I am stuck in a pixel matching algorithm for finding symbols in an image. I have two images of symbols that I intend to find in an image that has big resolution.
Instead of a pixel by pixel matching algorithm, is there a fast algorithm that gives the same result as that of pixel matching algorithm. The result should be similar to: (percentage of pixel matched) divide by (total pixels).
My problem is that I wish to find certain symbols in a 1 bit image. The symbol appear with exact similarity in the target image and 95% of total pixel match with the target block in the image. but it takes hours to do iterations. The image is 10k X 10k and the symbol size is 20 X 20, so it will 10 power of 10 calculations which is too much to handle. Is there any filter/NN combination or any other algorithm that can give same results as that of pixel matching in a few minutes?
The point here is that pixels are almost same in the but problem is that size is very large. I do not want complex features for noise handling or edges, fuzzy etc. just a simple algorithm to do pixel matching quickly and the result should be similar to: (percentage of pixel matched) divide by (total pixels)
object recognition is tricky in that any simple algorithm is generally going to be way too slow, as you've apparently realized.
Luckily, if you have a rather large collection of these images on hand that are already correctly labeled, then I have a very simply solution for you.
Simply make 3 layer feedforward network with one input unit per pixel, all of which connect to a much smaller hidden layer, and then those in turn connect to 1 output unit (representing which symbol is present in the image). Then just run the backpropagation algorithm on your dataset until the network learns to identify the symbols.
Unfortunately, this doesn't scale very well, so you might have to look into convolutional NNs for better performance.
Additionally, if you don't have any training data (i.e. labeled examples), then your best bet is probably to decompose your symbols into features and then sweep the image for those. If you can decompose them into lines, then a hough transform can do this quite rapidly.
Maybe an (Adaptive Resonance Theory) ART-1 network could help.
The algorithm can also be written that all Prototypes are checked in parallel in the same time and it can be blazing fast because it esentially uses binary math a lot.
Related
I'm trying to develop a way to count the number of bright spots in an image. The spots should be gaussian point sources, but there is a lot of noise. There are probably on the order of 10-20 actual point sources in this image. My first though was to use a gaussian convolution with sigma = 15, which seems to do a good job.
First, is there a better way to isolate these bright spots?
Second, how can I 'detect' the bright spots, i.e. count them? I haven't had any luck with circular hough transforms from opencv.
Edit: Here is the original without gridlines, here is the convolved image without gridlines.
I am working with thermal infrared images which subject to quantity of noises.
I found that low rank based approaches such as approaches based on Singular Value Decomposition (SVD) or Weighted Nuclear Norm Metric (WNNM) give very efficient result in terms of reducing the noise while preserving the structure of the information.
Their main drawback is the fact they are quite slow to compute (several minutes per image)
Here is some litterature:
https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7067415
https://arxiv.org/abs/1705.09912
The second paper has some MatLab code available, there is quite a lot of files but the translation to python is should not that complex.
OpenCV implement as well (and it is available in python) a very efficient algorithm on the Non-Local Means algorithm:
https://docs.opencv.org/master/d5/d69/tutorial_py_non_local_means.html
Im using GPUImage in my project and I need an efficient way of taking the column sums. Naive way would obviously be retrieving the raw data and adding values of every column. Can anybody suggest a faster way for that?
One way to do this would be to use the approach I take with the GPUImageAverageColor class (as described in this answer), only instead of reducing the total size of each frame at each step, only do this for one dimension of the image.
The average color filter determines the average color of the overall image by stepping down in a factor of four in both X and Y, averaging 16 pixels into one at each step. If operating in a single direction, you should be able to use hardware interpolation to get an 18X reduction in a single direction per step with good performance. Your final step might either require a quick CPU-based iteration on the much smaller image or a tweaked version of this shader that pulls the last few pixels in a column together into the final result pixel for that column.
You notice that I've been talking about averaging here, because the output values for any OpenGL ES operation will need to be in terms of colors, which only have a 0-255 range per channel. A sum will easily overflow this, but you could use an average as an approximation of your sum, with a more limited dynamic range.
If you only care about one color channel, you could possibly encode a larger value into the RGBA channels and maintain a 32-bit sum that way.
Beyond what I describe above, you could look at performing this sum with the help of the Accelerate framework. While probably not quite as fast as doing a shader-based reduction, it might be good enough for your needs.
I need to detect corners on grayscale images with the highest possible accuracy. Currently I am using the OpenCV function: cvFindCornerSubPix().
I prepared a simple test: got an image with a corner of black/white edges:
and then a series of the same object, moved 1/16pixel each. I did check pixel values manually, test images are just fine.
Detection results were disappointing:
Even though in TermCrit the condition is set to 100 iterations or 0.005 threshold, detection error gets as big as 0.08 pixel.
