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I'm using VLC to capture images on a webcam over time to build a timelapse video:

vlc -I dummy v4l2:// --video-filter scene -V dummy --scene-ratio 200 --scene-format png --scene-prefix img --scene-path ~/Pictures/timelapse/

I left this command running all day, but when I watched the video created from the images, I noticed the video speed increases right before sunset. Examining file timestamps for daylight hours:

-rw-r--r-- 1 mazur mazur 515282 2011-08-28 18:56 img427401.png
-rw-r--r-- 1 mazur mazur 515393 2011-08-28 18:56 img427601.png
-rw-r--r-- 1 mazur mazur 516564 2011-08-28 18:56 img427801.png
-rw-r--r-- 1 mazur mazur 513762 2011-08-28 18:56 img428001.png
-rw-r--r-- 1 mazur mazur 511077 2011-08-28 18:56 img428201.png
-rw-r--r-- 1 mazur mazur 506817 2011-08-28 18:56 img428401.png
-rw-r--r-- 1 mazur mazur 501374 2011-08-28 18:56 img428601.png
-rw-r--r-- 1 mazur mazur 499168 2011-08-28 18:56 img428801.png
-rw-r--r-- 1 mazur mazur 497036 2011-08-28 18:57 img429001.png
-rw-r--r-- 1 mazur mazur 494534 2011-08-28 18:57 img429201.png
-rw-r--r-- 1 mazur mazur 491844 2011-08-28 18:57 img429401.png
-rw-r--r-- 1 mazur mazur 488306 2011-08-28 18:57 img429601.png
-rw-r--r-- 1 mazur mazur 489109 2011-08-28 18:57 img429801.png
-rw-r--r-- 1 mazur mazur 490133 2011-08-28 18:57 img430001.png
-rw-r--r-- 1 mazur mazur 490122 2011-08-28 18:57 img430201.png
-rw-r--r-- 1 mazur mazur 487513 2011-08-28 18:57 img430401.png

Reveals that 8 photos or so are taken every minute. Looking a little bit later though:

-rw-r--r-- 1 mazur mazur  71436 2011-08-28 20:19 img500801.png
-rw-r--r-- 1 mazur mazur  71587 2011-08-28 20:20 img501001.png
-rw-r--r-- 1 mazur mazur  71111 2011-08-28 20:20 img501201.png
-rw-r--r-- 1 mazur mazur  72671 2011-08-28 20:21 img501401.png
-rw-r--r-- 1 mazur mazur  70913 2011-08-28 20:22 img501601.png
-rw-r--r-- 1 mazur mazur  70841 2011-08-28 20:23 img501801.png
-rw-r--r-- 1 mazur mazur  71105 2011-08-28 20:24 img502001.png
-rw-r--r-- 1 mazur mazur  71398 2011-08-28 20:25 img502201.png
-rw-r--r-- 1 mazur mazur  70392 2011-08-28 20:25 img502401.png
-rw-r--r-- 1 mazur mazur  70255 2011-08-28 20:26 img502601.png
-rw-r--r-- 1 mazur mazur  70640 2011-08-28 20:27 img502801.png
-rw-r--r-- 1 mazur mazur  68704 2011-08-28 20:28 img503001.png
-rw-r--r-- 1 mazur mazur  70236 2011-08-28 20:29 img503201.png

You can see that the rate drops to 1-2 per minute, causing the speed-up effect before sunset.

The documentation for --scene-ratio says:

--scene-ratio <integer>    Recording ratio
    Ratio of images to record. 3 means that one image out of three is recorded.

So what's going on here? The file names of the files show that there's nothing weird going on with whether the correct images are being recorded - it's still one image out of every 200.

Does the webcam take photos at different rates depending on the amount of light available or something? That's my only guess because it happened when it started to get dark, unless that was a coincidence.

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