Questions tagged [sift]

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46 questions
51 views

How to get formula (2) by formula(1)?
263 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 ...
84 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 ...
231 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 ...
669 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 ...
402 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 ...
1k 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. ...
35 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 ...
141 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 ...
38 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 ...
84 views

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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 ...
298 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 ...
1k 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 ...
4k 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 ...
133 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 ...
1k 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 ...
351 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 ...
174 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 ...
2k views

Detecting a filled glass object

I've been sent here from this question in stackoverflow, please excuse me if the question comes too specific and it's not in the manners in here:) The task is to find a glass with specific liquid in ...
971 views

How to proceed for BOW creation From sift features

I extracted sift feature to construct a bag-of-words model. I did the following to extract the feature vector: Extracted features from the key frames. After the feature extraction I tried using ...
66 views

Geometric models to discard image features

I am working on the implementation of a location recognition pipeline as detailed in the section 5.4 of the paper Leveraging 3D City Models for Rotation Invariant Place-of-Interest Recognition: ...
1k views

Object-Recognition using SIFT: Switching the order of images creates two different results

I've developed a large test-system which can use every interest-point-detector and -descriptor. I use it for evaluation of the given algorithms with the standford-image-collection of cd-covers. This ...
16k views

Image classification using SIFT features and SVM

I am hoping someone can explain how to use the bag of words model to perform image classification using SIFT/SURF/ORB features and a support vector machine? At the moment I can compute the SIFT ...
15k views

Scale and Rotation invariant feature descriptors

Can you list some scale and rotational invariant feature descriptors for use in feature detection. The application is for the detection of cars and humans in video captured by a UAV, using a multi-...
186 views

What scale should a detected SIFT point be extracted at?

I've heard this from a few different researchers but I can't find a definitive value. Given the "scale" extracted by David Lowe's SIFT detector, we need to extract a descriptor at a specific scale. ...
4k views

Is sift a good way to extract features from an image?

I am trying to extract features from an image, but I have failed to get the points that I want to extract, and hence my image fails to match with the template. Are there any checkpoints that I have ...