Why do we care about keypoints?

Image content analysis refers to the process of understanding the content of an image so that we can take some action based on that. Let's take a step back and talk about how humans do it. Our brain is an extremely powerful machine that can do complicated things very quickly. When we look at something, our brain automatically creates a footprint based on the interesting aspects of that image. We will discuss what interesting means as we progress through this chapter.

For now, an interesting aspect is something that's distinct in that region. If we call a point interesting, then there shouldn't be another point in its neighborhood that satisfies the constraints. Let's consider the following image:

Now, close your eyes and try to visualize this image. Do you see something specific? Can you recollect what's in the left half of the image? Not really! The reason for this is that the image doesn't have any interesting information. When our brain looks at something...