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learning target
Understand the characteristics of images
Know the corners of the image
1. Characteristics of images
Most of us have played jigsaw puzzles. First get the fragments of the complete image and then arrange the fragments in the correct way to reconstruct the image. If the principle of jigsaw puzzle is written into a computer program, then the computer will also be able to play jigsaw puzzle.
When putting together a puzzle, we are looking for unique features that are suitable for tracking and easy to compare. We search for such features in one image, find them, find them in other images, and stitch them together.
So how do you find these features in a computer?
If we look deeply into some images and search for different areas, take the following image as an example:
six small images are given above the image. Find the location of these small images in the original image. How many correct results can you find?
Although the approximate positions of A, B, C, and D can be known, the exact positions are still difficult to find. That's because: along the edge, it's the same everywhere.
So edges are better features than flat surfaces, but not good enough.
Finally, you can find that E and F are some corner points of the building. They can be found easily. Because at the corners, no matter which direction you move the small image, the result will be very different. So think of them as a good feature. To better understand this concept, let’s take another simple example.
In the picture above:
- The area in the blue box is a plane that is difficult to find and track. No matter which direction you move the blue box, it's the same.
- For the area in the black box, it is an edge. If you move the image vertically, it will change, but if you move it horizontally, it will not change.
- The corner points in the red box, no matter which direction you move, will get different results, which means that the area in the red box is unique to the green rectangular area . Therefore, we say that the corner point is a good image feature, which answers the previous question.
Corner points are very important features in images and play an important role in the understanding and analysis of image graphics.
Corner points play an important role in computer vision fields such as three-dimensional scene reconstruction, motion estimation, target tracking, target recognition, image registration and matching.
In the real world, corner points correspond to corners of objects, intersections of roads, T-shaped building openings, etc.
Below we use various algorithms in OpenCV to find the features of the image and describe them.
Summarize
In this chapter, we introduced the concepts of image features and corner points. The concept of corner points is very important and must be carefully written down and used accurately.