Locating Permanent Reference Points

Permanent reference points are typically part of the experimental set up and may be either white on a dark background or black on a light background. They may be sources of light superimposed using a front silvered mirror, or black dots affixed to the wall of a tank. In order for DigImage to use the permanent reference points, they must first be registered with DigImage. This is achieved by locating the cursor somewhere within each point and specifying the defining intensity. Once the permanent reference points are registered with DigImage, they may be located again automatically for each frame being processed. By comparing the location of the points in the current pixel coordinate system with their registered reference locations, DigImage is able to construct a mapping between the current pixel coordinates and a reference pixel coordinate system.

Locate reference points in which buffer ?

The permanent reference points may be specified using any valid buffer. Normally the user will have acquired previously the image to be used into the buffer.

Cursor Positioning: Specify Permanent Reference Point

The DigImage cursor is used to locate the permanent reference points, prior to specifying their threshold intensity. The cursor may be located at any pixel within the reference point so long as it satisfies the threshold to be used (see next section). Operation of the cursor is in the normal manner, with additional help available under the H help option within the cursor submenu.

Threshold for reference point ?

The reference point is defined as the island of pixels, including the pixel specified by the cursor, which satisfy the threshold specified by this entry. If the intensity at the cursor is less than the specified threshold, then the reference point will be assumed dark on a light background; if the intensity is more than the specified threshold then the point will be considered light.

Use Volume or Area centroid ?

DigImage has two options for determining the location of a permanent reference point. In addition to finding all the pixels within the reference point, the location of the centroid of the area (option A), and the centroid of the volume satisfying the threshold (option W; the volume centroid has each pixel weighted by its intensity excess or deficit). Either centroid may be used for the reference point (each reference point need not have the same type of centroid); generally the volume centroid is preferable as it locates the points more precisely. Note that with both centroid types the reference points may be located to subpixel accuracy (assuming they are more than one pixel in size).

Specify another reference point ?

By specifying yes (<Y>), the cursor menu will be brought up again in order to locate another reference point. Specifying no (<N>) will return to the Coordinate System Mappings submenu. Note that previously registered reference points are maintained and new points added each time the L (Locate permanent reference point) option is entered. To delete reference points the I (Initialise permeant reference point counter) option should be used. After the points have been specified the Mapping type should be specified using option M.

Errors, Warnings and Messages

Warning Message: The maximum number of permanent reference points have already been specified

It is probably best if you [I Initialise] the reference points and redefine them.

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DigImage User Documentation

Stuart Dalziel, last page update: 19 February 1996