Edge Detection via Zero Crossing. Figure: Three-dimensional Surface Representing a Selected Color Plane (Red) for a Region of the ad Image from. Edge detection includes a variety of mathematical methods that aim at identifying points in a . Looking for the zero crossing of the 2nd derivative along the gradient direction was first proposed by Haralick. It took less than two decades to find a. Gradient-based edge operators. ○ Prewitt. ○ Sobel. ○ Roberts. ▫ Laplacian zero-crossings. ▫ Canny edge detector. ▫ Hough transform for detection of straight.


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The image shows the zero crossings from this image. Note the large number of minor features detected, which are mostly due to noise or very faint detail.

Zero Crossing Detector

The image is the result of applying a LoG filter with Gaussian standard deviation 2. Note that there are far fewer detected crossings, zero crossing edge detection that those that remain are largely due to recognizable edges in the image. The thin vertical stripes on the wall, for example, are clearly visible.


Finally, is the output from a LoG filter with Gaussian standard deviation 3. The image is the zero crossings in this image. Note how only the strongest contours remain, due to the heavy smoothing.

Edge Detection via Zero Crossing

In particular, note how the thin vertical stripes on the wall no longer give rise to many zero crossings. All edges detected by the zero crossing detector are in the form of closed curves in the same way that contour lines on a map are always closed. The only exception to this is where the curve goes off the edge of the image.

Since zero crossing edge detection LoG filter is calculating a second derivative of the image, it is quite susceptible to noise, zero crossing edge detection if the standard deviation of the smoothing Gaussian is small.

  • Feature Detectors - Zero Crossing Detector
  • Edge detection - Wikipedia
  • How It Works

Thus it is common zero crossing edge detection see lots of spurious edges detected away from any obvious edges. One solution to this is to increase the smoothing of the Gaussian to preserve only strong edges. Another is to look at the gradient of the LoG at the zero crossing i.


Detection can be based on two criteria: First derivative is zero crossing edge detection sensitive to noise than the second derivative, but can give badly connected edges, due to a bad choice of the selection threshold.

Second derivative, more sensitive to noise, gives many zero crossings but also better connected edges.

Edge detection and zero crossing

The simplest signal-to-noise-based selection technique could be as follows: A natural use of the Canny filter in Figure The image is first segmented into Canny contours which are threshold as above and then labelled. For each contour, an average color is computed by integrating the color context enclosed by this zero crossing edge detection.

This reduced color palette is then used as input to the RGB clustering. This removes all the unwanted points and if applied carefully, results in one pixel thick edge elements.

Sharp and thin edges lead to greater efficiency in object recognition. If Hough transforms are used to detect lines and ellipses, then thinning could give much better results.

If the edge happens to be the boundary of a region, then thinning could easily zero crossing edge detection the image parameters like perimeter without much algebra.