capture , output 都是 /dev/video? 只是 index 不一樣。 capture 是 video0, output 是 video16. 在 v4l2-dev.c 的 video_register_device 有 /dev 下名字的分類： /** * video_register_device - register video4linux devices * @vdev: video device structure we want to register * @type: type of device to register * @nr: which device node number (0 == /dev/video0, 1 == /dev/video1, ... * -1 == first free) * @warn_if_nr_in_use: warn if the desired device node number * was already in use and another number was chosen instead. * * The registration code assigns minor numbers and device node numbers * based on the requested type and registers the new device node with * the kernel. * An error is returned if no free minor or device node number could be * found, or if the registration of the device node failed. * * Zero is returned on success. * * Valid types are * * %VFL_TYPE_GRABBER - A frame grabber * * %VFL_TYPE_VTX - A teletext device * * %VFL_TYPE_VBI - Vertical blank data (undecoded) * * %VFL_TYPE_RADIO - A radio card */ capture 和 output 的 type 都是 VFL_TYPE_GRABBER. capture device 的 register function , nr 是 -1 . output device 的 register function, nr 是