Spot vs. Corner Detection

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‌Spot vs. Corner Detection ‌

The SynthEyes auto-tracker normally looks for bright or dark spots in the images, which are tracked (linked) from one image to the next, then eventually made ("peeled") into automatic trackers.

Spots are very prevalent in a wide variety of indoor and outdoor scenes, and have the advantage of being largely unaffected by a wide variety of imaging conditions, including image rotation, viewing at an angle, defocus, motion blur, compression artifacts, and noise.

In the following situations, it is beneficial to detect corners instead:

 Clean sparse man-made interior sets may have only linear features and corners, instead of spots.

 Reconstructing building meshes, where we want trackers on the exterior corners of the building (spots are on the walls)

The corner auto-tracker is turned on from the Summary panel. It does add considerable processing time, so we recommend keeping it off unless suitable corners are present and desired.

The SynthEyes corner detector is specifically designed to produce corners features "at a distance." This is because the pixels that are physically located at a

corner location are frequently not particularly reliable, being subject to varios blurs and noise that have the effect of minimizing a corner, or shifting it from its true location.

Consequently the SynthEyes corner detector looks for suitable line segments at a distance from the corner, and intersects them to produce the location of the corner. So the accuracy derives from many smooth pixels further from the corner, rather than the few unreliable pixels located at it. Even still, corner features are inherently less accurate than spot features, which are based on a whole region of pixels. Since a tracker's 3-D position is based on many frames of data, generally an excellent position can still be obtained.

There will typically be fewer corners than spots located by an autotrack. For modeling, you may want to increase the number of corners. The Add Many Trackers panel can be set to selectively add only corner features. Or, the Features Control panel can be set to make only prospective trails of corner blips visible and eligible to be promoted to trackers (at your instruction).

You can adjust the corner detector's parameters from the Advanced dialog on the Features panel. To better understand the process, you can use the edge or corner view types, which will produce rather colorful displays in the main camera view.

The tooltips contain simple descriptions of the parameters. If you are using very high or low resolution images, you might consider changing the various pixel-based numbers such as edge width, minimum length, and intersect distance. In low-contrast situations you might experiment with Edge Threshold and Contrast. Be sure to experiment first, before doing an autotrack, so that you don't have to worry about trackers you've already worked on.

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