracks with points that change shape through rotation or lighting.


• Make sure the Ready to solve graph is all green for the frames you want to track.


• Before solving, point blast all point tracks to check for mistakes that are not detected by the tracker. One of these can completely ruin the Solve.


• Do the solving in the manual stages and look at the pixel slip numbers as you go. If you aren't getting a good solve, pointblast the tracking points again and correct any wandering points.