如何实现LCD显示摄像头图像

摄像头数据的读出

首先需要将摄像头采集到的数据读出，那么就需要摄像头采集到数据的基本格式以及相关操作函数：

typedef struct VideoDevice {
    int iFd;
    int iPixelFormat;
    int iWidth;
    int iHeight;

    int iVideoBufCnt;
    int iVideoBufMaxLen;
    int iVideoBufCurIndex;
    unsigned char *pucVideBuf[NB_BUFFER];

    /* 函数 */
    PT_VideoOpr ptOPr;
}T_VideoDevice, *PT_VideoDevice;

函数结构体：

typedef struct VideoOpr {
    char *name;
    int (*InitDevice)(char *strDevName, PT_VideoDevice ptVideoDevice);
    int (*ExitDevice)(PT_VideoDevice ptVideoDevice);
    int (*GetFrame)(PT_VideoDevice ptVideoDevice, PT_VideoBuf ptVideoBuf);
    int (*PutFrame)(PT_VideoDevice ptVideoDevice, PT_VideoBuf ptVideoBuf);
    int (*StartDevice)(PT_VideoDevice ptVideoDevice);
    int (*StopDevice)(PT_VideoDevice ptVideoDevice);
}T_VideoOpr, *PT_VideoOpr;

包括初始化，退出函数，从队列中获取buffer的函数以及将buffer放回队列的函数，还应该包括开启设备的函数以及停止设备工作的函数。·以上函数的实现都在v2l4.c中实现，下面就要实现这些函数。

v2l4.c的实现

static int V4l2InitDevice(char *strDevName, PT_VideoDevice ptVideoDevice)
{
    int i;
    int iFd;
    int iError;
    struct v4l2_capability tV4l2Cap;
	struct v4l2_fmtdesc tFmtDesc;
    struct v4l2_format  tV4l2Fmt;
    struct v4l2_requestbuffers tV4l2ReqBuffs;
    struct v4l2_buffer tV4l2Buf;

    int iLcdWidth;
    int iLcdHeigt;
    int iLcdBpp;

    iFd = open(strDevName, O_RDWR);
    if (iFd < 0)
    {
        DBG_PRINTF("can not open %s\n", strDevName);
        return -1;
    }
    ptVideoDevice->iFd = iFd;

    iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);
    memset(&tV4l2Cap, 0, sizeof(struct v4l2_capability));
    iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);      // 为什么调用两次
    if (iError) {
    	DBG_PRINTF("Error opening device %s: unable to query device.\n", strDevName);
    	goto err_exit;
    }

    if (!(tV4l2Cap.capabilities & V4L2_CAP_VIDEO_CAPTURE))
    {
    	DBG_PRINTF("%s is not a video capture device\n", strDevName);
        goto err_exit;
    }

	if (tV4l2Cap.capabilities & V4L2_CAP_STREAMING) {
	    DBG_PRINTF("%s supports streaming i/o\n", strDevName);
	}
    
	if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE) {
	    DBG_PRINTF("%s supports read i/o\n", strDevName);
	}

	memset(&tFmtDesc, 0, sizeof(tFmtDesc));
	tFmtDesc.index = 0;
	tFmtDesc.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
	while ((iError = ioctl(iFd, VIDIOC_ENUM_FMT, &tFmtDesc)) == 0) {
        if (isSupportThisFormat(tFmtDesc.pixelformat))
        {
            ptVideoDevice->iPixelFormat = tFmtDesc.pixelformat;
            break;
        }
		tFmtDesc.index++;
	}

    if (!ptVideoDevice->iPixelFormat)
    {
    	DBG_PRINTF("can not support the format of this device\n");
        goto err_exit;        
    }

    
    /* set format in */
    GetDispResolution(&iLcdWidth, &iLcdHeigt, &iLcdBpp);
    memset(&tV4l2Fmt, 0, sizeof(struct v4l2_format));
    tV4l2Fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    tV4l2Fmt.fmt.pix.pixelformat = ptVideoDevice->iPixelFormat;
    tV4l2Fmt.fmt.pix.width       = iLcdWidth;
    tV4l2Fmt.fmt.pix.height      = iLcdHeigt;
    tV4l2Fmt.fmt.pix.field       = V4L2_FIELD_ANY;

