Before selecting key frames, it should be observed that the key frames should not be too dense or too sparse. At the same time, it is necessary to ensure that sufficient local map points are generated. Based on these considerations, the key frame strategy is selected as follows:
（1）当前帧跟踪到的地图点太少，一般设置为 50。
（2）当前帧跟踪到的地图点中，包含上一关键帧的地图点太少。一般设置小于上一关键帧地图点数的一定比例，如 0.75。
Condition (1) describes the situation that the number of feature points tracked by the current image is small. It shows that the current scene has changed a lot from the previous key frame. At this point, you need to insert a new key frame as soon as possible to avoid the visual front-end tracking failure.
Condition (2) describes that there are fewer map points tracked to the key of the previous frame in the current image. This shows that the camera has moved a certain distance from the insertion of the last key frame at this time, and you can choose to insert a new key frame.
The key frame selection strategy in Orbslam:
Determine whether the current frame can be used as a key frame. Since there is a mechanism to filter redundant key frames in the process of local map construction, we need to insert new key frames as soon as possible to ensure that the tracking thread is more robust to the camera's movement, especially to rotational movement. We insert new key frames according to the following requirements:
(1) 距离上一次全局重定位后需要超过20帧图像。
(2) 局部地图构建处于空闲状态，或距上一个关键帧插入后，已经有超过20帧图像。
(3) 当前帧跟踪少于50个地图云点。
(4) 当前帧跟踪少于参考关键帧K_ref云点的90%。