PX4的PX4flow光流驱动文件分析
主要还是想找到他们发布的是哪种uORB，这样自己写光流驱动的时候直接用这个，这样也免得改动其他文件。
看头文件可以看出他们用了哪两种msg，和ROS话题一样，msg文件编译后会生成对应相同名称头文件，在cpp里面需要include，和ROS一样，《一本书看懂多旋翼无人机》里面也有讲，在讲msg的时候，比如P145面。
https://gitee.com/amovlab/prometheus_px4/blob/Prometheus_PX4_1.11.1/src/drivers/optical_flow/px4flow/px4flow.cpp
继而在msg文件夹找到对应msg文件
https://gitee.com/amovlab/prometheus_px4/blob/Prometheus_PX4_1.11.1/msg/optical_flow.msg
# Optical flow in XYZ body frame in SI units.
# http://en.wikipedia.org/wiki/International_System_of_Units
uint64 timestamp # time since system start (microseconds)
uint8 sensor_id # id of the sensor emitting the flow value
float32 pixel_flow_x_integral # accumulated optical flow in radians where a positive value is produced by a RH rotation about the X body axis
float32 pixel_flow_y_integral # accumulated optical flow in radians where a positive value is produced by a RH rotation about the Y body axis
float32 gyro_x_rate_integral # accumulated gyro value in radians where a positive value is produced by a RH rotation about the X body axis. Set to NaN if flow sensor does not have 3-axis gyro data.
float32 gyro_y_rate_integral # accumulated gyro value in radians where a positive value is produced by a RH rotation about the Y body axis. Set to NaN if flow sensor does not have 3-axis gyro data.
float32 gyro_z_rate_integral # accumulated gyro value in radians where a positive value is produced by a RH rotation about the Z body axis. Set to NaN if flow sensor does not have 3-axis gyro data.
float32 ground_distance_m # Altitude / distance to ground in meters
uint32 integration_timespan # accumulation timespan in microseconds
uint32 time_since_last_sonar_update # time since last sonar update in microseconds
uint16 frame_count_since_last_readout # number of accumulated frames in timespan
int16 gyro_temperature # Temperature * 100 in centi-degrees Celsius
uint8 quality # Average of quality of accumulated frames, 0: bad quality, 255: maximum quality
float32 max_flow_rate # Magnitude of maximum angular which the optical flow sensor can measure reliably
float32 min_ground_distance # Minimum distance from ground at which the optical flow sensor operates reliably
float32 max_ground_distance # Maximum distance from ground at which the optical flow sensor operates reliably
https://gitee.com/amovlab/prometheus_px4/blob/Prometheus_PX4_1.11.1/msg/distance_sensor.msg
# DISTANCE_SENSOR message data
uint64 timestamp # time since system start (microseconds)
float32 min_distance # Minimum distance the sensor can measure (in m)
float32 max_distance # Maximum distance the sensor can measure (in m)
float32 current_distance # Current distance reading (in m)
float32 variance # Measurement variance (in m^2), 0 for unknown / invalid readings
int8 signal_quality # Signal quality in percent (0...100%), where 0 = invalid signal, 100 = perfect signal, and -1 = unknown signal quality.
uint8 type # Type from MAV_DISTANCE_SENSOR enum
uint8 MAV_DISTANCE_SENSOR_LASER = 0
uint8 MAV_DISTANCE_SENSOR_ULTRASOUND = 1
uint8 MAV_DISTANCE_SENSOR_INFRARED = 2
uint8 MAV_DISTANCE_SENSOR_RADAR = 3
uint8 id # Onboard ID of the sensor
float32 h_fov # Sensor horizontal field of view (rad)
float32 v_fov # Sensor vertical field of view (rad)
