Feature extraction
Generally speaking, a feature is an interesting area of an image. It is a measurable property of an image that is very informative about what the image represents. Usually, the grayscale value of an individual pixel (the raw data) does not tell us a lot about the image as a whole. Instead, we need to derive a property that is more informative.
For example, knowing that there are patches in the image that look like eyes, a nose, and a mouth will allow us to reason about how likely it is that the image represents a face. In this case, the number of resources required to describe the data (are we seeing an image of a face?) is drastically reduced (does the image contain two eyes? a nose? a mouth?).
More low-level features, such as ...
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