安卓jni 使用V4L进行简单视频捕捉的基本步骤

1、open the VFL device

int deviceHandle;

char *devicename="/dev/video0";

deviceHandle=open(devicename,O_RDWR);

if(deviceHandle==-1)

{//fail to open device

}

调用成功，返回一个文件标示符；调用失败，返回-1。

2、查询设备属性(optional)

这一步可以省略，但是如果程序用在不同的机器和设备上，作为一个通用程序，最好进行这一步。

struct video_capability capability;

if(ioctl(deviceHandle,VIDIOCGCAP,&capability)!=-1)

{//query was successful

}

else

{//query failed

}

至此video_capability结构已经被填充，可以通过

if((capability.type & VID_TYPE_CAPTURE)!=0)

{//device can capture video

}

else

{//this device cann't capture video,exit.

}

video_capability结构的字段列表可以查询VFL API文档。

3、列举视频源的可用频道(optional)

如果频道数固定，此步骤可省略；要列举可用频道数，你必须在step.2中查询设备能力属性，然后继续下面

struct video_channel queryChannel;

i = 0;

while (i < capability.channels)

{

queryChannel.channel = i;

if (ioctl (deviceHandle, VIDIOCGCHAN, &queryChannel) != -1)

{       // ioctl success, queryChannel contains information about this channel

                printf ("%d. %s\n", queryChannel.channel, queryChannel.name);

}

       

else

{       // ioctl failure

}

++ i;

}

4、设置频道属性(optional)

如果不关心视频源来自哪个频道，此项可省略；

struct video_channel selectedChannel;

selectedChannel.channel=channelNumber;//用户定义的channelNumber

selectedChannel.norm=VIDEO_MODE_NTSC;//VIDEO_MODE_PAL|VIDEO_MODE_AUTO

if(ioctl(deviceHandle,VIDIOCSCHAN,&selectedChannel)==-1)

{//could not set channel

}

5、设置捕捉画面的高和宽(optional)

如果使用默认的图像尺寸，此项可省略；并不是每一种设备都支持尺寸裁减，所以要使用step2测试一下能力属性；假设已经完成step2，继续下面

if ((capability.type & VID_TYPE_SCALES) != 0)//经测试有点问题，每一个都不为0，但&后居然为0

{       // supports the ability to scale captured images

        struct video_window captureWindow;

        captureWindow.x = 0;

        captureWindow.y = 0;

        captureWindow.width = width;

        captureWindow.height = height;

        captureWindow.chromakey = 0;

        captureWindow.flags = 0;

        captureWindow.clips = 0;

        captureWindow.clipcount = 0;

       

if (ioctl (deviceHandle, VIDIOCSWIN, &captureWindow) == -1)

        {       // could not set window values for capture

        }

}//至此设置好了你想要的画面尺寸大小，但某些设备不支持scale，所以未必都成功设置

6、获取捕捉画面的实际尺寸

由于某些设备不支持scale，设置未必成功，所以有必要查询一下实际的画面尺寸，如下：

int width;

int height;

struct video_window captureWindow;

if (ioctl (deviceHandle, VIDIOCGWIN, &captureWindow) == -1)

{       // could not obtain specifics of capture window

}

width = captureWindow.width;

height = captureWindow.height;

7、设置捕捉画面的palette和bit depth(可选)

下面首先获取默认值，然后再设置想要改变的字段：

// get image properties

struct video_picture imageProperties;

if (ioctl (deviceHandle, VIDIOCGPICT, &imageProperties) != -1)

{       // successfully retrieved the default image properties==成功获取默认值

        // the following values are for requesting 8bit grayscale==改变为gray模式

        imageProperties.depth = 8;

        imageProperties.palette = VIDEO_PALETTE_GREY;

        if (ioctl (deviceHandle, VIDIOCSPICT, &imageProperties) == -1)

