All Questions
119 questions
35
votes
5
answers
6k
views
What factors should I consider in choosing an edge detection algorithm?
I've learned about a number of edge detection algorithms, including algorithms like Sobel, Laplacian, and Canny methods. It seems to me the most popular edge detector is a Canny edge detector, but is ...
- 1,610
22
votes
4
answers
8k
views
How do I detect distinct objects when their edges touch each other?
I need to find all the contours in an image retrieved from the camera. So I first use the canny edge detector to find the edges and then find the contours. Pretty simple.
However, my contours get "...
user650
19
votes
4
answers
14k
views
What Are the Limitations of a Canny Edge Detector?
Broadly a majority of the literature on edge detection algorithms and applications that uses edge detection, references Canny's edge detector. So much so that it looks like almost "the solution" to ...
- 5,597
14
votes
5
answers
26k
views
How Does a Convolution of an Image Can Be Expressed as a Matrix Multiplication (Matrix Form)?
I know this question may not be very relevant to programming, but if I don't understand the theory behind image processing I'll never be able to implement something in practice.
If I got it right ...
- 251
12
votes
2
answers
256
views
Identify the correct spot to place a label
In the image shown below, I have a 2D data set where I have identified four clusters labeled [0,1,2,3]. I'm looking for an algorithm to place the labels in a ...
- 509
12
votes
3
answers
11k
views
Can edge detection be done in the frequency domain?
Can we take advantage of the fact that high frequency components in the FFT of an image generally correspond to edges, to implement an edge detection algorithm in the fourier domain?
I did try ...
- 263
11
votes
2
answers
443
views
Explanation on Haarlets
Would someone be able to give me some info or a link etc ... regarding haarlets aka Haar wavelet-like features. I'm reading several papers for my master dissertation and several of these papers ...
- 392
10
votes
1
answer
2k
views
Could you describe the effects for varying different parameters of a canny edge detector?
Last couple of questions touched upon Canny edge detector
What are the limitations of a Canny edge detector? and
Best way of segmenting veins in leaves?
The basic outline of the algorithm is as ...
- 5,597
10
votes
2
answers
5k
views
How to implement a gradient based Hough transform
I am trying to use the Hough transform for edge detection, and would like to use gradient images as the basis.
What I have done so far, given the image I of size <...
9
votes
1
answer
12k
views
How can I detect elements of GUI using opencv?
Given a screenshot of an application, is there any way of finding GUI elements of that application using only opencv? (something like in the image)
I tried to play with blending the image and other ...
- 185
8
votes
1
answer
10k
views
Differences between OpenCV Canny and MatLab Canny?
does anyone know why the MatLab Canny (MLC) is so different compared to the OpenCV Canny (OCC)?
ML-C delivers precise and more connected edges than the OCC, but how is that possible?
The reason why I ...
- 587
8
votes
2
answers
13k
views
Straight lines with an edge detection algorithm
I've been thinking of an algorithm for object recognition but it would relies heavily on straight non-noisy lines and as far as I know this is hard to obtain with edge detection algorithms. What's the ...
- 83
7
votes
2
answers
6k
views
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.
...
- 171
6
votes
2
answers
1k
views
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 $[1,0,-1]$ ...
- 293
6
votes
2
answers
3k
views
Why Is Zero padding Required for Sobel Edge Detection?
Why do we leave two rows and two columns of an image (or) padding zeros to an image for detect edges while using Sobel operator and Prewitt operator?
5
votes
4
answers
3k
views
Do the DCT coefficients correspond to particular pixels?
Getting the DCT coefficient based on what I've read, is usually done through a matrix, usually 8x8 or 16x16 using these formulas:
$$B_{pq}=\alpha_p\alpha_q\sum_{m=0}^{M-1}\sum_{n=0}^{N-1}A_{mn}\cos\...
- 221
5
votes
3
answers
646
views
Are 2nd Order Edge Detectors More Susceptible to Noise?
I am reading about edge detection and I read that 2nd order detectors are more susceptible to noise. Is there a mathematical proof to this ?
- 219
5
votes
1
answer
1k
views
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 ...
- 213
5
votes
2
answers
10k
views
Edge Detection on a Color Image
I understand the process of using a Sobel kernel for edge detection in greyscale images. The input is a greyscale image, and the output is a greyscale image. I'm having trouble, however, figuring out ...
- 153
5
votes
3
answers
171
views
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 ...
- 53
5
votes
2
answers
1k
views
Why edge sharpening produces high frequency?
I have a low-resolution image in which the high frequencies are missing. When I apply an edge sharpening filter some of the missed high frequency is recovered.
I am wondering why this edge sharpening ...
- 51
5
votes
2
answers
1k
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)
- 1,952
5
votes
1
answer
637
views
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 ...
- 165
5
votes
2
answers
681
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.
- 73
5
votes
1
answer
923
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 ...
- 241
5
votes
2
answers
3k
views
Laplacian Operator with and without Diagonal Direction Elements in the Kernel
This is a general question on the laplacian operator, which has two different versions. The first version is :
\begin{matrix}
0 & 1 & 0 \\
1 & -4 & 1 \\
0 & 1 & 0
\end{...
