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Frames dropped during video recording?

I have done several recordings using the IntelĀ® RealSenseā„¢ Depth Camera D435 (https://www.intelrealsense.com/depth-camera-d435/).

I checked the BAG files using rosbag info, and got the number of frames from the number of "messages" for the /device_0/sensor_1/Color_0/image/data topic. Also, I know the duration of the video from rosbag info as well.

Is this the right process for checking if frames were dropped during recording? I know the D435 camera should be recording at 30 fps, so I multiplied the duration by 30 to get the number of expected frames. Then I subtract the number of "messages" from that to get the number of frames dropped.

For example, I have pasted an example output from rosbag info below. The number of "messages" is 20882. The duration is 699s. So the number of frames dropped is (699s x 30fps) - 20882 = 88 frames dropped.

Can anybody confirm this is correct?

Wed Sep 11 16:33:18 EDT 2019
path:         <BAG_FILENAME>
version:      2.0
duration:     11:39s (699s)
start:        Dec 31 1969 19:00:00.00 (0.00)
end:          Dec 31 1969 19:11:39.81 (699.81)
size:         35.9 GB
messages:     125361
compression:  lz4 [20883/20883 chunks; 66.67%]
uncompressed: 53.8 GB @ 78.7 MB/s
compressed:   35.9 GB @ 52.5 MB/s (66.67%)
types:        diagnostic_msgs/KeyValue  [cf57fdc6617a881a88c16e768132149c]
              geometry_msgs/Transform   [ac9eff44abf714214112b05d54a3cf9b]
              realsense_msgs/StreamInfo [311d7e24eac31bb87271d041bf70ff7d]
              sensor_msgs/CameraInfo    [c9a58c1b0b154e0e6da7578cb991d214]
              sensor_msgs/Image         [060021388200f6f0f447d0fcd9c64743]
              std_msgs/Float32          [73fcbf46b49191e672908e50842a83d4]
              std_msgs/String           [992ce8a1687cec8c8bd883ec73ca41d1]
              std_msgs/UInt32           [304a39449588c7f8ce2df6e8001c5fce]
topics:       /device_0/info                                                         9 msgs    : diagnostic_msgs/KeyValue 
              /device_0/sensor_0/info                                                1 msg     : diagnostic_msgs/KeyValue 
              /device_0/sensor_0/option/Depth Units/description                      1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Depth Units/value                            1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Emitter Enabled/description                  1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Emitter Enabled/value                        1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Enable Auto Exposure/description             1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Enable Auto Exposure/value                   1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Error Polling Enabled/description            1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Error Polling Enabled/value                  1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Exposure/description                         1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Exposure/value                               1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Frames Queue Size/description                1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Frames Queue Size/value                      1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Gain/description                             1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Gain/value                                   1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Inter Cam Sync Mode/description              1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Inter Cam Sync Mode/value                    1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Laser Power/description                      1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Laser Power/value                            1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Output Trigger Enabled/description           1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Output Trigger Enabled/value                 1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Stereo Baseline/description                  1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Stereo Baseline/value                        1 msg     : std_msgs/Float32         
              /device_0/sensor_0/option/Visual Preset/description                    1 msg     : std_msgs/String          
              /device_0/sensor_0/option/Visual Preset/value                          1 msg     : std_msgs/Float32         
              /device_0/sensor_1/Color_0/image/data                              20882 msgs    : sensor_msgs/Image        
              /device_0/sensor_1/Color_0/image/metadata                         104410 msgs    : diagnostic_msgs/KeyValue 
              /device_0/sensor_1/Color_0/info                                        1 msg     : realsense_msgs/StreamInfo
              /device_0/sensor_1/Color_0/info/camera_info                            1 msg     : sensor_msgs/CameraInfo   
              /device_0/sensor_1/Color_0/tf/0                                        1 msg     : geometry_msgs/Transform  
              /device_0/sensor_1/info                                                1 msg     : diagnostic_msgs/KeyValue 
              /device_0/sensor_1/option/Auto Exposure Priority/description           1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Auto Exposure Priority/value                 1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Backlight Compensation/description           1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Backlight Compensation/value                 1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Brightness/description                       1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Brightness/value                             1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Contrast/description                         1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Contrast/value                               1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Enable Auto Exposure/description             1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Enable Auto Exposure/value                   1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Enable Auto White Balance/description        1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Enable Auto White Balance/value              1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Exposure/description                         1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Exposure/value                               1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Frames Queue Size/description                1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Frames Queue Size/value                      1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Gain/description                             1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Gain/value                                   1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Gamma/description                            1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Gamma/value                                  1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Hue/description                              1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Hue/value                                    1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Power Line Frequency/description             1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Power Line Frequency/value                   1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Saturation/description                       1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Saturation/value                             1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/Sharpness/description                        1 msg     : std_msgs/String          
              /device_0/sensor_1/option/Sharpness/value                              1 msg     : std_msgs/Float32         
              /device_0/sensor_1/option/White Balance/description                    1 msg     : std_msgs/String          
              /device_0/sensor_1/option/White Balance/value                          1 msg     : std_msgs/Float32         
              /file_version                                                          1 msg     : std_msgs/UInt32