float32[4] q # Quaterion sensor orientation with respect to the vehicle body frame to specify the orientation ROTATION_CUSTOM
uint8 orientation # Direction the sensor faces from MAV_SENSOR_ORIENTATION enum
uint8 ROTATION_YAW_0 = 0 # MAV_SENSOR_ROTATION_NONE
uint8 ROTATION_YAW_45 = 1 # MAV_SENSOR_ROTATION_YAW_45
uint8 ROTATION_YAW_90 = 2 # MAV_SENSOR_ROTATION_YAW_90
uint8 ROTATION_YAW_135 = 3 # MAV_SENSOR_ROTATION_YAW_135
uint8 ROTATION_YAW_180 = 4 # MAV_SENSOR_ROTATION_YAW_180
uint8 ROTATION_YAW_225 = 5 # MAV_SENSOR_ROTATION_YAW_225
uint8 ROTATION_YAW_270 = 6 # MAV_SENSOR_ROTATION_YAW_270
uint8 ROTATION_YAW_315 = 7 # MAV_SENSOR_ROTATION_YAW_315
uint8 ROTATION_FORWARD_FACING = 0 # MAV_SENSOR_ROTATION_NONE
uint8 ROTATION_RIGHT_FACING = 2 # MAV_SENSOR_ROTATION_YAW_90
uint8 ROTATION_BACKWARD_FACING = 4 # MAV_SENSOR_ROTATION_YAW_180
uint8 ROTATION_LEFT_FACING = 6 # MAV_SENSOR_ROTATION_YAW_270
uint8 ROTATION_UPWARD_FACING = 24 # MAV_SENSOR_ROTATION_PITCH_90
uint8 ROTATION_DOWNWARD_FACING = 25 # MAV_SENSOR_ROTATION_PITCH_270
uint8 ROTATION_CUSTOM = 100 # MAV_SENSOR_ROTATION_CUSTOM
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应该是有两个uORB发布的，一个是距离，应该就是高度，一个是光流。光流模块本身就是要结合测高传感器来。
PX4flow上面是有超声波的。
https://gitee.com/amovlab/prometheus_px4/blob/Prometheus_PX4_1.11.1/src/drivers/optical_flow/px4flow/px4flow.cpp
https://gitee.com/amovlab/prometheus_px4/blob/Prometheus_PX4_1.11.1/src/drivers/optical_flow/px4flow/px4flow.cpp
我怀疑啊上面pubulish函数里面就是调用的orb_publish函数，这才是真正uORB的发布函数。
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在对应的cmakelists里面应该可以看到这个模块的名称，那么就可以在终端通过模块名称 status查看这个模块的数据。
https://gitee.com/amovlab/prometheus_px4/tree/Prometheus_PX4_1.11.1/src/drivers/optical_flow/px4flow
https://gitee.com/amovlab/prometheus_px4/blob/Prometheus_PX4_1.11.1/src/drivers/optical_flow/px4flow/CMakeLists.txt
当然GPS模块的名称就叫gps 不叫drivers___gps，看来是看main那个后面的名称？
看了《一本书看懂多旋翼无人机》P258上面写的注释清楚了，模块名称应该是横杠后面的部分。PX4FLOW的驱动模块名称就叫px4flow GPS的模型名称就叫gps
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px4flow.cpp里面百分之九十都是写PX4FLOW 这个类，先定义了这个类，类里面具体函数的实现放在类外面写的，也是放在这个cpp里面，所以这么看这个cpp文件就比较清晰了！！！也可以顺带学学C++面向对象 类的写法用法。他们这每个驱动似乎都是一个类，你看看你您不能写出自己的类，哪怕是继承这个PX4FLOW这个类也可以啊，自己写一个。
最后是单独写了个px4flow_main函数，可能是类似于main函数，在这里面对类创建实例化对象来用？
PX4的驱动的类的方法里面经常会看到方法前面加个virtual 虚函数，这也是C++里面的常用用法，不清楚说明你需要去补一补C++基础
拍自《一本书看懂多旋翼无人机》P262
拍自《一本书看懂多旋翼无人机》P281
真正明白虚函数的意义之后，就自然明白下面所说的 洗后函数一般声明成虚函数
拍自《C++程序设计教程》第3版 竞技版 P413
这里甚至建议，尽量将成员函数设计成虚函数，java里面所有的成员函数都是虚函数。这么看PX4的驱动类里面那么多虚函数也是很合理的。
拍自《C++程序设计教程》第3版 竞技版 P407
下面拍自 《C++primer plus》P403 这里有讲virtual
《C++ primer》P552 里面也有专门一小节讲虚析构函数
似乎虚函数是为了实现多态
https://blog.csdn.net/sinat_16643223/article/details/119423437
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我发现PX4的正点原子光流模块的驱动和PX4flow光流模块的驱动发布的uORB都是一个，这么我觉得优象光流也应该是可以用这个的。，这个不是单为PX4flow设计的。
optical_flow.msghttps://gitee.com/maxibooksiyi/prometheus_px4/blob/Prometheus_PX4_1.11.1/src/drivers/optical_flow/pmw3901/PMW3901.hpp
PX4官方手册里给了非常详细的使用方法，设置方法。
这样我连PX4怎么设置也知道了。我可以直接用优象光流替代这个光流，因为我看这个光流模块也没有带有什么测高传感器，那么高度这一块可以暂时不用管对不对。很多其他地方就不用管不用考虑，只需要和它发一样的uORB就可以了应该。
http://docs.px4.io/master/zh/sensor/pmw3901.html
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C++11 中的 override 关键字，可以显式的在派生类中声明，哪些成员函数需要被重写，如果没被重写，则编译器会报错。
https://www.cnblogs.com/xinxue/p/5471708.html
那么再看驱动代码时经常出现的override就好理解了。
