【问题描述】在友善之臂视频监控方案源码学习(5) - 输入控制一文中，介绍了input_run完成的功能。本文结合input_run实现的视频采集线程对视频采集进行详细分析。
【解析】
1 涉及到的文件和目录
mjpg-streamer-mini2440-read-only/start_uvc.sh
mjpg-streamer-mini2440-read-only/mjpg_streamer.c
mjpg-streamer-mini2440-read-only/mjpg_streamer.h
mjpg-streamer-mini2440-read-only/plugins/input.h
mjpg-streamer-mini2440-read-only/plugins/input_uvc
2 视频设备初始化
视频设备初始化在input_init函数中完成（mjpg-streamer-mini2440-read-only/plugins/input_uvc/input_uvc.c）：
/* open video device and prepare data structure */
if (init_videoIn(videoIn, dev, width, height, fps, format, 1) < 0) {
IPRINT("init_VideoIn failed\n");
closelog();
exit(EXIT_FAILURE);
}
init_videoIn函数中调用的视频初始化代码如下（mjpg-streamer-mini2440-read-only/plugins/input_uvc/V412uvc.c）：
if (init_v4l2 (vd) < 0) {
fprintf (stderr, " Init v4L2 failed !! exit fatal \n");
goto error;;
}
init_v412代码如下（mjpg-streamer-mini2440-read-only/plugins/input_uvc/V412uvc.c）：
static int init_v4l2(struct vdIn *vd)
{
int i;
int ret = 0;
if ((vd->fd = open(vd->videodevice, O_RDWR)) == -1) {
perror("ERROR opening V4L interface");
return -1;
}
memset(&vd->cap, 0, sizeof(struct v4l2_capability));
ret = ioctl(vd->fd, VIDIOC_QUERYCAP, &vd->cap);
if (ret < 0) {
fprintf(stderr, "Error opening device %s: unable to query device.\n", vd->videodevice);
goto fatal;
}
if ((vd->cap.capabilities & V4L2_CAP_VIDEO_CAPTURE) == 0) {
fprintf(stderr, "Error opening device %s: video capture not supported.\n",
vd->videodevice);
goto fatal;;
}
if (vd->grabmethod) {
if (!(vd->cap.capabilities & V4L2_CAP_STREAMING)) {
fprintf(stderr, "%s does not support streaming i/o\n", vd->videodevice);
goto fatal;
}
} else {
if (!(vd->cap.capabilities & V4L2_CAP_READWRITE)) {
fprintf(stderr, "%s does not support read i/o\n", vd->videodevice);
goto fatal;
}
}
/*
* set format in
*/
memset(&vd->fmt, 0, sizeof(struct v4l2_format));
vd->fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
vd->fmt.fmt.pix.width = vd->width;
vd->fmt.fmt.pix.height = vd->height;
vd->fmt.fmt.pix.pixelformat = vd->formatIn;
vd->fmt.fmt.pix.field = V4L2_FIELD_ANY;
ret = ioctl(vd->fd, VIDIOC_S_FMT, &vd->fmt);
if (ret < 0) {
perror("Unable to set format");
goto fatal;
}
if ((vd->fmt.fmt.pix.width != vd->width) ||
(vd->fmt.fmt.pix.height != vd->height)) {
fprintf(stderr, " format asked unavailable get width %d height %d \n", vd->fmt.fmt.pix.width, vd->fmt.fmt.pix.height);
vd->width = vd->fmt.fmt.pix.width;
vd->height = vd->fmt.fmt.pix.height;
/*
* look the format is not part of the deal ???
