AE6 Overview Part 1: Tracking and Stabilizing Motion
Page 4 of 4
Tracker Plug-in Options
One of the things that caused confusion among those new to tracking and stabilizing footage in After Effects was the large number of options that needed to be set in order to get a decent track. In After Effects 6.0 some of that initial fear is lessened by “hiding” those options in sub-panels. While setting a default tracking point in the Layer Window should work 90% of the time, chances are you will need to go in and make some fine adjustments in the Tracker Plug-In Options window.[an error occurred while processing this directive]
Use Channel: After Effects 6.0 can track data from one frame to another based on one of three options - RGB, Luminance, or Saturation. If you were tracking colored dots on a subject, RGB might be the best choice, as it uses information from all three-color channels. The Luminance option compares brightness values in the Feature region between frames, while Saturation looks at how much color is present from frame to frame.
The default channel is Luminance. This works great if you are tracking a region with a high contrast or brightness compared to the surrounding or background pixels. For example, a bright white building against a blue sky.
However, if your colors are similar to one another, Saturation might work better (a Granny Smith apple against a lime green background).
Process before match: If your video had been shot in low light, video grain will have been introduced and could make a pixel much harder to track causing the Feature and Search region boxes to jump all over the screen resulting in garbage. Grain will also be introduced if the production was shot on a high grain film stock.
If you ever have to deal with grainy footage, turn on Process Before Match, and check the Blur box and increase the pixel amount accordingly. Since video grain is fairly small, you will probably never have to increase the pixel blur higher than five.
The way this option works is by blurring the frame before After Effects begins the tracking process. The blurring causes the grain to be lowered, thus increasing the chances of getting a good track.
If the footage is slightly fuzzy, selecting Enhance will sharpen the video to make tracking easier.
It is important to know that the Process Before Match option only for tracking purposes. After tracking is done, After Effects will restore your footage to its original glory.
Two of the most difficult options to figure out in the Motion Tracker/Stabilizer of After Effects 5.5 was with Track Adaptiveness and Subpixel Matching. For most users this was a very hard concept to wrap their head around. To solve this (and again because of speed increases) Adobe has removed these features and made them somewhat automatic replacing them with Subpixel Positioning.
Subpixel Positioning specifies fractional position keyframes to help match the selected Feature region. It does this by automagically dividing the Feature region into smaller and smaller boxes to help track the pixels. By default, this option is turned on. If you turn the option off, the tracker will round the position of the feature region to the nearest pixel.
Extrapolate Motion if Accuracy is Below x%: This is the tolerance level After Effects uses to extrapolate motion if your tracked pixel becomes hidden momentarily. For example, if you are tracking a ball rolling down the street and it momentarily disappears behind a fire hydrant, the Motion Tracker and Stabilization feature will estimate the position of the ball during those frames when the ball is hidden.
Now that you understand the improvements to the tracker in After Effects 6.0, you can now move on to running the process and applying the results to a layer. Since Tracking and Stabilizing Motion can be frightening, I’ll walk you through a couple of exercises throughout this week to ease you into the process.
In the next installment we’ll stabilize some of that shaky footage a videographer has brought in. Here is the before and after.
| The before and after show what can happen when tracking data correctly. We'll do this as an exercise in Part 2. |
The best tracking starts with the best video
In order to track a feature region, After Effects likes to deal with areas of contrast. These contrast areas can be color differences (red against green), luminance differences (black against white), or saturation differences (color intensity).
In order to obtain the best contrast when tracking elements, it is important to start off with the best footage resolution possible. Try to avoid importing footage that is heavily compressed. In addition to the loss of image data, an end result of compressed footage is that it contains artifacts that create blocks or chunks that will cause the Motion Tracker and Stabilizer feature to act erratically. This is often the case in video that has been digitized for multi-media production.
It is always best to digitize and import footage that is as uncompressed as possible. Unless Adobe After Effects can properly track a pixel from one frame to another, the resulting data is worthless. Using the native compression formats found in the more popular non-linear edit systems usually will work best (QuickTime in Final Cut Pro for example). If your NLE system gives you the option of selecting the amount of compression during digitizing, and your system can handle it, go with uncompressed.
When not working deep in the labs of the DMN Central Division testing the latest and greatest software/hardware products Stephen Schleicher can be found at the local university teaching a few courses on video and web production. He can be reached at schleicher@mindspring.com. You can also visit him on the web at www.mindspring.com/~schleicher
Prev 1 2 3 4
Related forums:
Related sites: • Animation Artist • Animation Supplement • AV Video • Content Mastering • Corporate Media News • Creative Mac • Digital Animators • Digital Media Designer • Digital Post Production • Digital Producer • Digital Video Editing • The WWUG
[an error occurred while processing this directive]
