msg: Clarify sign conventions for optical flow message

msg/optical_flow.msg

@@ -1,16 +1,15 @@
-# Optical flow in NED body frame in SI units.
+# Optical flow in XYZ body frame in SI units.
 # @see http://en.wikipedia.org/wiki/International_System_of_Units
- 
+
 uint8 sensor_id			# id of the sensor emitting the flow value
-float32 pixel_flow_x_integral	# accumulated optical flow in radians around x axis
-float32 pixel_flow_y_integral	# accumulated optical flow in radians around y axis
-float32 gyro_x_rate_integral	# accumulated gyro value in radians around x axis
-float32 gyro_y_rate_integral	# accumulated gyro value in radians around y axis
-float32 gyro_z_rate_integral	# accumulated gyro value in radians around z axis
+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
+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
+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
 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
-