Questions tagged [sift]

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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 ...
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Registration of point cloud based on feature matching method

I recently find myself an interesting problem to solve. Basically I got 2 partially overlapped point clouds (in real world) that I want automate the registration process. My naive approach currently ...
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Difference of scale space and octave in SIFT

I am not sure about the difference of scale spaces and octaves in SIFT. Are they the same? When we blur the image with different sigma, are we creating an octave or a scale space?
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OpenCv: Homography transformation with perspectiveTransform results in strange data

I am trying to use SIFT to match a template in a scene image (to find an object). For most of my test data my algorithm works pretty well, but for a specific one, it doesn't. I am not completely sure ...
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Understanding Scale-space extrema detection in SIFT

Why do we say that minima in the scale-space domain make for good key points? It makes sense for maxima to be considered, but why do we consider it if it's a minima? This does not cohere with my ...
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Where is the Hessian matrix computed in SURF?

I understood that, in SIFT, the Hessian matrix is used to remove edges from previously detected keypoints. Hence, if I understood well, in SIFT the Hessian matrix is computed only in the locations ...
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Octaves and the SIFT algorithm

My current understanding is that the full algorithm is applied for each octave and then we have four images (the original image and its downsampled version) each with their key points and ...
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How to find objects via SIFT when there are multiple occurrences?

I am trying to find objects (defined via a template image) in smartphone screenshots. Since smartphones have different resolutions, I need scale invariance, so I can't use a template matcher. The use ...
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Are there algorithms that can match SIFT's key points of 2 images using their mutual arrangement?

I'm solving image matching task using SIFT as a feature extractor To match obtained descriptors I used FLANN based Matcher (provided by OpenCV) and result looks good: But applying same algorithm to ...
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1 vote
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feature extraction techniques for iris recognition

I want to ask how I can divide feature extraction techniques to feature detectors and feature descriptors. I have big problem how to understand it. For example I can use Gabor filters (feature ...
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2 votes
1 answer
345 views

Why do we need to construct Gaussian pyramid using SIFT detector

I am learning about SIFT detection and descriptor. I am slightly unsure about why a Gaussian pyramid is built for the image. I do understand that within each octave, we are applying the Difference of ...
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1 answer
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Invariances of FFT-based Image-Registration vs. SIFT-Features

TL;DR: I don't understand how invariant FFT-based image-registration techniques are to object alterations (scratches, marks etc.) in comparison to SIFT-features. I want to build kind of a feature-...
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2 answers
625 views

Why do we need multiple layers in each octave and multiple octaves in SIFT?

I skimmed through the SIFT paper. I understand that there are multiple octaves, which are composed of multiple layers. The layer $k$ of an octave (btw, where does this name come from?) corresponds to ...
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SIFT between 1 image and dataset (2 images)

I use SIFT, and I want to find the angle of inclination of an image compared to two reference images, in short: I have 3 images : Image x: input image Two images 1 and 2, reference images that are ...
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1 answer
205 views

SIFT About Difference-of-Gaussian function extrema?

How to get formula (2) by formula(1)?
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6 votes
1 answer
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Laplacian of Gaussian Approximation and Gaussian Blur as the Solution of Heat Equation

While I was reading SIFT paper(Lowe, 2004), I came across the method that he apply "heat diffusion equation" to Gaussian function to derive that $$ \frac{∂G}{∂σ} = σ∇^2G $$ I searched Wikipedia and ...
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How to obtain the relative position (coordinates) of SIFT feature from image center coordinates?

I have extracted SIFT features (keypoints and descriptors) of training set and saved in an inverted index form. My question is how can I calculate the coordinates of each visual features in an image ...
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VLFEat: How to extract the SIFT descriptor without SIFT feature detection?

I am using the MATLAB interface to the VLFeat toolbox in order to calculate a SIFT descriptor for image patches extracted around key points. Due to the greater framework of what I'm doing, I cannot ...
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Why is scale space (DoG) needed to detect scale invariant features?

I would have a theoretical question, about the detector part of the SIFT algorithm, which as it is explained by D. Lowe in his paper, used the DoG to detect keypoints in an input image. Problem ...
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1 vote
1 answer
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How to recognize an object from a small training set of images?

Given a small training set of images (say, around 6 max) of the same object, how to measure how likely is another query image to contain the given object? The training images are "clean", i.e. only ...
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3 answers
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Intuitive understanding of scale-space extrema detection

Can someone explain intuitively why local maxima and minima in the scale-space domain make for good keypoints? I understand using LoG or DoG zero-crossing points to identify spacial variations, i.e. ...
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Analytical expression for salient Blob detection in scale space

I am working on exact mathematical expression which can be obtained in image processing tasks.(I do not know much about image processing but my work is more mathmatical) In Gaussian scale space, we ...
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Hessian Matrix. Second partial derivative test

I have a question about the hessian matrix / determinant of the hessian matrix. For the Hessian matrix, one is able to determine the local curvature around a point. And with the Second partial ...
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Counting percentage of fouling on a surface

I am completely new to image processing and python. I hope I can find some help here. I am trying out a project on detecting fouling% on a ship hull using raspberry pi. I am trying to learn how this ...
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7 votes
1 answer
111 views

What Is "Description Vector" in Image Processing?

When we want to use classifiers like SVM, we first should extract descriptors using algorithms like SIFT. But I have a question which might call a silly one: Let's assume $$ \begin {equation} D_a=\{...
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Why optical flow ? why not tracking

I have a question that might be stupide ! Supposing that I try to detect object moving in a video or human action. Many works are based on optical flow computation. My question is why using OF is ...
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1 vote
1 answer
144 views

Image Projection with SIFT

Recently, I am thinking about a problem regarding image projection. I have a larger picture as bellow. I want to find the transformation matrix, x'=Hx. When the above image pass the transformation ...
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3 votes
2 answers
244 views

Would the locations of SIFT features generally agree with features detected by Shi-Tomasi method?