    /* 如果驱动程序发现无法某些参数(比如分辨率),
     * 它会调整这些参数, 并且返回给应用程序
     */
    iError = ioctl(iFd, VIDIOC_S_FMT, &tV4l2Fmt); 
    if (iError) 
    {
    	DBG_PRINTF("Unable to set format\n");
        goto err_exit;        
    }
    ptVideoDevice->iWidth  = tV4l2Fmt.fmt.pix.width;
    ptVideoDevice->iHeight = tV4l2Fmt.fmt.pix.height;

    /* request buffers */
    memset(&tV4l2ReqBuffs, 0, sizeof(struct v4l2_requestbuffers));
    tV4l2ReqBuffs.count = NB_BUFFER;
    tV4l2ReqBuffs.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    tV4l2ReqBuffs.memory = V4L2_MEMORY_MMAP;

    iError = ioctl(iFd, VIDIOC_REQBUFS, &tV4l2ReqBuffs);
    if (iError) 
    {
    	DBG_PRINTF("Unable to allocate buffers.\n");
        goto err_exit;        
    }
    
    ptVideoDevice->iVideoBufCnt = tV4l2ReqBuffs.count;
    if (tV4l2Cap.capabilities & V4L2_CAP_STREAMING)
    {
        /* map the buffers */
        for (i = 0; i < ptVideoDevice->iVideoBufCnt; i++) 
        {
        	memset(&tV4l2Buf, 0, sizeof(struct v4l2_buffer));
        	tV4l2Buf.index = i;
        	tV4l2Buf.type   = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        	tV4l2Buf.memory = V4L2_MEMORY_MMAP;
        	iError = ioctl(iFd, VIDIOC_QUERYBUF, &tV4l2Buf);
        	if (iError) 
            {
        	    DBG_PRINTF("Unable to query buffer.\n");
        	    goto err_exit;
        	}

            ptVideoDevice->iVideoBufMaxLen = tV4l2Buf.length;
        	ptVideoDevice->pucVideBuf[i] = mmap(0 /* start anywhere */ ,
        			  tV4l2Buf.length, PROT_READ, MAP_SHARED, iFd,
        			  tV4l2Buf.m.offset);
        	if (ptVideoDevice->pucVideBuf[i] == MAP_FAILED) 
            {
        	    DBG_PRINTF("Unable to map buffer\n");
        	    goto err_exit;
        	}
        }        

        /* Queue the buffers. */
        for (i = 0; i < ptVideoDevice->iVideoBufCnt; i++) 
        {
        	memset(&tV4l2Buf, 0, sizeof(struct v4l2_buffer));
        	tV4l2Buf.index = i;
        	tV4l2Buf.type  = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        	tV4l2Buf.memory = V4L2_MEMORY_MMAP;
        	iError = ioctl(iFd, VIDIOC_QBUF, &tV4l2Buf);
        	if (iError)
            {
        	    DBG_PRINTF("Unable to queue buffer.\n");
        	    goto err_exit;
        	}
        }
        
    }
    else if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE)
    {
        g_tV4l2VideoOpr.GetFrame = V4l2GetFrameForReadWrite;
        g_tV4l2VideoOpr.PutFrame = V4l2PutFrameForReadWrite;
        
        /* read(fd, buf, size) */
        ptVideoDevice->iVideoBufCnt  = 1;
        /* 在这个程序所能支持的格式里, 一个象素最多只需要4字节 */
        ptVideoDevice->iVideoBufMaxLen = ptVideoDevice->iWidth * ptVideoDevice->iHeight * 4;
        ptVideoDevice->pucVideBuf[0] = malloc(ptVideoDevice->iVideoBufMaxLen);
    }

    
    return 0;
    
err_exit:    
    close(iFd);
    return -1;    
}

最复杂的函数是是初始化函数，在初始化函数中需要涉及到应用层对硬件的操作，以上操作需要通过系统调用ioctl()来实现。
首先是打开设备：

 iFd = open(strDevName, O_RDWR);
 if (iFd < 0)
 {
     DBG_PRINTF("can not open %s\n", strDevName);
     return -1;
 }
 ptVideoDevice->iFd = iFd;

首先确认设备特性：cmd-VIDIOC_QUERYCAP

iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);
memset(&tV4l2Cap, 0, sizeof(struct v4l2_capability));
iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);      // 为什么调用两次
if (iError) {
	DBG_PRINTF("Error opening device %s: unable to query device.\n", strDevName);
	goto err_exit;
}

if (!(tV4l2Cap.capabilities & V4L2_CAP_VIDEO_CAPTURE))
{
	DBG_PRINTF("%s is not a video capture device\n", strDevName);
    goto err_exit;
}

if (tV4l2Cap.capabilities & V4L2_CAP_STREAMING) {
    DBG_PRINTF("%s supports streaming i/o\n", strDevName);
}
   
if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE) {
    DBG_PRINTF("%s supports read i/o\n", strDevName);
}

