Dave Mitchell's DigitalMicrograph™ Scripting Website

Function

An interactive edge finding filter based on the Canny Edge Detector method.

Version

version:20130824, v1.1

Author

D. R. G. Mitchell and T. C. Peteresen

Acknowledgements

The gaussian blur method used herein uses Fourier methods. Andrey Chuvlin's excellent warp method to apply FFTs to images of any size is used herein.

Comments

Updated in this version to add a ridge detection method. This locates edges as single lines, as opposed to the Sobel method which locates them as a pair of lines. Either method can be selected from within the dialog.

This interactive filter first applies a Gaussian Blur to reduce the effects of noise. Increasing blur eliminates spurious edge detections. Primary edge detection is done with a Sobel filter. The final step is hysteresis thresholding. First an arbitrary high threshold is set, above which the most intense features (strong edges) are retained. Finally a low threshold is set. The script works iteratively downwards in intensity, working from the high threshold to the low threshold, selecting only pixels of the given intensity which are in physical contact with pixels retained by the previous thresholding step. In this way, edge features which are contiguous but of varying intensity, and which were broken by the high threshold step are reconnected, preserving edge integrity. Isolated pixels arising from noise are eliminated. This is continued until no further changes occur during the low thresholding iteration, or until the limit set in the 'Iterations' field is reached. Images created at any step of the process can be output with the relevant '>' button. This script can therefore be used as a Gaussian Blur filter and a Sobel Filter, in addition to being a Canny Edge filter. A small version of the processed image is displayed in the dialog. The maximum image size displayed is 512 x 512. This size can be changed in DIgitalMicrograph (File/Global Info/Canny Edge Filter). Larger images will be downsampled for display, and will be shown in 8 bit mode. If this display is inadequate for seeing the effects of parameter changes, the relevant image should be displayed with the corresponding '>' button, and the processing monitored there. The default values can be set either in the Global Info as described above or by setting the desired values in the dialog and then closing it with the ALT key held down.

System Requirements

Tested on GMS 1.84 running under XP. Not tested on GMS 2, but should work.

Known Issues

The low thresholding step is slow, and the speed decreases with increasing image size and the amount of information selected in the image. Large Gaussian Blur sigma values and higher threshold values for the Low Threshold step will eliminate noise and spurious edge detections and speed up processing.

Supported

Yes

Included Files

Script file

Source Code