Questions tagged [edge-detection]

Edge detection is the process of identifying points in an image, where image brightness changes sharply.

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Why is Envelope noisy even after filtering and achievement of SNR gain (-6dB SNR to 14dB snr)?

A noisy signal with 500 MHz bandwidth, with negative SNR, when filtered to 6 MHz bandwidth, should attain a theoretical SNR gain of 20 dB. So if the original signal has -6 dB SNR, after filtering we ...
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Interpreting results of Sobel Edge Detection

I am implementing A No-Reference Perceptual Blur Metric paper. Somewhere in the prep steps they mention the use of vertical Sobel filter for finding vertical edges. The algorithm is summarized in Fig....
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Removing dark borders in Image Stitching for Panorama

I am working on a Matlab code for panoramic image stitching for multiple image frames and getting satisfactory results however there are always dark regions surrounding the stitched images as ...
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Implementation of Deriche filter

The Canny edge detector is expressed by the derivative of the Gaussian. Its practical implementation involves the Gaussian filter, then the Sobel filter to calculate the gradient in both directions. ...
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1 answer
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Finite/Infinite Impulse Response filters

Most image filters are of finite extension (which correspond to Finite Impulse Response in 1 dimension), like Laplacian, Canny, Gaussian... Instead of being of infinite extension (IIR in 1D). Why is ...
2 votes
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Numerical optimization method for optimal edge detector

John Canny, in his paper "A Computational Approach to Edge Detection" (PDF), finds the expression of an optimal edge detector by first using an analytical approach (Euler-Lagrange equation). ...
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How are the edges of an image obtained with anisotropic diffusion?

I already know how to implement the method and obtain a correctly diffused image, but in the papers related to the topic there is usually an edge map that's calculated after the diffusion is performed ...
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1 answer
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Edge Detection with a special algorithm

Studying for my finals in Image Processing. Trying to solve the following question: An image processing specialist recommended an algorithm for Edge Detection in a color image RGB. He argued that it ...
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Detecting bounded regions in JavaScript

I have an image of a crossword , and I would like to find the x,y coordinates of every valid square and the number associated with it, then I would feed it into crossword.js. Here are some sample ...
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48 views

MATLAB code to detect the corners of an object without using the built in functions

I need to write a MATLAB code to detect the corners of an object in a black and white image without the built-in MATLAB vision toolbox or bwlabel, imrotate, imresize, corner, edge, reshape, imfilter,...
2 votes
1 answer
248 views

Edge Detection Convolution Intuition

I was learning about convolution and how filtering helps us to detect an edge in an image;however I still cannot not understand how the convolution process in the image below does this. I understand ...
5 votes
1 answer
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How to Remove Double Lines Detected Along the Edges by Edge Detector?

I'm trying to take real time input for hand gestures with web cam, then processing the images to feed them to a neural network. I wrote this processing function to make the hand features look ...
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1 answer
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1st and 2nd Gaussian derivatives for edge detection

I have a project about edge detection, I made research on internet about filters and read many articles but there is an article talked about using Sobel and Gaussian derivatives then it mentioned &...
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1 answer
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Extracting ridges in automatically in image

I am trying to extract ridges from an image, calculated using the eigenvalues of the Hessian matrix (see answer in Best way of segmenting veins in leaves?). In this image I have zeroed out the ...
6 votes
2 answers
276 views

Should Edge Detection Be Applied in Spatial or in Frequency Domain?

I want to ask why everywhere is used to do edge detection in spatial domain and not in frequency domain with FFT of image and apply HP filter. Thank you.
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Help or suggestions with Line detection in microscopy images

I'm doing a personal project in which I want to select some very specific lines in a microscopy image. The first picture is the raw image, the second picture is the image for the lines (in yellow) I ...
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2 answers
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How to detect defects at a contour?

I am detecting a contour of a smooth metal. I do binarization first then use openCV contour function. But sometimes the metal has defects such as dots or dusts at edge. This leads a unsmooth edge. Is ...
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How Come the Low Pass Filter in Sobel Operator Isn't Normalized?

I am relatively new to the field of computer vision and I have just learnt about the sobel operator. The sobel operator in the x direction is a convolution of the finite difference kernel $[1,0,-1]$ ...
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Significance of outer product in structure tensor / second moment matrix

How does one interpret the outer product $\mathbf{M} = \nabla I \otimes \nabla I = \begin{bmatrix}\left(\frac{\partial I}{\partial x}(x)\right)^2 & \frac{\partial I}{\partial x}(x) \cdot \frac{\...
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3 answers
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Reversing the Order of Operators for Edge Detection?

Usually, for edge detection, we perform smoothing and then pass it through difference filter. What if application of difference filter happens first and then smoothing. How the math behind the same ...
6 votes
1 answer
563 views

What Is an Oriented Gaussian Second Derivative Filter

In the paper: Detecting and Localizing Edges Composed of Steps, Peaks and Roofs, the authors refer to an image filter as an oriented second-derivative Gaussian filter. I'm trying to figure out what ...
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2 answers
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Fitting a polynomial to a ridge in an array

I have an array of numerical data. Say, ...
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What is Maximally Stable Extremal Regions (MSER)?

In the paper , MSER is being defined. Please help me understand what is MSER and how to implement it. The paper says, The regions are defined solely by an extremal property of the intensity ...
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Maximally Stable Extremal Regions (MSER) Implementation in Document Image Character Patch Identification

My task is to identify character patches within the document image. Consider the image below: Based from the paper titled "CG-DIQA: No-reference Document Image Quality Assessment Based on ...
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644 views

Edge Detection vs. Contour Detection?