判定设备是否是V4L2_CAP_VIDEO_CAPTURE设备，判断设备是否支持STREAMING模式、READWRITE模式。

memset(&tFmtDesc, 0, sizeof(tFmtDesc));
tFmtDesc.index = 0;
tFmtDesc.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
while ((iError = ioctl(iFd, VIDIOC_ENUM_FMT, &tFmtDesc)) == 0) {
    if (isSupportThisFormat(tFmtDesc.pixelformat))
    {
        ptVideoDevice->iPixelFormat = tFmtDesc.pixelformat;
        break;
    }
	tFmtDesc.index++;
}

if (!ptVideoDevice->iPixelFormat)
{
	DBG_PRINTF("can not support the format of this device\n");
    goto err_exit;        
}

常见的摄像头支持的视频格式是：YUYV, MJPEG, RGB565，这就需要判断应用程序是否支持这三种格式，如果不支持任何一种格式则返回错误。

static int isSupportThisFormat(int iPixelFormat)
{
    int i;
    for (i = 0; i < sizeof(g_aiSupportedFormats)/sizeof(g_aiSupportedFormats[0]); i++)
    {
        if (g_aiSupportedFormats[i] == iPixelFormat)
            return 1;
    }
    return 0;
}

设置格式：

/* set format in */
GetDispResolution(&iLcdWidth, &iLcdHeigt, &iLcdBpp);
memset(&tV4l2Fmt, 0, sizeof(struct v4l2_format));
tV4l2Fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
tV4l2Fmt.fmt.pix.pixelformat = ptVideoDevice->iPixelFormat;
tV4l2Fmt.fmt.pix.width       = iLcdWidth;
tV4l2Fmt.fmt.pix.height      = iLcdHeigt;
tV4l2Fmt.fmt.pix.field       = V4L2_FIELD_ANY;

/* 如果驱动程序发现无法某些参数(比如分辨率),
 * 它会调整这些参数, 并且返回给应用程序
 */
iError = ioctl(iFd, VIDIOC_S_FMT, &tV4l2Fmt); 
if (iError) 
{
	DBG_PRINTF("Unable to set format\n");
    goto err_exit;        
}

申请buffer，

/* request buffers */
memset(&tV4l2ReqBuffs, 0, sizeof(struct v4l2_requestbuffers));
tV4l2ReqBuffs.count = NB_BUFFER;
tV4l2ReqBuffs.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
tV4l2ReqBuffs.memory = V4L2_MEMORY_MMAP;

iError = ioctl(iFd, VIDIOC_REQBUFS, &tV4l2ReqBuffs);
if (iError) 
{
	DBG_PRINTF("Unable to allocate buffers.\n");
    goto err_exit;        
}

获取内核buffer参数，映射到用户空间，并将申请的buffer放到队列中；

ptVideoDevice->iVideoBufCnt = tV4l2ReqBuffs.count;
if (tV4l2Cap.capabilities & V4L2_CAP_STREAMING)
{
    /* map the buffers */
    for (i = 0; i < ptVideoDevice->iVideoBufCnt; i++) 
    {
    	memset(&tV4l2Buf, 0, sizeof(struct v4l2_buffer));
    	tV4l2Buf.index = i;
    	tV4l2Buf.type   = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    	tV4l2Buf.memory = V4L2_MEMORY_MMAP;
    	iError = ioctl(iFd, VIDIOC_QUERYBUF, &tV4l2Buf);
    	if (iError) 
        {
    	    DBG_PRINTF("Unable to query buffer.\n");
    	    goto err_exit;
    	}

        ptVideoDevice->iVideoBufMaxLen = tV4l2Buf.length;
    	ptVideoDevice->pucVideBuf[i] = mmap(0 /* start anywhere */ ,
    			  tV4l2Buf.length, PROT_READ, MAP_SHARED, iFd,
    			  tV4l2Buf.m.offset);
    	if (ptVideoDevice->pucVideBuf[i] == MAP_FAILED) 
        {
    	    DBG_PRINTF("Unable to map buffer\n");
    	    goto err_exit;
    	}
    }        

