All Questions
Tagged with image-processing image-processing
373 questions
133
votes
5
answers
90k
views
What does frequency domain denote in case of images?
I was just learning about the frequency domain in images.
I can understand the frequency spectrum in case of waves. It denotes what frequencies are present in a wave. If we draw the frequency ...
- 2,397
5
votes
1
answer
945
views
Replicate MATLAB's `conv2()` in Frequency Domain
When conv2d is on same mode, the image needs no padding, because the result is the same size as the image.
When ...
- 53
5
votes
3
answers
2k
views
2D Cross-Correlation using 1D FFT
I need to calculate cross correlation between 2 images which I read in 2 vectors, both of them uni-dimensional.
As per my understanding I need to do the following operations:
FFT(1D) on vector 1 and ...
- 55
70
votes
12
answers
22k
views
Is deep learning killing image processing/computer vision?
I'm looking forward to enroll in an MSc in Signal and Image processing, or maybe Computer Vision (I have not decided yet), and this question emerged.
My concern is, since deep learning doesn't need ...
- 803
40
votes
3
answers
54k
views
What are the most common algorithms for adaptive thresholding?
Adaptive thresholding has been discussed in a few questions earlier:
Adaptive Thresholding for liver segmentation using Matlab
What are the best algorithms for document image thresholding in this ...
- 5,597
49
votes
3
answers
22k
views
What algorithm does Google use for its "Search By Image" site?
What is your best guess how Google Image Search works? I can upload a photo and can search for similar images. What algorithm does it use to identify similar images?
- 491
6
votes
2
answers
3k
views
Image Processing and applicability of 2D Fourier Transform
As a newbie in the world of signal processing, I am having a hard time in appreciating image 2-D fourier transforms.
I am fully able to appreciate the concept of 1-D Fourier transform. Essentially, ...
- 279
5
votes
1
answer
4k
views
Applying Image Filtering (Circular Convolution) in Frequency Domain
To filter an image we can:
Use a 3x3, 5x5, 7x7, etc. filter, that is convolve the image and the filter in the space domain.
Use a FFT on both the image and the filter, multiply them together in the ...
- 153
4
votes
1
answer
2k
views
Applying 2D Image Convolution in Frequency Domain with Replicate Border Conditions in MATLAB
I have created a function that filters an image (250x250) with a gaussian blur kernel (5x5) using FFT and IFFT. I am trying to get my filtered image to equal exactly the filtered image created by the '...
- 85
54
votes
6
answers
25k
views
Best way of segmenting veins in leaves?
I have done a lot of research and found out methods like adaptive thresholding , watershed etc that can be used of detecting veins in leaves . However thresholding isn't good as it introduces a lot of ...
- 2,192
16
votes
2
answers
35k
views
Image Reconstruction:Phase vs. Magnitude
Figure 1.(c) shows the Test image reconstructed from MAGNITUDE spectrum only. We can say that the intensity values of LOW frequency pixels are comparatively more than HIGH frequency pixels.
Figure 1.(...
- 549
12
votes
2
answers
5k
views
Generate the Convolution Matrix of 2D Kernel for Convolution Shape of `same`
I want to find a convolution matrix for a certain 2D kernel $ H $.
For example, for image Img of size $ m \times n $ , I want (in MATALB):
...
- 897
9
votes
3
answers
6k
views
How to Zero Pad in Order to Perform Filtering in the Fourier (Frequency) Domain?
Consider an $M\times N$ image $f$ and an $G \times K$ filter $h$. Given that convolution in the spatial domain corresponds to multiplication in the Fourier domain, then we can perform a convolution of ...
user40095
7
votes
2
answers
14k
views
How to Prove a 2D Filter Is Separable?
I want to prove that 2D Gaussian filter is separable and we can separate it into two dimensions, my problem is about the size of filters. we should prove that $G(x,y)*I$(where $G(x,y)=$$\begin{bmatrix}...
- 173
6
votes
1
answer
4k
views
The Meaning of the Terms Isotropic and Anisotropic in the Total Variation Framework
The isotropic TV is defined as the estimation of 2-norm of gradients $\sqrt{(y_{i+1,j}-y_{i,j})^2+(y_{i,j+1}-y_{i,j})^2}$, while the anisotropic TV is defined as the estimation of 1-norm of gradients $...
- 63
5
votes
1
answer
1k
views
Gaussian Equivalent of Convolving an Image 50 Times with a Box Filter
I have a image that I'm convolving 50 times with a box filter, and I like to replace this with a single gaussian filter. Because both box filter and gaussian filter are separable I can just study the ...