The graph shows error as a function of position within a pixel. Does not look random at all. Another thing worth a note: for other anular positions of the corner (when edges are not horizontal/vertical) results are better, but still not perfect.
Any idea, how to make this function work properly, why it does not, or what to use instead?
I would greatly appreciate any advice
Less than 10% of a pixel really isn't bad performance at all. For reference, a correlation peak detector suitable for the production of 3D model from satellite images will have the same order of magnitude of error.
As pointed out in the comments, the exact error pattern will depend on the interpolation method that you use to generate the subpixel pattern. In order to avoid the non-monotonicity introduced by higher-order interpolation methods (beyond order 2), I would suggest the following protocol:
Generate you input image in a high-res, 16 times bigger;
Move your target by 1 pixel at a time in this HR image;
Produce your test images by downsampling (careful: apply an appropriate blurring function like a PSF first if you go for brutal downsampling in order to avoid aliasing) to the correct size.
Finally, it is often not desirable to go to a smaller error magnitude. The subpixel corner detector was designed to be used in images where many (typically between 20 and 100) points are detected. These points are then used in a robust estimation process that should remove outliers and average the error on the valid point sets.
My lecturer has slides on edge histograms for image retrieval, whereby he states that one must first divide the image into 4x4 blocks, and then check for edges at the horizontal, vertical, +45°, and -45° orientations. He then states that this is then represented in a 14x1 histogram. I have no idea how he came about deciding that a 14x1 histogram must be created. Does anyone know how he came up with this value, or how to create an edge histogram?
Thanks.
The thing you are referring to is called the Histogram of Oriented Gradients (HoG). However, the math doesn't work out for your example. Normally you will choose spatial binning parameters (the 4x4 blocks). For each block, you'll compute the gradient magnitude at some number of different directions (in your case, just 2 directions). So, in each block you'll have N_{directions} measurements. Multiply this by the number of blocks (16 for you), and you see that you have 16*N_{directions} total measurements.
To form the histogram, you simply concatenate these measurements into one long vector. Any way to do the concatenation is fine as long as you keep track of the way you map the bin/direction combo into a slot in the 1-D histogram. This long histogram of concatenations is then most often used for machine learning tasks, like training a classifier to recognize some aspect of images based upon the way their gradients are oriented.
But in your case, the professor must be doing something special, because if you have 16 different image blocks (a 4x4 grid of image blocks), then you'd need to compute less than 1 measurement per block to end up with a total of 14 measurements in the overall histogram.
Alternatively, the professor might mean that you take the range of angles in between [-45,+45] and you divide that into 14 different values: -45, -45 + 90/14, -45 + 2*90/14, ... and so on.
If that is what the professor means, then in that case you get 14 orientation bins within a single block. Once everything is concatenated, you'd have one very long 14*16 = 224-component vector describing the whole image overall.
Incidentally, I have done a lot of testing with Python implementations of Histogram of Gradient, so you can see some of the work linked here or here. There is also some example code at that site, though a more well-supported version of HoG appears in scikits.image.
I´m trying to make an implementation of Gaussian blur for a school project.
I need to make both a CPU and a GPU implementation to compare performance.
I am not quite sure that I understand how Gaussian blur works. So one of my questions is
if I have understood it correctly?
Heres what I do now:
I use the equation from wikipedia http://en.wikipedia.org/wiki/Gaussian_blur to calculate
the filter.
For 2d I take RGB of each pixel in the image and apply the filter to it by
multiplying RGB of the pixel and the surrounding pixels with the associated filter position.
These are then summed to be the new pixel RGB values.
For 1d I apply the filter first horizontally and then vetically, which should give
the same result if I understand things correctly.
Is this result exactly the same result as when the 2d filter is applied?
Another question I have is about how the algorithm can be optimized.
I have read that the Fast Fourier Transform is applicable to Gaussian blur.
But I can't figure out how to relate it.
Can someone give me a hint in the right direction?
Thanks.
Yes, the 2D Gaussian kernel is separable so you can just apply it as two 1D kernels. Note that you can't apply these operations "in place" however - you need at least one temporary buffer to store the result of the first 1D pass.
FFT-based convolution is a useful optimisation when you have large kernels - this applies to any kind of filter, not just Gaussian. Just how big "large" is depends on your architecture, but you probably don't want to worry about using an FFT-based approach for anything smaller than, say, a 49x49 kernel. The general approach is:
FFT the image
FFT the kernel, padded to the size of the image
multiply the two in the frequency domain (equivalent to convolution in the spatial domain)
IFFT (inverse FFT) the result
Note that if you're applying the same filter to more than one image then you only need to FFT the padded kernel once. You still have at least two FFTs to perform per image though (one forward and one inverse), which is why this technique only becomes a computational win for large-ish kernels.