    /* Queue the buffers. */
    for (i = 0; i < ptVideoDevice->iVideoBufCnt; i++) 
    {
    	memset(&tV4l2Buf, 0, sizeof(struct v4l2_buffer));
    	tV4l2Buf.index = i;
    	tV4l2Buf.type  = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    	tV4l2Buf.memory = V4L2_MEMORY_MMAP;
    	iError = ioctl(iFd, VIDIOC_QBUF, &tV4l2Buf);
    	if (iError)
        {
    	    DBG_PRINTF("Unable to queue buffer.\n");
    	    goto err_exit;
    	}
    }
    
}

此处mmap的意义是，将内核空间中申请的buffer直接映射到用户空间，简化后续的操作流程。
streaming模式的处理方式是：将申请的buffer放在队里上，如果有数据读回，就获取队列上的buffer，读取完成后再将buffer放回队列。以下函数分别从队列中取出和放回buffer。

static int V4l2GetFrameForStreaming(PT_VideoDevice ptVideoDevice, PT_VideoBuf ptVideoBuf)
{
    struct pollfd tFds[1];
    int iRet;
    struct v4l2_buffer tV4l2Buf;
            
    /* poll */
    tFds[0].fd     = ptVideoDevice->iFd;
    tFds[0].events = POLLIN;

    iRet = poll(tFds, 1, -1);
    if (iRet <= 0)
    {
        DBG_PRINTF("poll error!\n");
        return -1;
    }
    
    /* VIDIOC_DQBUF */
    memset(&tV4l2Buf, 0, sizeof(struct v4l2_buffer));
    tV4l2Buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    tV4l2Buf.memory = V4L2_MEMORY_MMAP;
    iRet = ioctl(ptVideoDevice->iFd, VIDIOC_DQBUF, &tV4l2Buf);
    if (iRet < 0) 
    {
    	DBG_PRINTF("Unable to dequeue buffer.\n");
    	return -1;
    }
    ptVideoDevice->iVideoBufCurIndex = tV4l2Buf.index;

    ptVideoBuf->iPixelFormat        = ptVideoDevice->iPixelFormat;
    ptVideoBuf->tPixelDatas.iWidth  = ptVideoDevice->iWidth;
    ptVideoBuf->tPixelDatas.iHeight = ptVideoDevice->iHeight;
    ptVideoBuf->tPixelDatas.iBpp    = (ptVideoDevice->iPixelFormat == V4L2_PIX_FMT_YUYV) ? 16 : \
                                        (ptVideoDevice->iPixelFormat == V4L2_PIX_FMT_MJPEG) ? : 0 \ 
                                        (ptVideoDevice->iPixelFormat == V4L2_PIX_FMT_RGB565) : 16;
    ptVideoBuf->tPixelDatas.iLineBytes    = ptVideoDevice->iWidth * ptVideoBuf->tPixelDatas.iBpp / 8;
    ptVideoBuf->tPixelDatas.iTotalBytes   = tV4l2Buf->bytesused;
    ptVideoBuf->tPixelDatas.aucPixelDatas = ptVideoDevice->pucVideBuf[tV4l2Buf.index];    
    return 0;
}


static int V4l2PutFrameForStreaming(PT_VideoDevice ptVideoDevice, PT_VideoBuf ptVideoBuf)
{
    /* VIDIOC_QBUF */
    struct v4l2_buffer tV4l2Buf;
    int iError;
    
	memset(&tV4l2Buf, 0, sizeof(struct v4l2_buffer));
	tV4l2Buf.index  = ptVideoDevice->iVideoBufCurIndex;
	tV4l2Buf.type   = V4L2_BUF_TYPE_VIDEO_CAPTURE;
	tV4l2Buf.memory = V4L2_MEMORY_MMAP;
	iError = ioctl(ptVideoDevice->iFd, VIDIOC_QBUF, &tV4l2Buf);
	if (iError) 
    {
	    DBG_PRINTF("Unable to queue buffer.\n");
	    return -1;
	}
    return 0;
}

除了streaming模式，还有readwrite模式，read、write模式操作较为简单，如下：

else if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE)
{
    g_tV4l2VideoOpr.GetFrame = V4l2GetFrameForReadWrite;
    g_tV4l2VideoOpr.PutFrame = V4l2PutFrameForReadWrite;
    
    /* read(fd, buf, size) */
    ptVideoDevice->iVideoBufCnt  = 1;
    /* 在这个程序所能支持的格式里, 一个象素最多只需要4字节 */
    ptVideoDevice->iVideoBufMaxLen = ptVideoDevice->iWidth * ptVideoDevice->iHeight * 4;
    ptVideoDevice->pucVideBuf[0] = malloc(ptVideoDevice->iVideoBufMaxLen);
}

