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Update the README on geotagging videos with a GPX - #857
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Interval sampling stamped each frame with the start time of the video plus its time in the video. A timelapse packs several seconds of capture into each second of video, so its frames were stamped that many times too close together. Geotagged from a GPX by those stamps, they all landed near the start of the track. Geotagged from the video's own track, they moved so far per second that the speed check could reject them: a 30x timelapse of a drive read as over 1,000 km/h. Interval sampling now reads the rate from the video's GPS clock, as GPX sync does, and stamps the frames at it. The start time is mapped back from the first GPS timestamps at the same rate. --video_duration_ratio, which used to apply to every video, now paces only the videos without a GPS clock, so that a ratio given for those leaves the other videos in the batch alone. Each video whose clock overrides a given ratio says so in a warning. The samples are then geotagged from the video's own track (the gopro, camm and blackvue sources) by their time since the first sample, while the track is timed in video seconds. It is now timed in GPS seconds, at the rate of its clock. That also fixes distance samples of a timelapse, which were already stamped with their capture times.
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The previous commit timed the GPS points of a timelapse at one rate from
its first timestamp. Distance samples are stamped with the GPS time of
their video time instead, and the first timestamp of a GoPro MAX
timelapse can be about 3 s early: in one, the camera moved 6 m at 8 m/s
in the 3.9 s to the next one. On two such timelapses (30x and 15x), the
frames that geotagged were placed a median of 36 m and 5 m from where
the sampler picked them, and the last 2 and 1 frames were stamped past
the end of the track:
The image time is 1.801 seconds beyond the GPX end point
Time the track by the timestamps, interpolated between them and
extrapolated beyond them at the clock's rate. Stamp interval samples of
a timelapse the same way, from the GPS time at video time 0, as distance
samples are.
On those two timelapses, distance samples now all geotag where they
were picked. Interval samples (0.5 s) stay where they were with
--geotag_source gopro, and with a GPX of the same track they move from
a median of 0.6 m and 0.04 m (4.4 m and 1.4 m at most) to within 1 cm.
Videos without a timelapse clock are unchanged.
--interpolation_use_gpx_start_time places the GPX from the start of the video instead of by the video's GPS clock. It skipped reading the clock altogether, and with it the clock's rate, so a timelapse got its GPX laid out at 1x: a 5 s, 30x timelapse got 157 s of GPX, and distance sampling picked 5 frames along its first 12 m instead of 148 along the 1.4 km route. The clock is now read in that mode too, and placed at the start of the GPX, so that only its rate is used. A video that is not a timelapse has a rate of 1, and is placed as before. The coverage check still sees such a track as one that no clock placed: the clock gave it only its pace.
A timelapse packs several seconds of capture into each second of video. With a GPS clock of its own, a GPX is synced at the clock's rate. Without one, nothing in the video tells the rate, and the GPX was laid out at 1x, so the frames got only the start of the track: an Insta360 ONE X timelapse, 1.8 s of video with a 110 s GPX, gave 1 frame. --video_duration_ratio, the seconds of real time per second of video, used to apply to interval sampling only, and was marked deprecated. It now paces the GPX of a video without a GPS clock in distance sampling too, whether the GPX is placed by --video_start_time, by --interpolation_use_gpx_start_time or by assuming that it starts with the video. The GPS clock of a video, if it has one, still wins, and says so in a warning when it overrides a ratio. The ratio is required where a track runs on long after the video and nothing else accounts for it. Without a start time, a track more than twice as long as the video was already refused; it is now measured against the time the video spans at the ratio, and the error names the ratio. With --interpolation_use_gpx_start_time, such a track is now refused as well, in interval sampling too: the video is a timelapse, or the track was left running after it or covers more than this video. For a track of less than a day, which may be a GPS logger left running, the error gives the start of the track to pass as --video_start_time instead, which places the video the same way. A longer track is a log of several trips, which that would put the video on the first of: the error asks for the time the video started. With --video_start_time nothing is required, so that one GPX can serve many videos, each placed by its start time. The ratio divides GPS time now, so one that is not positive is rejected. process is unchanged.
The previous commit checks a GPX that --interpolation_use_gpx_start_time
says started with a video without a GPS clock. Interval sampling reads
the clock from the video's telemetry, which is empty when its GPS is
rejected as noise. GPX sync reads the clock from that GPS all the same,
and syncs the GPX at that clock's pace, as in process and in distance
sampling. So a 1.24 s GoPro MAX 2 clip with noisy GPS and a 12x clock,
which main samples with its 12 s GPX, was refused:
The GPX track in GS018205.360.gpx lasts 12 seconds and the video only
1.24 seconds, so it runs on long after the video ends.
Skip the check when GPX sync reads a clock from the video. The clip
samples as on main again.
It said that a GPX works with interval sampling only, which has not been true since distance sampling honours --geotag_source, and that the start time of a video is always read from its GPS or its creation time, which distance sampling does not do. It now says how each kind of video is placed along a GPX, that --video_start_time applies to every video in a command, so one GPX serves several videos run one at a time, how --video_duration_ratio paces a timelapse without a GPS clock, and what to pass instead of --interpolation_use_gpx_start_time for a GPS logger left running. The examples sample one video, and the --interpolation_use_gpx_start_time one drops the interval sampling options it no longer needs.
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The README said that a GPX works with interval sampling only, which hasn't been true since #841, and that a video's start time is always read from its GPS or creation time, which distance sampling doesn't do. It now says:
--video_start_timeapplies to every video in a command, so one GPX serves several videos run one at a time;--video_duration_ratiopaces a timelapse without a GPS clock;--interpolation_use_gpx_start_timefor a GPS logger left running (Sync a GPX to a video without a GPS clock at --video_duration_ratio #856 prints it).The examples sample one video, and the
--interpolation_use_gpx_start_timeone drops the interval sampling options it no longer needs.It doesn't touch "Limitations of
--video_geotag_source", which #851 rewrites.