All Questions
Tagged with image-processing image-processing
3,214 questions
0
votes
2
answers
244
views
Downsample (aggregate) raster by a non-integer factor, using a Gaussian filter kernel
The task is to downsample (aggregate) a raster from 100m pixel size to 460m. The aggregation should be performed using a Gaussian filter. To better understand the task, I am following the paper ‘The ...
- 111
0
votes
1
answer
45
views
Frequency domain equivalent to center oriented rotated convolution?
I have a convolution kernel which I need to apply to an image. However, at each pixel in the image, I'm convolving using a rotated kernel, with respect to the center (think hands on a clock, with the ...
- 111
1
vote
1
answer
252
views
How many directions are there in the Non Maximum Suppression part of the Canny Edge Detector
In the Non-maximum suppression algorithm, each pixel has a total of 4 directions since there are 8 neighboring pixels. But why there is not 8 directions instead, please? I see below there are 8 ...
- 113
2
votes
1
answer
924
views
Convolving Image with Kernel with Fourier
I am trying to convolve an image with the code above using Convolution Theory and numpy's Fourier transform. However, my output seems to be slightly different than the result from scipy. I am not sure ...
- 23
3
votes
1
answer
66
views
Edge / Pixel Type (Homogenous, Edge, Texture) Classification as Part of an Image Denoising Procedure
For most noise reduction algorithm, the same process is applied to every pixel no matter
the pixel belongs to one of three types of pixels such as homogeneous regions, edges or textures.
Different ...
- 425
1
vote
1
answer
628
views
Fast Way to Remove Isolated Pixels in a Highly Quantized Image
I have an image that I've quantized like so:
And I would like to get rid of pixels that aren't much like their neighbors (basically do a low-pass filter). The goal would be to get rid of isolated ...
- 11
-1
votes
1
answer
126
views
Can FFT2 be used as a randomness test for image encryption?
Based on this link
Is it possible to consider a model to introduce FFT2 as a parameter to determine how random the image is? (randomness test like Entropy)
consider Lena's image and its FFT2:
If we ...
- 51
4
votes
3
answers
439
views
Any Relationship Between the Entropy of an Image and Its Spectrum?
Is there a relationship between the Shannon entropy of image and the output of the 2D Fourier transform (DFT) of the image?
- 51
1
vote
2
answers
569
views
What's the FFT2 of white noise image?
I try to compute the FFT2 of the white noise image.
I use this image
with Entropy 7.995 and
pixel scatter plot:
based on this python code:
...
- 51
6
votes
1
answer
922
views
Why is scaling of images / pixels into `[0, 1]` range performed before SIFT (Scale Invariant Feature Transform) algorithm?
The SIFT paper and the paper of Anatomy of the SIFT Method do not mention that the input images should be preprocessed (normalized, re-scaled) before feeding images into the standard SIFT algorithm. ...
- 213
4
votes
1
answer
251
views
Choice of Laplacian Filter for 2D Images
Context: I am solving a 2-dimensonal discrecte Poission equation ($\mathbf{\nabla}^2x = f$) by assembling a system of linear equations $A x = b$ and then solving it using numpy's integrated solver (np....
- 41
1
vote
1
answer
86
views
Minimum number of patches needed to find color correction matrix (CCM)
The chart should have at least 9 distinct color patches for 3×3 CCMs (which are generally recommended). Excellent results can usually be achieved with the inexpensive, widely-available 24-patch X-Rite ...
- 11
4
votes
4
answers
150
views
What Measure to Compare the Color Depth (Distribution of Colors) of Images
I am looking to compare the color depth of multiple images, in order to see which one has the best color depth. I was wondering what is the best way to compute color depth computationally?
I've seen ...
- 53
1
vote
0
answers
314
views
How to Measure Image Quality in an Objective Way
I need a method to measure image quality via computer programming (like Python). I'm doing this so as to compare quality of multiple images. I have found stuff like NMSE, PSNR and SSIM, but don't ...
- 53
0
votes
1
answer
122
views
Measurement of blocking artifacts in images
I want to define a threshold for maximal blocking artifacts in an image.
What is a good key value to measure blocking artifacts?
I have no reference image
non natural image
Example:
I looked into ...
1
vote
2
answers
1k
views
What does convolving an image with kernel [1 -2 1] do? How is it different than convolving with the derivative kernel [1 -1]?
What does convolving an image with the filter [1 -2 1] do? I see that it's a form of edge detection but it's also very similar to the results I get when convolving with the derivative kernel [1 -1] so ...