读数据函数：

static int V4l2GetFrameForReadWrite(PT_VideoDevice ptVideoDevice, PT_VideoBuf ptVideoBuf)
{
    int iRet;

    iRet = read(ptVideoDevice->iFd, ptVideoDevice->pucVideBuf[0], ptVideoDevice->iVideoBufMaxLen);
    if (iRet <= 0)
    {
        return -1;
    }
    
    ptVideoBuf->iPixelFormat        = ptVideoDevice->iPixelFormat;
    ptVideoBuf->tPixelDatas.iWidth  = ptVideoDevice->iWidth;
    ptVideoBuf->tPixelDatas.iHeight = ptVideoDevice->iHeight;
    ptVideoBuf->tPixelDatas.iBpp    = (ptVideoDevice->iPixelFormat == V4L2_PIX_FMT_YUYV) ? 16 : \
                                        (ptVideoDevice->iPixelFormat == V4L2_PIX_FMT_MJPEG) ? : 0 \ 
                                        (ptVideoDevice->iPixelFormat == V4L2_PIX_FMT_RGB565) : 16;
    ptVideoBuf->tPixelDatas.iLineBytes    = ptVideoDevice->iWidth * ptVideoBuf->tPixelDatas.iBpp / 8;
    ptVideoBuf->tPixelDatas.iTotalBytes   = iRet;
    ptVideoBuf->tPixelDatas.aucPixelDatas = ptVideoDevice->pucVideBuf[0];    
    
    return 0;
}

最后还要实现的是开关设备。开设备。

static int V4l2StartDevice(PT_VideoDevice ptVideoDevice)
{
    int iType = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    int iError;

    iError = ioctl(ptVideoDevice->iFd, VIDIOC_STREAMON, &iType);
    if (iError) 
    {
    	DBG_PRINTF("Unable to start capture.\n");
    	return -1;
    }
    return 0;
}

关设备。

static int V4l2StopDevice(PT_VideoDevice ptVideoDevice)
{
    int iType = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    int iError;

    iError = ioctl(ptVideoDevice->iFd, VIDIOC_STREAMOFF, &iType);
    if (iError) 
    {
    	DBG_PRINTF("Unable to stop capture.\n");
    	return -1;
    }
    return 0;
}

最后还要将结构体注册(放到链表上)。

static T_VideoOpr g_tV4l2VideoOpr = {
    .name        = "v4l2",
    .InitDevice  = V4l2InitDevice,
    .ExitDevice  = V4l2ExitDevice,
    .GetFrame    = V4l2GetFrameForStreaming,
    .PutFrame    = V4l2PutFrameForStreaming,
    .StartDevice = V4l2StartDevice,
    .StopDevice  = V4l2StopDevice,
};

int V4l2Init(void)
{
    return RegisterVideoOpr(&g_tV4l2VideoOpr);
}

video_manager.c的实现

这部分函数基本不需要修改，沿用display设备即可。

#include <config.h>
#include <video_manager.h>
#include <string.h>

static PT_VideoOpr g_ptVideoOprHead = NULL;

int RegisterVideoOpr(PT_VideoOpr ptVideoOpr)
{
	PT_VideoOpr ptTmp;

	if (!g_ptVideoOprHead)
	{
		g_ptVideoOprHead   = ptVideoOpr;
		ptVideoOpr->ptNext = NULL;
	}
	else
	{
		ptTmp = g_ptVideoOprHead;
		while (ptTmp->ptNext)
		{
			ptTmp = ptTmp->ptNext;
		}
		ptTmp->ptNext     = ptVideoOpr;
		ptVideoOpr->ptNext = NULL;
	}

	return 0;
}

void ShowVideoOpr(void)
{
	int i = 0;
	PT_VideoOpr ptTmp = g_ptVideoOprHead;

	while (ptTmp)
	{
		printf("%02d %s\n", i++, ptTmp->name);
		ptTmp = ptTmp->ptNext;
	}
}

PT_VideoOpr GetVideoOpr(char *pcName)
{
	PT_VideoOpr ptTmp = g_ptVideoOprHead;
	
	while (ptTmp)
	{
		if (strcmp(ptTmp->name, pcName) == 0)
		{
			return ptTmp;
		}
		ptTmp = ptTmp->ptNext;
	}
	return NULL;
}

int VideoInit(void)
{
	int iError;

    iError = V4l2Init();

	return iError;
}

来源：CSDN

作者：不当大白菜

链接：https://blog.csdn.net/weixin_40474683/article/details/103920460