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A Prototype Optical Tracking System Investigation and Development - page 38 / 170

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Theory and Design

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Figure 3.2 A coordinate frame is defined as a translation and rotation with respect to a parent frame. Two sets of axes are shown here. The X’, Y’, Z’ axes are defined with respect to the X, Y, Z axes (the parent frame). A translation from the parent frame origin to the X’, Y’, Z’ origin is given by T.

3.2.1

Regions of interest

region of interest (ROI) is a rectangular window with sides parallel to the image frame

that defines an important area of an image.

ll image processing operations in the Black

Spot firmware operate on ROIs. The firmware is responsible for identifying these regions, creating them, and removing them. By using ROIs the number of pixels that must be analysed per frame is significantly reduced. ROI contains the image of a marker. s the marker’s location within the Black Spot’s FOV changes, the position of the ROI is updated to follow the marker’s image. Figure 3.3 shows the composition of a ROI. The marker is shown in the middle of the region. n area of background pixels surround the marker. By making the ROI larger than the marker, the firmware can track changes in location. This process is described in Section 3.2.5. During the remainder of this thesis the term ‘marker’

will be used to describe both a marker and a marker’s image.

3.2.2

Marker segmentation

In order to track the positions of markers in a stream of images, the markers must be iden- tified and segmented from the background. trivial method to segment an object from its surroundings is by background subtraction. This method uses a reference background image frame and this is subtracted from the given frame. The result of this subtraction is an image that contains the difference between the foreground and the background. Provided

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