- 435
5
votes
1
answer
3k
views
Generate the Matrix Form of 2D Convolution Kernel
We know that a convolution can be replaced by a multiplication with a Toeplitz / Circulant Matrix. Meaning, assume I have convolution kernel $ h $ and matrix $ I $ (Of size $ m \times m $ for example),...
- 193
4
votes
3
answers
2k
views
Vein extraction from this image
I have this image:
What methods should I use to extract the veins from this image.
Thank you.
- 41
78
votes
4
answers
62k
views
What are some free alternatives to SIFT/ SURF that can be used in commercial applications?
As far as I understand, both SURF and SIFT are patent protected.
Are there any alternative methods that can be used in a commercial application freely?
For more info on the patent check out:
http://...
- 2,880
78
votes
9
answers
110k
views
Why do we use the HSV colour space so often in vision and image processing?
I see the HSV colour space used all over the place: for tracking, human detection, etc... I'm wondering, why? What is it about this colour space that makes it better than using RGB?
- 781
41
votes
2
answers
22k
views
What's the difference between the Hough and Radon transforms?
I'm familiar with the Radon transform from learning about CT scans, but not the Hough transform. Wikipedia says
The (r,θ) plane is sometimes referred to as Hough space for the set of straight ...
- 16k
28
votes
1
answer
5k
views
Mathematics of Harris corner point detection
This is the mathematical expression for Harris corner detection:
But I have the following doubts:
What is the physical significance of $u$ and $v$? Many references say it is the magnitude by which ...
- 601
24
votes
6
answers
28k
views
What Is the Walsh Hadamard Transform? How Could One Use It in Image Processing?
I am trying to teach myself about the WHT but there dont seem to be many good explanations of it online anywhere. I think I have figured out how to calculate the WHT, but I am really trying to ...
- 9,897
22
votes
1
answer
46k
views
Difference Between Correlation and Convolution in the Context of Image Processing
Could you please clearly explain what is the difference between correlation and convolution that is done by a filter on an image?
I mean in terms of signal processing definition I know that ...
- 363
15
votes
4
answers
5k
views
List of possible image features for content based image retrieval
I am trying to find a list of possible image features like color, oriented edges and so on for measuring their usability in case of finding same/similar objects in images. Does anyone know such a list ...
- 867
7
votes
2
answers
2k
views
Kernel Convolution in Frequency Domain - Cyclic Padding
I don't know whether this is the right place to post this, but I suppose it is.
I know that frequency multiplication = circular convolution in time space for discrete signals (vectors).
I also know ...
- 223
6
votes
1
answer
1k
views
Filtering $ n \times n $ Images by Separable $ m \times m $ Filters. Computation Time for Filtering Using FFT, 2D Convolution and Two 1D Convolutions
Consider filtering square $n\times n$ images by square, separable $m\times m$ filters.
Give general equations for the computation time for the following approaches to this:
Filtering using FFT
2-D ...
- 61
6
votes
1
answer
2k
views
Why a Convolution Matrix Is Called a Kernel?
According to Wikipedia:
In image processing, a kernel, convolution matrix, or mask is a small matrix.
I am wondering, why the matrix is called a kernel? Does it has anything to do with kernel ...
- 213
6
votes
2
answers
2k
views
How to Solve Image Denoising with Total Variation Prior Using ADMM?
I was looking at some articles or Wikipedia on denoising images using the Total Variation norm. The setup is the Rudin Osher Fatemi (ROF) scheme, and the corresponding equation is:
$$
F(u)=\int_{\...
- 257
5
votes
1
answer
429
views
Why Sparse Priors Like Total Variation Opts to Concentrate Derivatives at a Small Number of Pixels?
When performing image deconvolution (deblurring), people often make use of priors to get rid of the illness of the problem. One very common prior is total variation, a sparse prior. Compared to ...
- 497
4
votes
1
answer
2k
views
2D Frequency Domain Convolution Using FFT (Convolution Theorem)
In the time domain I have an image matrix ($256x256$) and a gaussian blur kernel ($5x5$). I've used FFT within Matlab to convert both the image and kernel to the frequency domain as zero padded $...
- 85
4
votes
2
answers
5k
views
Parameters of Gaussian Kernel in the Context of Image Convolution
Hi Everyone i am new at image processing. I copy code from Code with C - Gaussian Filter Generation in C++, I have image $600 \times 480$ gray scale.
