Global and local image features

7 important questions on Global and local image features

What properties should feature vectors of a global image descriptor be in order to allow comparing under different sized images?

Same feature vector size and normalized to have the same order of magnitude.

Name an approach to allow comparing images with a variable number of local image features.

Bag of words

What local image feature descriptors does the "Bag of Words" aproach use, and how is this converted into a comparable (fixed) feature vector?

Words. Converted by building a histogram on word frequency.
  • Higher grades + faster learning
  • Never study anything twice
  • 100% sure, 100% understanding
Discover Study Smart

What is a local image descriptor?

A feature of an image that can be present or absent, that allows to be collected and combined/shaped into a global image descriptor.

What are three important things in consideration of the number and quality of codewords for available local image features?

  • The number of codewords should be sufficiently many in such that classes can be distinguished by them.
  • The number of codewords should be limited general in such that resulting codeword-histograms are dense.
  • Codewords should be sufficiently discriminative to be able to distinguish classes.

What role can K-Means play in the process of converting local image descriptors into classifying images?

K-Means clustering can be applied to find clusters of similar images by the (Chi-squared) distance of their codewords-histogram.

What are the five steps for the Bag of Words approach?

  1. Find keypoints in image
  2. Calculate local feature descriptors (with e.g. SIFT)
  3. Map local descriptors to codewords by computing distance to codebook cluster centers.
  4. Normalize the BoW histogram to unit length.

The question on the page originate from the summary of the following study material:

  • A unique study and practice tool
  • Never study anything twice again
  • Get the grades you hope for
  • 100% sure, 100% understanding
Remember faster, study better. Scientifically proven.
Trustpilot Logo