Apple MA434Z/A User Manual - Page 743

• 1/32: Area is sampled at every .03125 pixels (1024 times more than with a sampling of 1). • 1/64: Area is sampled at every .015625 pixels (4096 times more than with a sampling of 1). tolerances The tolerances subtree contains subparameters that let you control this node's behaviors when the tracking quality goes down. • matchSpace: The pixels are matched according to the correlation between the selected color space-luminance, hue, or saturation. When an image has roughly the same luminance, but contrasting hues, you should switch to hue-based tracking. You can also adjust the weight of the color channels in the matchSpace subtree: • redWeight, greenWeight, blueWeight: Three subparameters with sliders let you weight how closely the tracking operation follows each color channel of the image being tracked. In general, the color channels with the most contrast for the feature you're tracking should be weighed most heavily. Color channels with minimal contrast for the feature you're tracking should be de-emphasized. • referenceTolerance: A tracking correlation of 1 is a perfect score-there is an exact match between the original reference frame and the sampled area. When the referenceTolerance is lowered, you accept greater inaccuracy in your track. If tracked keyframes are between the referenceTolerance and the failureTolerance, they are highlighted in the Viewer. Also, in some cases, referenceBehavior is triggered if the tracking correlation is below the referenceTolerance. • referenceBehavior: A pop-up menu that sets the tracking area reference sample. By default, the reference pattern is the first frame from which the track is started, not necessarily the first frame of the trackRange. The last two behaviors in the referenceBehavior list measure the tracking correlation and match it to the referenceTolerance to decide an action. The referenceBehavior pop-up menu contains the following options: • use start frame: The new samples are compared to the reference pattern from the first frame of the track. If you stop tracking midway, and start again at a later frame, the later frame is used as the reference sample. • update every frame: The source sample is updated from the previous frame. This usually creates an inherent drift in the track, as tiny errors accumulate. This method is for movements that have drastic changes in perspective and scale. • update from keyframes: If you are using a failureBehavior of "predict location and don't create keys" or "don't predict location," a keyframe is not necessarily saved every frame. In this case, you may only want to update from the last frame with a valid keyframe. Chapter 25 Image Tracking, Stabilization, and SmoothCam 743