Image noise filtering - image-processing

I am looking for inputs on an image noise filter method. A 9-pixel median filter does not work very well with dense noise. Noise is periodic (periods of 50 lines) and additive.
Thanks,
Bi

What about filtering in the Fourier domain? If the noise is periodic then with any luck your noise will appear as a pair of nice pointy features in Fourier space, where you can filter them with a couple of Gaussians then transform back to real space and your periodic noise should be gone.

I like to use selective blurs, which finds the average of only the surrounding pixels whos values are within a certain range from the value of the center pixel.
Gimp has a weighted version of this called "selective gaussian blur" you could try to see what this looks like.

Related

I have to detect a drastic change of intensity within an image

I have a thermal image of human standing either carrying a cold tool or a hot tool. I want to find the place this tool is. So basically i am trying to make an image processing filter which would give me the area of the place where drastic change of intensity of gray color occurs in the relatively smoother background. I have tried canny edge detector but it gives a lot of noise.
Hot Object To be detected: https://imgur.com/0ZyK6WP
Cold object to be detected: https://imgur.com/YYT9rHW
You might increase the Gaussian smoothing kernel to filter out the noise, but that might result in losing out the edges. So in that case you might want to use filter that would preserve the edge and also smooth out the image. Something like Bilateral filter could help in that case. It replaces the intensity of each pixel with a weighted average of intensity values from nearby pixels.
Also have you tried different threshold values foe non-max suppression. As that might be helpful when dealing with false positives.

Noise reduction using histograms in opencv

I want to implement a simple noise correction scheme for RGB images. The image contain some garbage pixels at random locations. So, I am thinking of doing this:
Segment the image.
Calculate histograms for each segment.
Analyze the histogram and dump the pixels which are negligible in histogram distribution over a segment.
I am using openCV. I have implemented steps 1 and 2, but I am not able to find out the number of pixels in each histogram bin. Please help
In order to analyze a histogram, you have to make a few assumptions about it. One good assumption is that the histogram will be roughly modeled as noise + gaussian bell curves.
Check this out.
http://en.wikipedia.org/wiki/Root-finding_algorithm
Finding the roots of the derivative function of the histogram will give you the location of the peaks. You can then find the boundaries of each peak, possibly by finding the roots of the second derivative function.
After you identify the location and span of the histogram peaks, you can classify pixels as being signal or noise pixels.

How to calculate an image has noise and Geometric distortion or not?

I need to make an application in iphone which needs to calculate noise, geometric deformation other distortions in an image. How to do this? I have done some image processing stuff with opencv + iphone. But I dont know how to calculate these parameters.
1) How to calculate noise in an image?
2) What is geometric deformation and how to calculate geometric deformation of an image?
3) Is geometric deformation and distortion are same parameters in terms of image filter? or any other distortions available to calculate an image is good quality or not?
Input: My image is a face image in live video stream.
I advise you to read some literature about image processing, for example Gonzalez & Woods.
1) The simplest method of noise calculation by single image is to compute standard deviation between image and its smoothed copy. For smoothing I recommend you to use simple median filter by sample of 3x3 pixels (or more). Median is non-sensitive to outbursts of data, so noice like "salt-n-pepper" won't worsen statistics.
In cases of overexposed or underexposed images such method can give you bad results, in that case you can calculate FFT of image and use a high frequency components for noise estimation.
2), 3) Calculation of geometric deformation is possible only if you know, what should be on image. For example, if you use mire (optical etalon) with quadratic grid, you can find lines on your image (for example by Canny edge detector) and compute distortion, astigmatism and some other aberrations. This could be done also if you sure that image have some straight lines.
Defocusing can be computed from analysis of edges on image or with help of image wavelet transform.
There also much more different methods for image analysing. For example, by analysis of colour image you can estimate chromatic aberration and so on.
But I repeat: in common case this operations are impossible. They all have some particular cases of application.
Read about image quality: there are no standard for this term, in every particular case you can use one or more simple characteristics to recognize whether image good or not.
In you case I'd advice you to make a lot of photos with different kind of artefacts and quality, then make simple analysis of their statistics, wavelet compositions and R-G-B components correlation. BTW, to make analysis of colour image less sensitive to its brightness I recommend you to work in HSV colorspace (but to estimate chromatic aberration you need to work exactly with RGB components).

How to implement despeckle in OpenCV?

If histogram equalization is done on a poorly-contrasted image then its features become more visible. However there is also a large amount of grains/speckles/noise. using blurring functions already available in OpenCV is not desirable - i'll be doing text-detection on the image later on and the letters will get unrecognizable.
So what are the preprocessing techniques that should be applied?
Standard blur techniques that convolve the image with a kernel (e.g. Gaussian blur, box filter, etc) act as a low-pass filter and distort the high-frequency text. If you have not done so already, try cv::bilateralFilter() or cv::medianBlur(). If neither of these algorithms work, you should look into other edge-preserving smoothing algorithms.
If you imagine the image as a three-dimensional space, traditional filtering replaces the value of each pixel with the weighted average of all filters in a circle centered around the pixel. Bilateral filtering does the same, but uses a three-dimensional sphere centered at the pixel. Since a well-defined edge looks like a plateau, the sphere contains only one point and the pixel value remains unchanged. You can get a more detailed explanation of the bilateral filter and some sample output here.

Gaussian blur and convolution kernels

I do not understand what a convolution kernel is and how I would apply a convolution matrix to pixels in an image (I am talking about doing a Gaussian Blur operation on an image).
Also could I get an explanation on how to create a kernel for a Gaussian Blur operation?
I am reading this article but I cannot seem to understand how things are done...
Thanks to anyone who takes time to explain this to me :),
ExtremeCoder
The basic idea is that the new pixels of the image are created by an weighted average of the pixels close to it (imagine drawing a circle around the pixel).
For each pixel in the image you are going to create a little square around the pixel. Lets say you take the 8 neighbors next to a pixel (including diagonals even though do not matter here), and we perform a weighted average to get the middle pixel.
In the Gaussian blur case it breaks down to two one dimensional operations. For each pixel take the some amount of pixels next to a pixel in the row direction only. Multiply the pixel values time the weights computed from the Gaussian distribution (or if you are doing this for an visual effect and not for a scientific reason, the weights can anything that looks good) and sum them up. Another way to look at it is the pixel make a vector and the weights make a vector and your are taking the dot product. Repeat this process in the column direction as a separate pass.
A convolution kernel is a matrix of values that specify how the neighborhood of a pixel contribute to that pixel's state in the final image. There's a fair description of the basics here. A gaussian blur is a convolution function that uses a really ugly (you've seen the wikipedia page) function to compute a convolution kernel to pass over the image. You'll find an example kernel for a gaussian in that wikipedia page.
The point of all the math in there is to produce a soft blur that resembles the scatter pattern produced by a mesh screen placed between the viewer and the image. You can think of the 'size' (the standard deviation) of the gaussian as being related to the distance between the image and the screen.
Here's an awesome tool, if you don't want to calculate it all by yourself (like me):
http://www.embege.com/gauss/
EDIT
Since the link seems to be broken now, here's a link to archive.org:
http://web.archive.org/web/20150217075657/http://www.embege.com/gauss

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