synch problem [closed]

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I am using 2 webcams with openCV on Raspberry Pi 3 model B (not simultaneously). If I use "Logitech" one, my python program makes nice pictures and writes them to a timestamped PNG file. Now I start using a "NGS XpresCam 300". I take one picture and it is OK. I wait 2 minutes without moving the camera, take another picture and the left part (1 centimeter) of it has "moved" to the right of the pic. I wait 2 minutes more, take another, and now half of the picture (split vertically) is moved between right side and left side of the pic. Do you have any clue about the reason for this behavior ? In /var/log/messages there is nothing special, neither in "dmesg" I attach a "god" pic and a "bad" one.

Good picture : C:\fakepath\webcam_20160815_090626_almost_good.png +++

Bad picture : C:\fakepath\webcam_20160815_090207_2_halfs_reversed.png +++

Sometimes it gets a bit worse, losing synchronism few times in the same pic : C:\fakepath\webcam_20160815_091614_sync_error.png +++

This last pic is quite smaller because I used less resolution - width (320), height (240). Previous were width (640), height (480).

Thanks.