*/
}
if(vd->fmt.fmt.pix.pixelformat!=vd->formatIn)
{
char fourcc1[5]={0,0,0,0,0};
char fourcc2[5]={0,0,0,0,0};
memmove(fourcc1,(char*)&vd->formatIn,4);
memmove(fourcc2,(char*)&vd->fmt.fmt.pix.pixelformat,4);
fprintf(stderr, " requested %s but got %s format instead\n",fourcc1,fourcc2);
vd->formatIn = vd->fmt.fmt.pix.pixelformat;
}
/*
* set framerate
*/
struct v4l2_streamparm *setfps;
setfps = (struct v4l2_streamparm *) calloc(1, sizeof(struct v4l2_streamparm));
memset(setfps, 0, sizeof(struct v4l2_streamparm));
setfps->type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
setfps->parm.capture.timeperframe.numerator = 1;
setfps->parm.capture.timeperframe.denominator = vd->fps;
ret = ioctl(vd->fd, VIDIOC_S_PARM, setfps);
/*
* request buffers
*/
memset(&vd->rb, 0, sizeof(struct v4l2_requestbuffers));
vd->rb.count = NB_BUFFER;
vd->rb.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
vd->rb.memory = V4L2_MEMORY_MMAP;
ret = ioctl(vd->fd, VIDIOC_REQBUFS, &vd->rb);
if (ret < 0) {
perror("Unable to allocate buffers");
goto fatal;
}
/*
* map the buffers
*/
for (i = 0; i < NB_BUFFER; i++) {
memset(&vd->buf, 0, sizeof(struct v4l2_buffer));
vd->buf.index = i;
vd->buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
vd->buf.memory = V4L2_MEMORY_MMAP;
ret = ioctl(vd->fd, VIDIOC_QUERYBUF, &vd->buf);
if (ret < 0) {
perror("Unable to query buffer");
goto fatal;
}
if (debug)
fprintf(stderr, "length: %u offset: %u\n", vd->buf.length, vd->buf.m.offset);
vd->mem[i] = mmap(0 /* start anywhere */ ,
vd->buf.length, PROT_READ, MAP_SHARED, vd->fd,
vd->buf.m.offset);
if (vd->mem[i] == MAP_FAILED) {
perror("Unable to map buffer");
goto fatal;
}
if (debug)
fprintf(stderr, "Buffer mapped at address %p.\n", vd->mem[i]);
}
/*
* Queue the buffers.
*/
for (i = 0; i < NB_BUFFER; ++i) {
memset(&vd->buf, 0, sizeof(struct v4l2_buffer));
vd->buf.index = i;
vd->buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
vd->buf.memory = V4L2_MEMORY_MMAP;
ret = ioctl(vd->fd, VIDIOC_QBUF, &vd->buf);
if (ret < 0) {
perror("Unable to queue buffer");
goto fatal;;
}
}
return 0;
fatal:
return -1;
}
该部分主要完成了下述功能：
(1) 打开视频设备
if ((vd->fd = open(vd->videodevice, O_RDWR)) == -1) {
perror("ERROR opening V4L interface");
return -1;
}
(2) 开启捕获功能
memset(&vd->cap, 0, sizeof(struct v4l2_capability));
ret = ioctl(vd->fd, VIDIOC_QUERYCAP, &vd->cap);
if (ret < 0) {
fprintf(stderr, "Error opening device %s: unable to query device.\n", vd->videodevice);
goto fatal;
}
if ((vd->cap.capabilities & V4L2_CAP_VIDEO_CAPTURE) == 0) {
fprintf(stderr, "Error opening device %s: video capture not supported.\n",
vd->videodevice);
goto fatal;;
}
if (vd->grabmethod) {
if (!(vd->cap.capabilities & V4L2_CAP_STREAMING)) {
fprintf(stderr, "%s does not support streaming i/o\n", vd->videodevice);
goto fatal;
}
} else {
if (!(vd->cap.capabilities & V4L2_CAP_READWRITE)) {
fprintf(stderr, "%s does not support read i/o\n", vd->videodevice);
goto fatal;
}
}
(3) 设置视频格式
/*
* set format in
*/
memset(&vd->fmt, 0, sizeof(struct v4l2_format));
vd->fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
vd->fmt.fmt.pix.width = vd->width;
vd->fmt.fmt.pix.height = vd->height;
vd->fmt.fmt.pix.pixelformat = vd->formatIn;
vd->fmt.fmt.pix.field = V4L2_FIELD_ANY;
ret = ioctl(vd->fd, VIDIOC_S_FMT, &vd->fmt);
if (ret < 0) {
perror("Unable to set format");
goto fatal;
}
if ((vd->fmt.fmt.pix.width != vd->width) ||
(vd->fmt.fmt.pix.height != vd->height)) {
fprintf(stderr, " format asked unavailable get width %d height %d \n", vd->fmt.fmt.pix.width, vd->fmt.fmt.pix.height);
vd->width = vd->fmt.fmt.pix.width;
vd->height = vd->fmt.fmt.pix.height;
/*
* look the format is not part of the deal ???
