point matching

Point features can be used to find a sparse set of corresponding locations in different images.

There are two main approaches to finding feature points and their correspondences:

  1. The first is to find features in one image that can be accurately tracked using a local search technique such as correlation or least squares; -more suitable when images are taken from nearby viewpoints or in rapid succession (e.g., video sequences)
  2. The second is to independently detect features in all the images under consideration and then match features based on their local appearance – more suitable when a large amount of motion or appearance change is expected, e.g., in stitching together panoramas

keypoint detection and matching pipeline four separate stages:

  1. feature detection (extraction) stage – each image is searched for locations that are likely to match well in other images
  2. second feature description stage – each region around detected keypoint locations in converted into a more compact and stable (invariant) descriptor that can be matched against other descriptors
  3. third feature matching stage – efficiently searches for likely matching candidates in other images
  4. fourth feature tracking stage – an alternative to the third stage that only searches a small neighborhood around each detected feature and is therefore more suitable for video processing

Two steps:

  1. point detection – finds points or patches in the image that have saliency – interest points
  2. point description – assigns a feature vector to each point

The NNs of a point in one image are found in another image:

nn

 

There are many existing point detection and description algorithms, SIFT return not only the location of the points, but also the scale and orientation.

Point matching as classification – instead of computing point feature vectors and finding the NNs, each point in the ‘training’ image is a class.

Why corner?

– junctions of the contours

more stable features over the changes of viewpoint

large variations in the neighbourhood of the point in all directions

– Corner is the good feature to match!

Corner points

Looking at intensity values within a small window, shifting the window in any direction should yield a large change in appearance.

Harris Corner detector

Harris corner detector gives a mathematical approach for determining which case holds:

  • flat region: no change in all directions
  • edge: no change along the edge direction
  • corner: significant change in all directions

 

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