- 13
3
votes
1
answer
3k
views
Removing white reflective pixels from scanned RGB image (Python - preferably OpenCV)
The images above are tiles taken from a scanned painting. It's easy to see where there are tiny reflections scattered throughout. I wish to remove (or diminish) the tiny reflections somehow, across a ...
- 161
0
votes
2
answers
214
views
Why we need convolution in image processing?
To my understanding, there are two methods to do linear filtering. One is cross-correlation, and another is convolution. Convolution requires 'flipping' the kernel when you do the calculation.
I think ...
- 23
3
votes
4
answers
832
views
Effect of gaussian blur on FFTs
I am trying to figure out the difference between the FFT of an image, say, cameraman.tif in MATLAB and the FFT of image when the image is Gaussian blurred. What is the effect of the averaging filter ...
7
votes
1
answer
176
views
Why Is PDE Based Image Processing Not as Active as It Used to Be?
I can see from the literature that PDE-based image processing (especially inpainting) was a "hot topic" at one point. Why does it not seem to be an active area of research anymore, yet it ...
- 123
1
vote
3
answers
251
views
What is the frequency response of binning 2x2 pixels of an image into 1 pixel in software?
What is the frequency response of binning 2x2 pixels into 1 pixel in software?
Can the binning introduce aliasing?
Since the Fourier of the 2D boxcar function is a 2D sinc I would intuitively think ...
- 1,783
1
vote
0
answers
37
views
Removing the background of a postcard scan
I'm trying to build a tool to automatically create a "background mask" of a scan of a postcard, allowing pixel information not relating to the postcard itself (ie. the ceiling of the scanner)...
- 111
3
votes
2
answers
1k
views
Why use sinc function to downsample an image in fourier domain?
I'm very confused about downsampling in image processing and the use of sinc function to do it.
I read this post [1]: 2D Fourier downsampling some time ago that talked about my own doubt, that is to ...
- 31
1
vote
1
answer
83
views
bias-variance trade-off in image denoising
I am reading the paper A Bias-Variance Approach for the Nonlocal Means.
One sentence from the paper is as follow:
To discuss the tuning of parameters of the NLM, we interpret this
choice as a bias-...
- 425
1
vote
1
answer
277
views
How to reconstruct image from an Under Sampled MRI k-space data?
Hardware
We have a homemade MRI machine (from the open source project openimaging). It has a single coil and B0 = 0.5 Tesla (enough for teaching).
Sampling Mask
We used a randomly generated sampling ...
- 123
1
vote
1
answer
296
views
How to upscale an image using a Gaussian filter in R?
I want to resample a raster from 15m to 460m using a Gaussian filter.
The goal
I am having a coarse image which I want to downscale. I also have a fine resolution band to assist the downscaling. The ...
- 111
0
votes
0
answers
186
views
I just computed an DFT and FFT of a sinc function, but when I visualise them the results are strangely different? what am I missing
The idea here is to improve a Sinc function by manipulating its Fourier transform.
As you can see, the tophat has ripples in the bands of interest and I want to eliminate them by smoothing them with a ...
- 1
2
votes
0
answers
20
views
Debugging an IP in Vivado
I have a custom-made image processing IP in a Vivado FPGA project which does not work properly. The VHDL logic is OK, I have simulated it outside the IP. The fault must be with the AXI interface and ...
- 53
0
votes
2
answers
154
views
Problem with transforming a tophat to obtain a sinc?
I have observed that the Fourier transform of a tophat function is a Sinc-like function with higher peak sidelobes than an actual Sinc.
However, the Fourier transform of a Sinc is a tophat-like ...
- 1
3
votes
1
answer
49
views
Explain the Terms: Semantically Richer and Spatially More Precise in the HRNet Model Paper
High-resolution representations are essential for position-sensitive vision problems, such as human pose estimation, semantic segmentation, and object detection.
From HR-Nets we get semantically ...
3
votes
1
answer
150
views
What image processing software or technique does google's usercontent website use to serve an image in any size?
I've noticed that Stack Overflow uses lh3.googleusercontent.com to store and serve the profile images of each user. One of the features this website has is that you can easily request a resized ...
- 133
2
votes
2
answers
78
views
Distinguishing between intensity image plots
I have a series of intensity images generated by varying different scatterer geometries. An example of variation observed by varying the aspect ratio of these geometries is shown below in the three ...