- 283
5
votes
1
answer
202
views
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.
...
- 53
5
votes
1
answer
630
views
Image Edge Preserving Smoothing
I'm searching for a method that can smooth a 3D volume whilst preserving the edges in my volume.
I researched anisotropic diffusion filtering and bilateral filtering, but I'm having trouble to ...
- 121
5
votes
2
answers
245
views
detecting "deformed" lines
I have a project in which I have to recognize 2 lines in different "position", the lines are orthogonal but can be projected on different surfaces. I'm using opencv.
I'm not asking for code, but ...
- 409
5
votes
2
answers
2k
views
What's the best technique to determine if the contents of two images are the same?
I need to run comparisons on the output of hundreds of frames of simple 3D-rendered characters. Perceptually, many of the frames are exact duplicates. Based on this, I assumed something like a ...
user15190
4
votes
3
answers
4k
views
Bitmap border/stroke alogirthm?
I am looking for an algorithm that adds borders to a bitmap image(very much like the photoshop stroke effect) but one that will also have antialiasing to the borders(or one that copies alpha from the ...
- 319
4
votes
3
answers
9k
views
Which sigma to use for edge detection
I know that in many edge detection algorithm the first step is choosing a scale to work on.
We actually operate on the chosen scale by applying a gaussian blur on the original image.
I wanted to ...
- 355
4
votes
2
answers
129
views
Separating dominoes pieces from one another and counting them
I Have a picture of Dominoes on a table, which is noisy.
I am trying to do the following:
removing the background.
counting how many dominoes pieces are there.
identifying what each piece represent.
...
4
votes
1
answer
808
views
Keypoint orientation with the oFAST from Rublee's ORB
I have just read the papers from Rublee et al. (ORB: an efficient alternative to SIFT or SURF) and Rosten et al. (Machine learning for high-speed corner detection) (=fast-detector). Rublee tries to ...
- 241
4
votes
1
answer
326
views
Detect Longest Vertical Lines (Edges) in an Image
I have a task in image processing, which need to detect vertical lines from matrix. For example, there is a pair of white and black vertical lines segmenting the picture below. I need to find it. So I ...
- 43
4
votes
1
answer
2k
views
Edge linking using local processing - why is this criterion imposed
I'm reading Digital Image Processing by R. Woods and R. Gonzalez. In it, a section on edge linking using local processing in an image details a simplified algorithm, that imposes a criterion on the ...
- 41
4
votes
1
answer
6k
views
Sobel Vs Gaussian Derivative
What would be the most significant difference when using either a sobel filter or a gaussian-derivative filter, apart from the obvious difference of the size of the 2 filters?
- 41
4
votes
1
answer
991
views
OpenCV 2 Connect Incomplete Contours edges
I have to connect the unfinished edges of the contours in the red color bounding box.
I used distance transform, watershed, and morphology, but still, edges are not connecting.
...
- 41
4
votes
1
answer
602
views
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 ...
- 145
4
votes
1
answer
4k
views
Difference of Gaussians
I use the following formula for DoG:
$$\frac{1}{\sigma} (\frac{x^2}{2\sigma^2}-1.0) e^{\frac{-x^2}{2\sigma^2}}$$
What is the relationship between this formula and the difference of two Gaussian ...
- 141
4
votes
1
answer
178
views
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....
- 41
3
votes
2
answers
22k
views
In opecv function Hough circles how does parameter 1 and 2 affect circle detection and how can I adjust them to increase accuracy?
I'm using opencv houghcircle function to detect circles.
http://docs.opencv.org/doc/tutorials/imgproc/imgtrans/hough_circle/hough_circle.html
Problem is if I set minimum radius to null it detects ...
- 33
3
votes
2
answers
2k
views
Why is Prewitt filter High Pass
Prewitt filters are popular filters in image processing for edge detection
http://en.wikipedia.org/wiki/Prewitt_operator
Can anyone give a proof on why Perwitt and other edge detecting filters are ...
- 395
3
votes
2
answers
232
views
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 ...
- 167
3
votes
1
answer
2k
views
Algorithm for finding length and end points of a curly human hair
I'm trying to do some analysis of a curly human hair on an iPhone app. I've been using the openCV library.
Particularly I want to be able to estimate the length of the hair, and locate its end points ...
- 258
3
votes
1
answer
554
views
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 &...
- 31
3
votes
1
answer
303
views
What is a segmentation mask in the paper "Fast Edge Detection Using Structured Forests"
I've read the paper about edge detection, in this paper they treat edge detection as a learning problem which takes an image patch as input and output a label, a binary edge map or a segmentation mask,...
- 31
3
votes
2
answers
102
views
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.
Basically ...
- 435
3
votes
1
answer
64
views
How Last Edge Image Can Be Achieved from Law Masks
as we know thechnique of law edge detection has 25 2d masks that is obtained form 5 different 1d filters. we can use each of those masks on image with convolution but how can we obtain final result? ...
- 277
3
votes
1
answer
3k
views
Ridge finding in noisy 2D image
I'm using MATLAB, and have experience with it but not with this kind of problem. I'm trying to identify white lines in noisy 2D images such as the one below. I can separate the brighter ones just by ...
- 31