Motion - Config Option Pre Capture

pre_capture

  • Type: Integer
  • Range / Valid values: 0 - 100s
  • Default: 0 (disabled)
  • Group: Output - General

Specifies the number of previous frames to be outputted at motion detection. Recommended range: 0 to 5, default=0. Do not use large values! Large values will cause Motion to skip video frames and cause unsmooth mpegs. To smooth mpegs use larger values of post_capture instead., Specifies the number of previous frames to be outputted at motion detection. Recommended range: 0 to 5, default=0. Do not use large values! Large values will cause Motion to skip video frames and cause unsmooth mpegs. To smooth mpegs use larger values of post_capture instead.

Motion buffers the number of picture frames defined by 'pre_capture'. When motion is detected the pictures in the buffer are included in the video clip generated by ffmpeg. The effect is that it seems the program knew in advance that the event was going to take place and started the recording before it actually happened. This is a nice feature that give more complete video clips of an event.

If pre_capture is set to 0 the feature is disabled. Keep this value below 5.

The recommended value would be approx 0.5 second of video so the value should be defined so it fits the framerate and the desired pre-capture time. E.g. 0.5 second at 20 frames pr second would mean a value of 5. You should never use a value larger than 10.

You can in theory have up to 100s of pre-captured frames but naturally this makes motion leave a larger footprint in the memory of the computer. More important Motion is processing all the buffered images including saving jpegs, encoding mpegs, writing to databases and executing external programs after the first image is detected as Motion.

Motion will not grab another image until this is done. This means that even moderate values for pre_capture combined with high framerates will mean that you will miss quite many frames of Motion. It is therefore recommended to use relative small values for pre_capture. Depending on your chosen framerate and depending on the features enabled values from 1-5 are sensible.

If you wish to create smooth mpegs during events using large pre_capture values will do the opposite! It will create a long pause where a lot of action is missed.

To get a smooth mpeg use a large value for post_capture which does not cost any performance hit or RAM space.
Topic revision: r8 - 07 Aug 2016, KennethLavrsen
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