*/
}
if(vd->fmt.fmt.pix.pixelformat!=vd->formatIn)
{
char fourcc1[5]={0,0,0,0,0};
char fourcc2[5]={0,0,0,0,0};
memmove(fourcc1,(char*)&vd->formatIn,4);
memmove(fourcc2,(char*)&vd->fmt.fmt.pix.pixelformat,4);
fprintf(stderr, " requested %s but got %s format instead\n",fourcc1,fourcc2);
vd->formatIn = vd->fmt.fmt.pix.pixelformat;
}
(4) 设置帧速率
/* * set framerate */ struct v4l2_streamparm *setfps; setfps = (struct v4l2_streamparm *) calloc(1, sizeof(struct v4l2_streamparm)); memset(setfps, 0, sizeof(struct v4l2_streamparm)); setfps->type = V4L2_BUF_TYPE_VIDEO_CAPTURE; setfps->parm.capture.timeperframe.numerator = 1; setfps->parm.capture.timeperframe.denominator = vd->fps; ret = ioctl(vd->fd, VIDIOC_S_PARM, setfps);
(5) 设置缓存
/*
* request buffers
*/
memset(&vd->rb, 0, sizeof(struct v4l2_requestbuffers));
vd->rb.count = NB_BUFFER;
vd->rb.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
vd->rb.memory = V4L2_MEMORY_MMAP;
ret = ioctl(vd->fd, VIDIOC_REQBUFS, &vd->rb);
if (ret < 0) {
perror("Unable to allocate buffers");
goto fatal;
}
/*
* map the buffers
*/
for (i = 0; i < NB_BUFFER; i++) {
memset(&vd->buf, 0, sizeof(struct v4l2_buffer));
vd->buf.index = i;
vd->buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
vd->buf.memory = V4L2_MEMORY_MMAP;
ret = ioctl(vd->fd, VIDIOC_QUERYBUF, &vd->buf);
if (ret < 0) {
perror("Unable to query buffer");
goto fatal;
}
if (debug)
fprintf(stderr, "length: %u offset: %u\n", vd->buf.length, vd->buf.m.offset);
vd->mem[i] = mmap(0 /* start anywhere */ ,
vd->buf.length, PROT_READ, MAP_SHARED, vd->fd,
vd->buf.m.offset);
if (vd->mem[i] == MAP_FAILED) {
perror("Unable to map buffer");
goto fatal;
}
if (debug)
fprintf(stderr, "Buffer mapped at address %p.\n", vd->mem[i]);
}
/*
* Queue the buffers.
*/
for (i = 0; i < NB_BUFFER; ++i) {
memset(&vd->buf, 0, sizeof(struct v4l2_buffer));
vd->buf.index = i;
vd->buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
vd->buf.memory = V4L2_MEMORY_MMAP;
ret = ioctl(vd->fd, VIDIOC_QBUF, &vd->buf);
if (ret < 0) {
perror("Unable to queue buffer");
goto fatal;;
}
3 视频采集
在input_run中调用了视频采集的线程函数，如下所示：
int input_run(void) {
pglobal->buf = malloc(videoIn->framesizeIn);
if (pglobal->buf == NULL) {
fprintf(stderr, "could not allocate memory\n");
exit(EXIT_FAILURE);
}
pthread_create(&cam, 0, cam_thread, NULL);
pthread_detach(cam);
return 0;
}
cam_thread定义如下：
void *cam_thread( void *arg ) {
/* set cleanup handler to cleanup allocated ressources */
pthread_cleanup_push(cam_cleanup, NULL);
while( !pglobal->stop ) {
/* grab a frame */
if( uvcGrab(videoIn) < 0 ) {
IPRINT("Error grabbing frames\n");
exit(EXIT_FAILURE);
}
DBG("received frame of size: %d\n", videoIn->buf.bytesused);
/*
* Workaround for broken, corrupted frames:
* Under low light conditions corrupted frames may get captured.
* The good thing is such frames are quite small compared to the regular pictures.
* For example a VGA (640x480) webcam picture is normally >= 8kByte large,
* corrupted frames are smaller.
*/
if ( videoIn->buf.bytesused < minimum_size ) {
DBG("dropping too small frame, assuming it as broken\n");
continue;
}
/* copy JPG picture to global buffer */
pthread_mutex_lock( &pglobal->db );
/*
* If capturing in YUV mode convert to JPEG now.
* This compression requires many CPU cycles, so try to avoid YUV format.