What is the difference between edge detection and contour detection? Apparently both techniques are used to detect boundaries of close shapes(whether oval shape or r rectangular shape)
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1 answer
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Identify if a Specific Edge Pixel Belongs to Horizontal, Vertical or Diagonal Edge Line

I have a code below that identifies the edge of the text image. Input image is: Output display is: I want to select random points near the edge border of the text. I was thinking to identify if the ...
1 vote
1 answer
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Understanding the Threshold Process in Harris Corner and Edge Detector

I have read the paper Chris Harris , Mike Stephens - A Combined Corner and Edge Detector about the Harris Corner and Edge Detector, but I didn't understand the point that after low and high threshold ...
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Is HSV Color Space Sufficient for Rudimentary Color ID and Edge Detection

I want to, as part of a C++ school project (no OpenCV), work with an RGB image taken from a camera filming smarties (round colored pellets) from above and extract the number of candies in each color. ...
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Why we need Laplacian for edge detection?

I am studying some trivial computer vision processing techniques and I came across edge detection algorithms. IMO sharp changes in the gradient are enough indications to detect the edges in an image ...
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Finding defects in wall paint (finding difference in textures)

I have been experimenting with computer vision techniques to find defects in paintings, more specifically defective brush strokes. I have tried a handful of techniques such as different filters, One ...
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2 answers
692 views

How to find position, scale and rotation of a known shape/contour in an image?

I have an image of an object. The image shows a high contrast outline of the object. It is guaranteed that the image shows exactly one object. The shape/contour of that outline is known. It is not ...
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1 answer
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Intuition behind image derivative using Fourier Transform for edges detection

This equation can be shown mathematically: $\frac{\partial f}{\partial x}=\frac{2\pi i}{N} \mathcal F^{-1}\left(u\cdot \mathcal F(f(x,y)\right)$ I am struggling to understand the intuition behind it ...
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Help understand the results of hough transform

I'm trying to apply the Hough transform of the following image using MATLAB: The result is: How could I predict that there are 6 bright spots in the resulted image? I expected 8 spots since I ...
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2 answers
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why do directional derivatives work better with a gaussian filter?

I've been working on image segmentation and computing the directional derivatives of a grey-scaled image, with the objective of detecting contours and edges. I have realised that if I apply a gaussian ...
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1 answer
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Recommended approach for extracting specific edges from image?

I have several sets of images with a typical image being: and from each image, I would like to extract the top and bottom curves of the object, shown in red here: What are some possible tools I ...
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What are the limitations of Watershed Segmentation Algorithm?

I am developing an Android application using Opencv to detect objects from images. I used Canny edge detector to detect the edges and extract contours, and it's working good so far. But I need to ...
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Estimate gap between two adjacent images

I have two images of a panoramic view but there is a small gap (<24 pixels) in the horizontal dimension between them. I would like to do interpolation/inpainting to fill the gap, but the exact size ...
1 vote
2 answers
159 views

How to discard deviating values from standard deviation and running average

I'm building a inductive vehicle loop detector. By measuring the frequency of a LC-oscillator where the cars will pass over the inductor of the LC-Oscillator. If the cars pass over the inductor the ...
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1 answer
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Does flat surface detection and orientation require special hardware to compute?

Modern augmented reality platforms such as Google's ARCore and Apple's ARKit seem to only operate on mobile devices, I'm guessing, because their underlying algorithms require specialized hardware that ...
6 votes
2 answers
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Is the Sobel filter a high pass filter, and if not, what is the difference between them?

I am an artist with an interest in signal processing. I realize that maybe this question very basic, but the web banner on the frontpage says "anyone can ask", so I assume that means beginners too. ...
2 votes
1 answer
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How to Clean Black and White Images Which Are Results of Edge Detection

I want to extract text from some images with messy backgrounds. First step was switching to gray scale, applying an edge detection algorithm, and then cleaning up a little bit. Here's an example of ...
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1 answer
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How to segment multiple overlapping coins (ellipses)

I am doing a program that counts coins (and tells their value by their size) with OpenCV (C++). Let's say we have an image of some coins. If there is an overlap of two coins at the same time, it is ...
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1 vote
1 answer
2k views

How can I trace a path around an image consisting of only black and white lines?

I'm looking for a way to trace a closed curve (i.e. a long list of pixel values all connected by lines in between) that approximates a bitmap image of black and white lines. The image starts out as an ...
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edge detection of 1-D signals

Would you please introduce me to some methods (or references) to do edge detection on some pulses (rectangular pulses embedded in noise). At first I apply some filters to smooth the signals, then I ...
1 vote
2 answers
2k views

Why are edges in spatial images represented as edges in their Fourier transform image?

Here is a well-known image and its Fourier Transform (magnitude). If I understand correctly the theory behind the FFT, each pixel in the FFT image represents a certain 2D sine wave with frequency ...
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What's Logic Behind the Construction of Sobel's Filter in Image Processing?

I am basically very new to this image processing field. I am presently working on edge detection on colour images. While learning the basics of edges and edge detection in images, I encountered image ...
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2 answers
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Is it possible to combine two Sobel kernels into one?

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1 answer
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Automatically detect an image collage

I'm trying to automatically detect if an image is a collage vs a single photograph. I'm not too concerned with difficult cases of artistic collages. What I'm trying to solve is rectangular collages ...
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Chapter 4, section 4.2.1. in Szelisky computer vision, is there a mistake?

I've already asked a similar question in mathematics exchange without getting an answer, I read a bit more and I think the question might be more suitable for the signal processing exchange. ...
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Finding the edge pixels on only one side of the object

For my work on lung images, I get a binary image mostly resembling the pattern shown below. I need to find the pixels on the borders shown by yellow arrows. The edges on the other end are shown in ...