# 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 ...
9 views

### 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 ...
9 views

### 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?
44 views

### 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 ...
19 views

### 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 ...
24 views

### 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 ...
23 views

### 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 ...
21 views

### 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 ...
1 vote
74 views

### 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 ...
1 vote
24 views

### 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 ...
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 ...
1 vote
77 views

### 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-...
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 ... 51 views

### 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 ...
205 views

### SIFT About Difference-of-Gaussian function extrema?

How to get formula (2) by formula(1)?
811 views

### 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 ...
226 views

### 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 ...
1 vote
261 views

### 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 ...
1 vote
2k views

### 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 ...
1 vote
417 views

### 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 ...
2k views

### 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. ...
1 vote
45 views

### 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 ...
225 views

### 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 ...
43 views

### 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 ...
111 views

2k views

### 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 ...
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 ...
2k views

### SIFT - why s+3 scales per octave?

I have a problem with SIFT that I do not understand. Lowe  proposed in his work the s=3 levels of scale are enough for one octave. Afterwards, he mentioned that ...
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 ...
1 vote
171 views

### 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 ...
2k views

### 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 ...
1 vote
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 ...