        {       // failed to set the image properties==不支持改变

        }

}

深度depth和palette的值对应关系如下：

imageProperties.depth imageProperties.palette

8bit  GREY VIDEO_PALETTE_GREY

15bit RGB 15 VIDEO_PALETTE_RGB555

16bit RGB 16 VIDEO_PALETTE_RGB565

24bit RGB 24 VIDEO_PALETTE_RGB24

32bit RGB 32 VIDEO_PALETTE_RGB32

8、获取捕捉画面实际的depth和palette

int depth;

int palette;

struct video_picture imageProperties;

if (ioctl (deviceHandle, VIDIOCGPICT, &imageProperties) == -1)

{       // failed to retrieve default image properties

}

depth = imageProperties.depth;

palette = imageProperties.palette;

比如要求24bit RGB模式，可进行如下校验：

if ((depth != 24) || (palette != VIDEO_PALETTE_RGB24))

{       // not a format our program supports

}

9、设置内存映射MMIO，把硬件视频buffer映射到内存空间

第一步，获取MMIO所需的信息：

struct video_mbuf memoryBuffer;

if (ioctl (deviceHandle, VIDIOCGMBUF, &memoryBuffer) == -1)

{       // failed to retrieve information about capture memory space

}

video_mbuf这个数据结构包括了内存映射区域的尺寸、捕捉设备缓存帧的数目、偏移地址；

第二步：获取内存映射区域的首地址

// obtain memory mapped area

char* memoryMap;

memoryMap = (char*)mmap (0, memoryBuffer.size, PROT_READ | PROT_WRITE, MAP_SHARED, deviceHandle, 0);

if ((int)memoryMap == -1)

{       // failed to retrieve pointer to memory mapped area

}

内存映射首地址和偏移确定了每个缓存帧的地址：

Buffered Frame 0 is located at: memoryMap + memoryBuffer.offsets[0]

Buffered Frame 1 is located at: memoryMap + memoryBuffer.offsets[1]

Buffered Frame 2 is located at: memoryMap + memoryBuffer.offsets[2]

etc...

The number of buffered frames is stored in memoryBuffer.frames.

捕捉过程的每一个buffer用到了video_mmap结构，定位、填充这个结构：

// allocate structures

struct video_mmap* mmaps;

mmaps = (struct video_mmap*)(malloc (memoryBuffer.frames * sizeof (struct video_mmap)));

// fill out the fields

int i = 0;

while (i < memoryBuffer.frames)

{

mmaps[i].frame = i;

mmaps[i].width = width;

mmaps[i].height = height;

mmaps[i].format = palette;

++ i;

}

//变量width、height、palette来自前面的设置过程。

10、使用MMIO进行捕捉

下面代码请求每一帧进行捕捉，除了最后一帧：

int i = 0;

while (i < (memoryBuffer.frames-1))

{

        if (ioctl (deviceHandle, VIDIOCMCAPTURE, &mmaps[i]) == -1)

        {       // capture request failed

        }

        ++ i;

}

设置一个index追踪正在捕捉的帧：

int bufferIndex;

bufferIndex = memoryBuffer.frames-1;//为何初始值指向最后一帧？

写一个循环过程控制捕捉过程，返回当前可用帧的地址：

char* NextFrame()

{

        // send a request to begin capturing to the currently indexed buffer

//向当前帧缓存发送采集请求

        if (ioctl (deviceHandle, VIDIOCMCAPTURE, &mmaps[bufferIndex]) == -1)

        {       // capture request failed

        }

//移到下一帧

        // move bufferIndex to the next frame

++ bufferIndex;

        if (bufferIndex == memoryBuffer.frames)

        {       // bufferIndex is indexing beyond the last buffer

                // set it to index the first buffer

                bufferIndex = 0;

        }

//等待当前帧完成采集过程

        // wait for the currently indexed frame to complete capture

        if (ioctl (deviceHandle, VIDIOCSYNC, &mmaps[bufferIndex]) == -1)

        {       // sync request failed

        }

//当前帧采集完成，返回帧数据的地址

        // return the address of the frame data for the current buffer index

        return (memoryMap + memoryBuffer.offsets[bufferIndex]);

}//成功的调用NextFrame()，返回当前帧的地址

11、清理工作

//free the video_mmap structures

free(mmaps);

//unmap the capture memory

munmap(memoryMap,memoryBuffer.size);

12、关闭视频设备

close (deviceHandle);

 
(jacky)