What will be the value of standard deviation or $\...
- 41
3
votes
3
answers
5k
views
recover image by only magnitude of image fourier transform
I have a image and I want calculate fft2 of it, after it I want recover image only by magnitude of it. how can I achieve this work?
...
- 133
3
votes
1
answer
708
views
How to Validate Bilateral Filter Implementation?
For this may be a X-Y question, I'm providing the full background story:
I'm trying to test for the correctness for a fixed point implementation of bilateral filter written in C.
My current approach ...
- 135
2
votes
3
answers
8k
views
What is the advantage of weighted median filter over median filter?
I could not find any proof or comparison of weighted median and median filters. Yes the more occurring pixel contributes much but what should be the criteria of choosing weight function. And any ...
1
vote
1
answer
4k
views
How to find out if a transform matrix is separable?
In image processing, when we have a series of basis images, how could we know if the transformation is separable or not?
For example, I know that following bases are separable and transformation can ...
- 399
0
votes
1
answer
1k
views
Recursive Implementation of the Gaussian Filter (1D & 2D)
I'm trying to implement an IIR form to approximate the Gaussian Blur Filter.
I'm working with the article "Recursive Implementation of the Gaussian Filter" by Ian T. Young and Lucas J. van Vliet.
...
- 20.5k
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
34
votes
5
answers
21k
views
What Are the Best Algorithms for Document Image Thresholding (Example Inside)?
I'm trying to implement various binarization algorithms to the image shown:
Here's the code:
clc;
clear;
x=imread('n2.jpg'); %load original image
% Now ...
- 475
33
votes
6
answers
3k
views
Detecting Trail in Forest Images
Is anyone aware of any research/papers/software for identifying a trail (as a line or point-to-point curve) in an image of a forest scene (from the perspective of the camera standing somewhere along ...
- 628
28
votes
4
answers
2k
views
Find the stomata in a plant microscopy image
Here is a question for image processing experts.
I am working on a difficult computer vision problem. The task is to count the stomata (marked below) in DIC microscopy images. These images are ...
- 1,088
25
votes
3
answers
45k
views
What Is the Difference between Difference of Gaussian, Laplace of Gaussian, and Mexican Hat Wavelet?
There are three techniques used in CV that seem very similar to each other, but with subtle differences:
Laplacian of Gaussian: $\nabla^2\left[g(x,y,t)\ast f(x,y)\right]$
Difference of Gaussians: $ \...
- 398
22
votes
1
answer
31k
views
How to Extract High Frequency and Low Frequency Component Using Bilateral Filter?
What does High frequency and low frequency component signify in an image.
How to separate out the high frequency as well as low frequency component of an image using Bilateral filter.
- 457
12
votes
2
answers
2k
views
Auto-focus estimation?
What kind of analysis might be used on sets of 2D monochrome image matrices or bitmaps to estimate or compare their relative quality of being "in focus"? Is there an efficient way of computing a ...
- 35.7k
10
votes
5
answers
6k
views
Auto Detection of Rotation Angle on Arbitrary Image with Orthogonal Features
I have a task at hand where I need to detect angle of an image like the following sample (part of microchip photograph). The image does contain orthogonal features, but they could have different size, ...
- 155
8
votes
2
answers
4k
views
Understanding the Bilateral Filter - Neighbors and Sigma
I have been reading about the Bilateral Filter in - Fast Bilateral Filtering
for the Display of High Dynamic- Range Images and have felt crippled and confused while understanding its working and the ...
- 213
8
votes
1
answer
2k
views
How to Check Separability of 2D Filter / Signal / Matrix
Given:
x(n1,n2) = {1 ,n1=0,n2=0 ;
2 ,n1=1,n2=0 ;
3 ,n1=0,n2=1 ;
6 ,n1=1,n2=1 }
How could one prove it is ...
- 135
7
votes
4
answers
6k
views
Which domain used for denoising additive and multiplicative noises
I want to know what is the difference between additive noise, multiplicative noise.. In what domain these noises are handled to remove?
I am interested particularly in image dataset
Does different ...
- 71
7
votes
2
answers
400
views
Locate Non Homogeneous Areas in an Image
I need to choose some points on an image. These points should be chosen more where there is lots of color changes, transitions and variations. Which techniques can I use for determining where most ...
- 194
6
votes
2
answers
15k
views
Gaussian Blur In MATLAB and Connection to Image Resolution
I am calling fspecial() in MATLAB to create a Guassian blur of an image.
For example,
...
- 163