- 21
1
vote
0
answers
72
views
how to understand the gradient vector flow formula
Currently, I am learning gradient vector flow. I have one question about the calculation.
After Euler equations, the GVF can be found by solving the following formula:
Then, the paper said: "...
- 11
2
votes
1
answer
497
views
The difference between convolution and cross-correlation from image processing point of view
The difference between convolution and cross-correlation from a signal-analysis point of view
The above question gives very good information from signal processing point of view. But what about images?...
- 1,952
2
votes
3
answers
3k
views
Any good alternatives of ImageJ (Fiji)?
I worked on some medical image processing research two decades ago and I used ImageJ then. I started to pick up some medical image processing projects again (some registration and segmentation). I ...
- 43
0
votes
2
answers
79
views
Signal/image processing using python?Additional IDE must required or default pyhton IDLE is sufficent?
Can we perform signal processing and image processing using default pyhton IDLE(that is downloaded from pyhton website) for scenarios where we may also need plotting etc? or is it necesssary to ...
- 9
4
votes
2
answers
94
views
Wider sharpening filter
The kernel of a sharpening filter of size 3x3 may look like
$$
\frac{1}{4}\begin{bmatrix}
0 & 1 - \alpha & 0\\
1 - \alpha & 4\alpha& 1 - \alpha\\
0 & 1 - \alpha & 0
\end{...
- 205
1
vote
0
answers
86
views
Guided bilinear interpolation: is it a known algorithm?
I have a function that's sampled relatively coarsely along one direction, $x$, and much more finely along another, $y$. Sampling grid is regular. I need to interpolate between all these samples, with ...
- 249
3
votes
2
answers
368
views
Which Blur is being used in this effect
I'm trying to recreate an effect used on photoshop called satin effect, which has an option that control the size of the satin, which I think it's just a special blur applied to the satin pattern
So ...
- 71
2
votes
3
answers
336
views
Why are real-world digital images not bandlimited?
In the materials about image resampling, it always mentions that real-world digital images not bandlimited. However no explanation
is provided.
For example,
Sinc resampling in theory provides the ...
- 425
1
vote
0
answers
253
views
Bring two Fourier transforms to same range to add them
I have Fourier transforms of two images which I wish to add (Basically I have an input Fourier transform which I mask, reconstruct the underlying image using an algorithm, and then try to replace the ...
- 71
3
votes
1
answer
254
views
Why resizing an image smoothed by Gaussian by factor of 2 also increase sigma by factor of 2
In the paper about SIFT algorithm (Distinctive Image Features
from Scale-Invariant Keypoints) it says at the end of section 3.3:
We double the size of the input image using linear interpolation prior ...
- 31
3
votes
0
answers
257
views
Improving the intuition for the 2d fourier transform
As far as I understand, the 2d fourier transform is calculated as following:
...
- 131
3
votes
2
answers
999
views
Metric for image sharpness?
Suppose I have a blurry image: a photo convolved with a gaussian blur kernel of unknown sigma. I would want to deconvolve the blurry image using several gaussian kernels (with different sigmas). Is ...
- 131
2
votes
2
answers
105
views
Image warping with heat map that "pulls" on pixels
I am looking for an efficient image warping function $$f: (\text{image}\in \mathbb R ^{M\times N\times3}, \text{heatMap} \in \mathbb R ^{M\times N}) \to \text{warpedImage} \in \mathbb R ^{M\times N\...
- 143
1
vote
1
answer
887
views
What does hue, saturation and intensity means in image processing?
I ran some code in matlab to show what is hue, saturation and intensity to get an intuition of what they mean.
But the result of the code does not match the explanation i got from here
Hue is a color ...
- 111
2
votes
1
answer
585
views
Noise Reduction - noisy lines
I'm trying to reduce the noise in this photo. What type of filters/techniques should I use to cancel out the noise?
I've tried using box filter and Gaussian filter to blur out some of the noise, but i ...
- 39
2
votes
1
answer
330
views
What's the difference between SIFT and general stereo matching algorithm (eg, sgbm)?
I am working on an open source project s2p which creates digital height model from satellite stereo imagery. The procedure of how s2p works can be roughly summarized with the following steps.
Split a ...
- 213
4
votes
1
answer
177
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
0
votes
1
answer
430
views
Copy an object (along with bounding box) from one image and paste on different image (only background)
I have a dataset of images with bounding boxes.
I also have another dataset of images with different backgrounds.
I want to copy the images from one dataset (with their ground truth boxes) and paste ...
- 31