Locate Edge in Window

Locate Edge in Window

The purpose of this option is to allow specific intensity contours to be located within a window and their position written out to a data file. Unlike [C Contours], only one contour of sufficient length is located and plotted or written out. This contour may optionally be smoothed.

Find edge on which buffer ?

This entry specifies the buffer containing the image on which the contour is to be located. The selected buffer will be made the current output buffer. If the buffer is in extended memory, then it will be copied to buffer 0.

Edge in Window or Screen ?

The edge may be located in a subwindow, or the entire frame buffer.

Window Specification: Region containing edge If a window is chosen, then sections of the contour falling outside the window will not be included. Moreover, if a section of a continuous contour falls outside the window between two sections inside the window, then those two sections will be treated as different contours. The choice of window may also affect the section of contour found (see below). Control over the window selection is in the normal DigImage manner. Further details are available from [H Help] within the window selection menu.

Find edge in Window or search from Point ?

The start of the contour to be found may be specified by either locating a single point on it (<P>), or by automatically searching the specified window or entire screen (<W>). If the <P> option is chosen, then the intensity of the point specified will dictate the threshold at which the contour is to be drawn. If <W> is chosen, then DigImage will perform an ordered search from one of the corners of the window (see below).

Start searching from 0: top left, 1: top right 0 1 2: bottom left, or 3: bottom right ? 2 3

The contour may be search for starting at any one of the four corners of the window by selecting the appropriate option. The search proceeds from a given corner in an ordered manner. For example, if the search starts from the top left (<0>), then first the top line of the window is scanned for an appropriate contour. If one is not found, then the left hand column is scanned, then the second row, second column, etc. until the contour is found.

Cursor Positioning: Locate start of edge

Position the cursor on the pixel through which the edge must pass. For further details on how to operate the cursor, refer the [H Help] within the cursor submenu.

Threshold value ?

The contour being searched for is specified by a threshold value. DigImage will position the contour at pixels with an intensity less than or equal to this threshold, where their neighbour is greater than the threshold.

Minimum number of points in edge ?

The contour returned by DigImage may be filtered on the basis of its length. This entry specifies the minimum length the contour may have: if a contour is located with a shorter length, then it is disregarded and the search continued.

Smooth over how many pixels ?

Once the pixels belonging to a contour satisfying the minimum length have been found, DigImage provides the option of passing these points through a low pass filter before outputting them onto a buffer. The low pass filter operates by computing the average position of the pixel and its neighbours. This entry specifies the number of pixels to be included in this average. A value of zero will suppress the smoothing. If a nonzero value is specified, the averaging will utilise an odd number of pixels (the odd number greater than or equal to the specified value). The smoothing operator looks after both closed and open contours in a rational manner. Note that this form of smoothing will tend to decrease the area contained within a closed contour; this may be significant for very small closed contours.

Copy image, draw edge in Pixel coords, draw edge in Reference coords or Nothing ?

This prompt will only come up if a contour satisfying the threshold and length requirements is found. If no such contour could be located, then a warning message is generated. These options provide a number of different ways of displaying the contour on a frame buffer: 

           C       The copy of the buffer on which the contour was located and
                   marked is copied back to the original frame buffer. Note
                   that this may have contours which did not satisfy the
                   length criterion marked on it in addition to the desired
                   contour. Intensity values of 0 or 1 in the original image
                   will have been set to 2 by the contouring routines, and the
                   contours will be marked with the value 0.

           P       This option allows the contour to be drawn on a separate
                   frame buffer, using a specified intensity, in the current
                   pixel coordinate system.

           R       This option is the same as <P> except that the reference
                   pixel coordinate system will be used.

           N       The contour will not be drawn on a buffer if this option is
                   selected.

Copy/Draw edge to which buffer ?

The required image/contour may be copied/drawn to any frame buffer. Note that the buffer is not erased before drawing the contour, so multiple contours may be drawn on the same buffer.

Draw edge with what intensity ?

The contour may be drawn on the selected buffer with any desired intensity.

Draw edge with what intensity ?

The contour may be drawn on the selected buffer with any desired intensity.

File to contain coordinates (blank to suppress, .DAT default extension) ?

The coordinates of the pixels defining the contour may be written out to a formatted file by specifying a file name to this prompt. When written the file contains the current pixel coordinates (integer), the reference pixel coordinates (real), the current world coordinates (real) and the reference world coordinates (real). Additional comments are included, prefixed with the DigImage comment character '#'.

Append or New ? If the file already exists, choosing <A> will cause the data to be appended to that already in the file, while <N> forces DigImage to start a new file. If the file does not exist, then the answer to this question has no effect.

Title for data ?

An optional title may be included in the data file. It will be prefixed by the DigImage comment character automatically '#' when written out.

Error Message:
Unable to open output file

The specified output file could not be opened. This may be due to an invalid filename or path, or insufficient space on the disk drive.

Warning Message:
No edge found

The routine could not locate an edge satisfying the required criteria.

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DigImage User Documentation

Stuart Dalziel, last page update: 19 February 1996