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Preprocessing

All preprocessing is applied at load time by passing arguments to MgVideo. Steps execute in a fixed order: trim → skip → fix → rotate → contrast/brightness → crop → grayscale.

import musicalgestures as mg

mv = mg.MgVideo(
    '/path/to/video.avi',
    starttime=5,
    endtime=15,
    skip=3,
    rotate=90,
    contrast=100,
    brightness=20,
    crop='auto',
    color=False,
)

Trimming

Use starttime and endtime (in seconds) to work on a portion of the video:

mv = mg.MgVideo('/path/to/video.avi', starttime=5, endtime=15)

Frame skipping

skip=n keeps every (n+1)th frame, reducing processing time:

mv = mg.MgVideo('/path/to/video.avi', skip=2)

Fixing a frame count

frames extracts a fixed number of frames, useful for batch processing files of different lengths:

mv = mg.MgVideo('/path/to/video.avi', frames=1000)
mv = mg.MgVideo('/path/to/video.avi', frames=-1)    # keyframes only

Rotation

rotate accepts any angle in degrees:

mv = mg.MgVideo('/path/to/video.avi', rotate=90)
mv = mg.MgVideo('/path/to/video.avi', rotate=5.31)

Contrast and brightness

Both parameters are percentages in the range −100 to 100. A value of 0 leaves the video unchanged:

mv = mg.MgVideo('/path/to/video.avi', contrast=100, brightness=20)

Cropping

crop='auto' detects the region of motion and crops to it. crop='manual' opens a window where you draw the crop rectangle, then press c to confirm or r to reset:

mv = mg.MgVideo('/path/to/video.avi', crop='auto')
mv = mg.MgVideo('/path/to/video.avi', crop='manual')

Grayscale

color=False converts the video to grayscale and keeps all subsequent processes in grayscale mode, which can reduce processing time:

mv = mg.MgVideo('/path/to/video.avi', color=False)

Resampling (frame rate, speed, frame decimation)

Unlike the options above, resample() is a method called on an already-loaded MgVideo. It returns a new MgVideo and leaves the original object untouched, so you can branch off a re-timed copy:

mv = mg.MgVideo('/path/to/video.avi')

mv25 = mv.resample(fps=25)         # retime to 25 fps (duration-preserving)
fast = mv.resample(speed=2.0)      # play 2× faster (video + audio retimed in sync)
slow = mv.resample(speed=0.5)      # play 2× slower / longer
dec  = mv.resample(skip=2)         # discard 2 frames for every one kept
mv25.show()

Three independent, combinable operations:

  • fps: retime to a target frame rate via FFmpeg's fps filter — duration-preserving (frames are dropped/duplicated to hit the rate), e.g. 30 → 25 fps.
  • speed: change playback speed by a factor (>1 faster/shorter, <1 slower/longer); the video (setpts) and the audio (atempo) are retimed together so they stay in sync.
  • skip: integer frame decimation — discard skip frames for every one kept (this also shortens/speeds up the clip), matching the loader's skip parameter.

When more than one is given they are applied in order: skip → speed/fps. The output filename defaults to the input name with a _resampled suffix; target_name and overwrite work as for the other methods.

Keeping intermediate files

By default only the final preprocessed video is kept. Set keep_all=True to retain the result of each step:

mv = mg.MgVideo('/path/to/video.avi', starttime=5, endtime=15, skip=3,
                rotate=90, contrast=100, brightness=20, crop='auto',
                color=False, keep_all=True)

This produces files like video_trim.avi, video_trim_skip.avi, video_trim_skip_rot.avi, and so on.

Output file names and overwriting

Every analysis method accepts target_name and overwrite:

  • target_name sets the output file path. If None (default), a suffix is appended to the source name — for example, history on dance.avi produces dance_history.avi in the same directory.
  • overwrite=True (default) replaces the existing file in place — re-running a method overwrites the previous result instead of leaving stale copies behind.
  • overwrite=False silently increments the filename if one already exists: dance_history.avi → dance_history_0.avi → dance_history_1.avi, and so on (the old auto-increment behaviour).
mv = mg.MgVideo('/path/to/video.avi')
mv.history(target_name='/output/my_history.avi', overwrite=False)   # keep every run

Next steps