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# Tag Info

Accepted

### Why should an image be blurred using a Gaussian Kernel before downsampling?

An image "should not be blurred using a Gaussian Kernel" in general. This however can be a safe bet for a lot of basic image processing needs, and a smoothing is almost mandatory when you ...
• 31.9k
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### What is different between Euclidean distance and the Geodesic distance?

Simple: the Euclidean distance completely ignores the shape when finding a path from the start point to the end point while, for the geodesic distance, the path is constrained to be within the given ...
• 25.8k
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### What Is the Difference between Difference of Gaussian, Laplace of Gaussian, and Mexican Hat Wavelet?

Laplace of Gaussian The Laplace of Gaussian (LoG) of image $f$ can be written as $$\nabla^2 (f * g) = f * \nabla^2 g$$ with $g$ the Gaussian kernel and $*$ the convolution. That is, the Laplace ...
• 2,584

### Why color is coded with two numbers rather than one?

Note: also check out the comments for more pointers. I'll assume that you think of wavelengths, or hue and saturation, which is why you think you could maybe represent color as a single value. You ...
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### Why real part of FFT converts image into rotation + original?

Let's say your image is given by $I(x,y)$. Then its Fourier transform is given by $$I^f(\omega_x,\omega_y) = \int_x\int_yI(x,y)e^{j\omega_xx}e^{j\omega_yy}dxdy$$ Now you take the real part and ...
• 1,460
Accepted

### what does "derivative" means in image processing?

Look at the numbers in the filter kernel in just 1 dimension for simplicity. For a Sobel and Prewitt matrix you have something that roughly looks like this ...
• 4,134
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### Which image filter can be applied to remove gridded pattern from corrupt jpegs?

You can use a standard inpainting algorithm. These algorithms replace marked pixels in an image with the pixel values that surround these marked pixels. The challenge here is to detect the grid (my ...
• 6,218
Accepted

### DSP or signal/image/data processing jokes

An airplane is leaving Warsaw (the capital of Poland), and gets caught in a terrible winter storm. The plane rolls, pitches and yaws. The crew is expecting the plane to crash or break up at any time. ...
• 31.9k
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### Does a simple photograph contain more information than a complex painting?

It depends how you define the term "information" or "entropy". The conventional definition of entropy of an image is to think the image as a two-dimensional matrix of pixels and H = - \sum_k p_k \...
• 6,595
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### Why does this Moiré pattern look like this?

You'll need to understand the sampling theorem. In short, each signal has what we call a spectrum¹, which is the Fourier transform of the signal as it comes in time domain (if it is a time signal), or ...
• 31.1k
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### Auto Detection of Rotation Angle on Arbitrary Image with Orthogonal Features

If I understand your method 1 correctly, with it, if you used a circularly symmetrical region and did the rotation about the center of the region, you would eliminate the region's dependency on the ...
• 13.5k

### Is deep learning killing image processing/computer vision?

Today we had a discussion with a friend of mine. It was a rainy day here in Munich, while a large portion of Europe was having a kind of sunny atmosphere. People were sharing photographs in social ...
• 5,465

### In opecv function Hough circles how does parameter 1 and 2 affect circle detection and how can I adjust them to increase accuracy?

Intuition for parameters of HoughCircles: image: 8-bit, single channel image. If working with a color image, convert to grayscale first. method: Defines the method to detect circles in images. ...
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### 2D Convolution as a Doubly Block Circulant Matrix Operating on a Vector

The point is that circular convolution of two 1-D discrete signals can be expressed as the product of a circulant matrix and the vector representation of the other signal. The circulant matrix is a ...
• 4,295

### Peak Signal to Noise Ratio (PSNR) in Python for an Image

You can use cv2.PSNR like this example: import cv2 img1 = cv2.imread('img1.bmp') img2 = cv2.imread('img2.bmp') psnr = cv2.PSNR(img1, img2)
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### Python vs Matlab? Which one better for image processing?

[Comparison with cars at the end] For learning from scratch, I would not suggest a programming language alone, but instead the couple "teaching materials" (book, lecture notes) + "...
• 31.9k

### How do I get the most accurate camera calibration?

I decided to post this answer here because a while back, this came up as the top result in Google and its suggestions helped me. So I decided to share my experience too. Having spent countless hours ...
• 201
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### When is a network called end-to-end training?

From feature extraction to learning the desired result, deep learning algorithms can act as full pipelines for solving tasks at hand. End-to-end learning usually refers to omitting any hand-crafted ...
• 5,465
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### Subpixel - what is it?

Images are technically limited to the original pixel resolution. Objects in an image are spatially quantized at this resolution. However, an edge for instance may be localized "between" native pixels. ...
• 31.9k
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### Why does the separable filter reduce the cost of computing the operator?

Assume you have a $N\times M$ sized image. If you know take what is classically used, a square filter kernel, of let's say size $L\times L$, you'd need to convolve that with the picture – which gives ...
• 31.1k

### DSP or signal/image/data processing jokes

In high school math class I finally got the courage up to ask the cute and brainy girl at the front row out on a date. She just looked at me with a discouraging face and as she waved her hand face ...
• 52k
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### Which Noise Reduction Algorithms Are Used in Commercial RAW Image Processors?

Common Approaches for Commercial Denoisers Commercial denoisers are different than what you'd see on most papers. While on papers the results are mostly using objective metrics (PSNR / SSIM) and are ...
• 19.7k
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### Pros, Cons, and Applications of SVD-based Image Compression (Singular Value Decomposition)

The idea of using PCA with images is to look at the margin ensembles generated by the image. That means, we consider two distributions: The distribution of the column over the realisations of the rows....
• 4,583

### Best way of segmenting veins in leaves?

Following on from the above excellent answer, here is how to do it in python using scikit funcitons. ...
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### How to calculate particles sizes for spheric particles with overlapping and superposition? (Example image included)

Here is what I experimented with: Use ELSD to generate elliptic contours. You could basically use any edge detector, but since in the following stages I will benefit from circle detectors, it is good ...
• 5,465

### Why real part of FFT converts image into rotation + original?

The result ThP provided can also be stated in very simple terms: If you have a data set which is purely real, its (inverse) Fourier transform will have Hermitian symmetry: If you find the value $z$ at ...
• 1,736

### DSP or signal/image/data processing jokes

Crazy Audio Fx ilustration lol more:
• 2,038

### Why should an image be blurred using a Gaussian Kernel before downsampling?

According to (digital) sampling theorem, signals should be properly bandlimited, before they are (down) sampled. A practical digital filter approximately limits the bandwidth of the signal and makes ...
• 28.3k