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Preface
Optical flow uses image change processing to detect the state of the ground to monitor the movement of the aircraft; it is mainly used to maintain the horizontal position of the aircraft and achieve fixed height and fixed point flight indoors.
In fact, optical flow is part of the theory of digital image processing. For details, you can read this article. I think it is well explained: Computer Vision-Optical Flow.
PX4 Autonomous Driving User Guide
1. Optical flow sensor
What is optic flow? The name is very professional and feels strange, but in essence, we are most familiar with it. Because of this visual phenomenon we experience every day. In essence, optical flow is the obvious visual movement you feel in this moving world (haha, theory of relativity, there is no absolute stillness and no absolute movement). For example, when you are sitting on a train and you look out the window. You can see the trees, the ground, the buildings, etc., they are all moving back. This movement is optical flow. Moreover, we will all find that their movement speeds are actually different? This provides us with a very interesting piece of information: judging the distance between different targets and us by their movement speed. Some distant targets, such as clouds and mountains, move very slowly and feel like they are stationary. But some objects that are relatively close, such as buildings and trees, recede faster, and the closer they are to us, the faster they recede. Some very close objects, such as road markings, grass, etc., are so fast that they seem to make a whooshing sound in our ears.
In addition to providing distance, optical flow can also provide angle information. The speed of objects moving at 90 degrees to the direction our eyes are facing is faster than that at other angles. When it is as small as 0 degrees, that is, the object directly hits us in the direction, and we just can't feel it. The movement (optical flow) of the object appears to be stationary. As it gets closer to us, it gets bigger and bigger (of course, we still feel speed when we see it, because the object is larger, and its edge still has an angle greater than 0 with our human eyes).
The concept of optical flow was first proposed by Gibson in 1950. It is the instantaneous speed of the pixel movement of a spatially moving object on the observation imaging plane. It uses the changes in the time domain of pixels in the image sequence and the correlation between adjacent frames to find the relationship between the previous frame and the current frame. Correspondence relationship to calculate the motion information of objects between adjacent frames. Generally speaking, optical flow is caused by the movement of the foreground object itself in the scene, the movement of the camera, or the joint movement of both.
2. px4FLOW
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