安卓系統採用v4l2接口打開YUYV和MJPEG攝像頭，支持熱插拔。
1､需求：安卓5.1系統，視頻監控門禁主板(rk3288)上要用到兩個攝像頭，且兩個攝像頭要同時打開。
2､背景：開始使用安卓系統的camera 2接口時，無法同時打開兩個攝像頭。
然後有個攝像頭廠商幫改了一下攝像頭固件，可以用camera 2接口同時打開兩個攝像頭了，但有時攝像頭死機。
聽說uvc接口非常穩定，可是我們的安卓系統不支持uvc。（如果你的系統支持uvc，這篇文章可以不用看了。如果是linux系統可以參考一下）
於是想採用v4l2接口。
完整工程下載地址：https://download.csdn.net/download/alterli/12050693
v4l2接口支持兩種攝像頭數據格式“V4L2_PIX_FMT_YUYV”和“V4L2_PIX_FMT_MJPEG”，網上大部分示例給出的是採用“V4L2_PIX_FMT_YUYV”格式的，
但我們這個攝像頭只支持“V4L2_PIX_FMT_MJPEG”，於是有了這個示例。
本示例基於github上的工程擴展的，回過頭來想寫引用出處，同名稱的太多了，https://github.com/search?q=simplewebcam，不記得是哪個項目了。（兄弟，對不住了）
3､攝像頭支持的數據格式：
我的理解是攝像頭可以同時支持“V4L2_PIX_FMT_YUYV”和“V4L2_PIX_FMT_MJPEG”，或只支持其中一個，實際情況是大部分只支持V4L2_PIX_FMT_YUYV。
我手上有7款攝像頭，6個只支持“V4L2_PIX_FMT_YUYV”（其中一個是羅技的），1個只支持“V4L2_PIX_FMT_MJPEG”（就是攝像頭廠商幫我們修改過固件的）。
4､帶寬
V4L2_PIX_FMT_MJPEG相比V4L2_PIX_FMT_YUYV，輸出的圖像數據格式不一樣，帶寬小了很多。
在我的測試中，640*480像素，V4L2_PIX_FMT_MJPEG一幀只有8k(這個值會變的),
而V4L2_PIX_FMT_YUYV一幀是614K（614400＝640*480*2)。
爲什麼我的系統中無法同時打開兩個V4L2_PIX_FMT_YUYV的攝像頭，大概是與usb hub的帶寬有關係吧？或者ＡＰＰ中使用方法有問題？
可能的解決方法有1､攝像頭直接接到rk3288上，而不是掛在下一級hub上；2､換一個hub芯片,把GL850換成GL852；3、改用V4L2_PIX_FMT_MJPEG格式。
方法1,2沒試過，我們採用了方法3。
5､攝像頭設備名
以只接一個攝像頭爲例，有的安卓系統中，插拔攝像頭時，攝像頭設備名在/dev/video0和/dev/video1之間變換，有的一直固定爲0.（有的系統中，攝像頭名是從/dev/video4開始增長的。）
6､V4L2_PIX_FMT_MJPEG幀數據示例
測試時順便打印了一個完整的V4L2_PIX_FMT_MJPEG幀，只有8106字節，貼一下有個直觀印象（湊稿費）：
FFD8 (Start of image)
FFDB (Define quantization table) 008400120C0D0F0D0B120F0E0F141312151A2C1D1A18181A362629202C403843423F383E3C464F6556464B604C3C3E58785960686C71727144557C7F7B6E7F656F716D011314141A171A341D1D346D493E496D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D6D
FFC0 (Baseline DCT) 00110801E0028003012100021101031101
FFDD (Define restart interval) 00040000
FFC4 (Define Huffman table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
FFDA (Start of scan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