YUV2摄像头相关数据大小计算

视备支持的分辨率格式如下：

当我们使用YUV2 1280x720x10格式打开摄像头，通过BUSHOUND抓包如下：

Length    Phase  Data                                                                          Description   
--------  -----  ----------------------------------------------------------------------------------- ------
    CTL    21 01 00 05  00 02 01 00                                                            SET CUR       
 1  OUT    02                                                                                  .             
    CTL    a1 81 00 01  01 00 1a 00                                                            GET CUR       
    CTL    21 01 00 01  01 00 1a 00                                                            SET CUR       
26  OUT    00 00 02 01  40 42 0f 00  00 00 00 00  00 00 1e 00  00 00 00 00  00 00 00 00  00 00 
    CTL    a1 81 00 01  01 00 1a 00                                                            GET CUR       
    CTL    a1 83 00 01  01 00 1a 00                                                            GET MAX       
    CTL    a1 82 00 01  01 00 1a 00                                                            GET MIN       
    CTL    21 01 00 01  01 00 1a 00                                                            SET CUR       
26  OUT    00 00 02 01  40 42 0f 00  00 00 00 00  00 00 1e 00  00 00 00 20  1c 00 00 00  00 00 
    CTL    a1 81 00 01  01 00 1a 00                                                            GET CUR       
    CTL    21 01 00 02  01 00 1a 00                                                            SET CUR       
26  OUT    00 00 02 01  40 42 0f 00  00 00 00 00  00 00 1e 00  00 00 00 20  1c 00 00 0c  00 00 
    CTL    01 0b 06 00  01 00 00 00                                                            SET INTERFACE 
    CTL    a1 83 00 14  00 01 0a 00                                                            GET MAX       
    CTL    21 01 00 14  00 01 0a 00                                                            SET CUR       
10  OUT    a3 01 5d 02  9a 02 53 03  1d 00                                                    
    CTL    a1 81 00 14  00 01 0a 00                                                            GET CUR       
    CTL    a1 83 00 14  00 01 0a 00                                                            GET MAX       
    CTL    a1 87 00 14  00 01 0a 00                                                            GET DEF       
    CTL    21 01 00 14  00 01 0a 00                                                            SET CUR       
10  OUT    00 00 00 00  00 00 00 00  10 00                                                      
    CTL    a1 81 00 14  00 01 0a 00                                                            GET CUR       
    CTL    a1 83 00 14  00 01 0a 00                                                            GET MAX       
    CTL    21 01 00 14  00 01 0a 00                                                            SET CUR       
10  OUT    9d 01 2d 02  98 02 27 03  1d 00                                                     
    CTL    a1 81 00 14  00 01 0a 00                                                            GET CUR       
`

通过SET_CUR提交到视频流接口的数据长度为26可知，此UVC摄像头使用的UVC版本为1.0。

• wPFrameRate:
• bmHint = 00 00
• bFormatIndex = 02
• bFrameIndex = 01
• dwFrameInterval = 40 42 0f 00 = 0x000f4240 = 1,000,000 ns = 100ms即每秒10帧
• wKeyFrameRate = 00 00
• wPFrameRate = 00 00
• wCompQuality = 00 00
• wCompWindowSize = 1e 00 = 30
• wDelay = 00 00
• dwMaxVideoFrameSize =00 20 1c 00 = 0x001C2000==1,843,200
• dwMaxPayloadTransferSize = 00 0c 00 00 = 0x00000c00=3072，即一个USBD_ISO_PACKET_DESCRIPTOR的最大数据长度。

其对应的描述符如下：

---> This is the Default (optimum) Frame index
bLength                  : 0x1E (30 bytes)
bDescriptorType          : 0x24 (Video Streaming Interface)
bDescriptorSubtype       : 0x05 (Uncompressed Frame Type)
bFrameIndex              : 0x01
bmCapabilities           : 0x00
wWidth                   : 0x0500 (1280)
wHeight                  : 0x02D0 (720)
dwMinBitRate             : 0x08CA0000 (147456000 bps -> 18.432 MB/s) = 1280*720*2*8*10=147,456,000
dwMaxBitRate             : 0x08CA0000 (147456000 bps -> 18.432 MB/s) 
dwMaxVideoFrameBufferSize: 0x001C2000 (1843200 bytes)                = 1280*720*2=1,843,200
dwDefaultFrameInterval   : 0x000F4240 (100.0000 ms -> 10.000 fps)
bFrameIntervalType       : 0x01 (1 discrete frame interval supported)
adwFrameInterval[1]      : 0x000F4240 (100.0000 ms -> 10.000 fps)
Data (HexDump)           : 1E 24 05 01 00 00 05 D0 02 00 00 CA 08 00 00 CA   .$..............
                           08 00 20 1C 00 40 42 0F 00 01 40 42 0F 00         .. ..@B...@B..

下面对照 UVC 视频流接口控制请求 进行数据分析:

dwMaxVideoFrameSize(0x18):00 20 1c 00 即0x001c2000=1843200=1280*720*2,由于这里为YUV2格式，所以2个Y对应一个UV,所以数据帧的总大小为1280*720*2，具体可参见 YUV数据格式 一节。

打开摄像头时，选择接口的内容如下：

 CTL    01 0b 06 00  01 00 00 00                      SET INTERFACE

可见选择接口ID=1,转换接口为6的接口内的端点，其描述符如下：

        ---------------- Interface Descriptor -----------------
bLength                  : 0x09 (9 bytes)
bDescriptorType          : 0x04 (Interface Descriptor)
bInterfaceNumber         : 0x01
bAlternateSetting        : 0x06
bNumEndpoints            : 0x01 (1 Endpoint)
bInterfaceClass          : 0x0E (Video)
bInterfaceSubClass       : 0x02 (Video Streaming)
bInterfaceProtocol       : 0x00
iInterface               : 0x00 (No String Descriptor)
Data (HexDump)           : 09 04 01 06 01 0E 02 00 00                        .........

        ----------------- Endpoint Descriptor -----------------
bLength                  : 0x07 (7 bytes)
bDescriptorType          : 0x05 (Endpoint Descriptor)
bEndpointAddress         : 0x81 (Direction=IN EndpointID=1)
bmAttributes             : 0x05 (TransferType=Isochronous  SyncType=Asynchronous  EndpointType=Data)
wMaxPacketSize           : 0x1400
 Bits 15..13             : 0x00 (reserved, must be zero)
 Bits 12..11             : 0x02 (2 additional transactions per microframe -> allows 683..1024 bytes per packet)
 Bits 10..0              : 0x400 (1024 bytes per packet)
bInterval                : 0x01 (1 ms)
Data (HexDump)           : 07 05 81 05 00 14 01                              .......

可以看到，该USB使用的是同步/等时传输，端点最大包为1024字节，可额外传输2次，所以最大传输的字节数为1024*(2+1)=0xc00