I am having a comparison of different feature detection methods. So if we compare the SIFT (Scale-invariant feature transform) and Shi-Tomasi method, will their feature locations agree and why? Are ...
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1 answer
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Using ROC curves for comparing the performance of SIFT and SURF

I want to compare the performance of SIFT and SURF against some standard image set. After a bit of research, I found that ROC curves are one of the best ways to do this task. In order to plot ROC ...
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Is there two Types of Scale Space in SURF algorithm?

I couldn't figure out below paragraph on SURF paper and hope that someone can help me to understand it. Bay H., Ess A., Tuytelaars T. Van Gool L. - Speed-Up Robust Features (SURF), page 4, ...
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2 votes
1 answer
237 views

Can SIFT run in realtime?

What is the best possible runtime for extracting SIFT keypoints and Descriptors? I know it depends on #keypoints extracted. So, say for image of size 640x640, the code that I have been using requires ...
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6 votes
1 answer
182 views

Why Does the Odd Multiple of $ \frac{\pi}{4} $ on Gaussian Cause Loss in Repeatability Under Image Rotations?

I couldn't figure out below paragraph on SURF paper and hope that someone can help me to understand it. Why image rotations around odd multiples of $\frac{\pi}{4}$ lead to a loss of repeatability? ...
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need general resource about image matching

i'm working on image matching mainly SIFT and SURF for my thesis. i read Moravec's and harris corner detectors and understand them quiet well in my own opinion.A COMBINED CORNER AND EDGE DETECTOR[1] ...
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k-NN match for object recognition

The $\frac{d_1}{d_2}<0.6$ gives a large number of false matches and it assumes the nearest neighbour is a correct match. Anybody knows any good method to reduce number of false matches or filter ...
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0 votes
1 answer
491 views

Which is the best method for comparing the histogram obtained after BOW?

Euclidian distance Cosine similarity Histogram Intersection Or any other? Consider the importance of the method in case of images. Eg, Cosine Similarity matches angle of two vectors... What impact ...
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1 vote
2 answers
1k views

what is the output of BoW after an image has been trained with SIFT algorithm and k-means

SIFT algorithm provides a 128 dimensional feature vector that is used for image classification.When all the interest points(key points) are taken together and K-means clustering is applied,the image ...
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1 answer
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What steps come after finding the gradients of the SIFT key points?

I am using a SIFT algorithm to extract features from an image. I understand that the SIFT descriptor first finds the extrema and then finds the gradient and direction of each of these interest points....
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608 views

Distance-metric for SIFT descriptors matching

Will using Cosine-Similarity or Histogram Intersection as distance-metric, for matching SIFT descriptors, help to get better results? Anybody tried on that ever?
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2 answers
220 views

Increasing Recall rate for SIFT

Is there a better way to increase Recall Rate when using SIFT features? I am thinking a way to replace the NN1/NN2 ratio to account for slightly distorted objects. Moving towards clustering and using ...
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2 votes
3 answers
1k views

Visualize visual words in bag of words model

I am implementing visual bag of words through these steps: Find interest points using SIFT Calculate SIFT descriptor Build codebook through kmeans clustering of SIFT descriptors. How can I visualize ...
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1 answer
567 views

Segmentation step when using feature descriptors

In my final year project, we are using a SIFT feature descriptor to classify and recognize objects, but I have never come across segmentation in the process of image classification/categorization. ...
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2 votes
1 answer
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SIFT - Taylor Expansion

I'm trying to implement SIFT algorithm. After I found the min/max points in DOG images I need to find the real min/max subpixels by taylor exapnsion: $$ \begin{equation} D(\mathbf{x}) = D + \frac{\...
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3 votes
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Creating a Gaussian Pyramid

I'm trying to recreate a gassuain pyramid using the following scales: There seems to be two concepts used here: 1) When an image is halved, applying a gaussian kernel of σ will apply as 2σ. 2) When ...
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9 votes
1 answer
379 views

What is the story behind the story about SIFT descriptor?

The following is from Lowe 2004 paper ( http://www.cs.ubc.ca/~lowe/papers/ijcv04.pdf ). One obvious approach would be to sample the local image intensities around the keypoint at the appropriate ...
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6 votes
2 answers
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SIFT - why s+3 scales per octave?

I have a problem with SIFT that I do not understand. Lowe [1] proposed in his work the s=3 levels of scale are enough for one octave. Afterwards, he mentioned that ...
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4 votes
2 answers
6k views

Why is it necessary to implement octaves in sift

I've been studying sift very hard for two weeks. I found much materials about scale space. It is very hard to understand scale space depthly. What I've found and confusing things are that 'scale' in ...
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1 vote
1 answer
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I have a question on SIFT

I am reading a paper on SIFT by Lowe. In it, gradient magnitudes and orientations are calculated using these formulae: $$$$ I don't know why m(x,y) is calculated above. I heard that gradient ...
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taylor expansion of scale space function

I see the following expression from http://en.wikipedia.org/wiki/Scale-invariant_feature_transform The quadratic Taylor expansion of the Difference-of-Gaussian scale-space function, with the ...
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  • 457
1 vote
1 answer
416 views

Keypoint position and scale in SIFT

If I understood it right, a keypoint is a tuple $$(x,y,\sigma, r),$$ where $x,y$ define the position of the keypoint and $r$ an orientation (given by the most domiant gradiant vector around the ...
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180 views

Affine detector + SIFT descriptor

I am approaching to the Oxford Buildings dataset which contains already computed SIFT descriptors from affine detected keypoint locations. Usually SIFT detector provides to its detected keypoints ...
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