* Getting JPEGs straight from the webcam, is one of the major advantages of
* Linux-UVC compatible devices.
*/
if (videoIn->formatIn != V4L2_PIX_FMT_MJPEG) {
DBG("compressing frame\n");
pglobal->size = compress_yuyv_to_jpeg(videoIn, pglobal->buf, videoIn->framesizeIn, gquality, videoIn->formatIn);
}
else {
DBG("copying frame\n");
pglobal->size = memcpy_picture(pglobal->buf, videoIn->tmpbuffer, videoIn->buf.bytesused);
}
#if 0
/* motion detection can be done just by comparing the picture size, but it is not very accurate!! */
if ( (prev_size - global->size)*(prev_size - global->size) > 4*1024*1024 ) {
DBG("motion detected (delta: %d kB)\n", (prev_size - global->size) / 1024);
}
prev_size = global->size;
#endif
/* signal fresh_frame */
pthread_cond_broadcast(&pglobal->db_update);
pthread_mutex_unlock( &pglobal->db );
DBG("waiting for next frame\n");
/* only use usleep if the fps is below 5, otherwise the overhead is too long */
if ( videoIn->fps < 5 ) {
usleep(1000*1000/videoIn->fps);
}
}
DBG("leaving input thread, calling cleanup function now\n");
pthread_cleanup_pop(1);
return NULL;
}
该函数完成下述任务：
(1) 视频抓取
while( !pglobal->stop ) {
/* grab a frame */
if( uvcGrab(videoIn) < 0 ) {
IPRINT("Error grabbing frames\n");
exit(EXIT_FAILURE);
}
uvcGrab函数实现如下：
int uvcGrab(struct vdIn *vd)
{
#define HEADERFRAME1 0xaf
int ret;
if (!vd->isstreaming)
if (video_enable(vd))
goto err;
memset(&vd->buf, 0, sizeof(struct v4l2_buffer));
vd->buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
vd->buf.memory = V4L2_MEMORY_MMAP;
ret = ioctl(vd->fd, VIDIOC_DQBUF, &vd->buf);
if (ret < 0) {
perror("Unable to dequeue buffer");
goto err;
}
switch (vd->formatIn) {
case V4L2_PIX_FMT_MJPEG:
if (vd->buf.bytesused <= HEADERFRAME1) { /* Prevent crash
* on empty image */
fprintf(stderr, "Ignoring empty buffer ...\n");
return 0;
}
/* memcpy(vd->tmpbuffer, vd->mem[vd->buf.index], vd->buf.bytesused);
memcpy (vd->tmpbuffer, vd->mem[vd->buf.index], HEADERFRAME1);
memcpy (vd->tmpbuffer + HEADERFRAME1, dht_data, sizeof(dht_data));
memcpy (vd->tmpbuffer + HEADERFRAME1 + sizeof(dht_data), vd->mem[vd->buf.index] + HEADERFRAME1, (vd->buf.bytesused - HEADERFRAME1));
*/
memcpy(vd->tmpbuffer, vd->mem[vd->buf.index], vd->buf.bytesused);
if (debug)
fprintf(stderr, "bytes in used %d \n", vd->buf.bytesused);
break;
case V4L2_PIX_FMT_YUYV:
default:
if (vd->buf.bytesused > vd->framesizeIn)
memcpy (vd->framebuffer, vd->mem[vd->buf.index], (size_t) vd->framesizeIn);
else
memcpy (vd->framebuffer, vd->mem[vd->buf.index], (size_t) vd->buf.bytesused);
break;
//goto err;
break;
}
ret = ioctl(vd->fd, VIDIOC_QBUF, &vd->buf);
if (ret < 0) {
perror("Unable to requeue buffer");
goto err;
}
return 0;
err:
vd->signalquit = 0;
return -1;
}
该函数主要完成了下述功能：
(a) 使能视频设备
if (!vd->isstreaming)
if (video_enable(vd))
goto err;
实质上是调用了下述方法：
ret = ioctl(vd->fd, VIDIOC_STREAMON, &type);
if (ret < 0) {
perror("Unable to start capture");
return ret;
}
(b) 视频采集
ret = ioctl(vd->fd, VIDIOC_DQBUF, &vd->buf);
if (ret < 0) {
perror("Unable to dequeue buffer");
goto err;
}
注：采集的视频信息放在vd->buf中。
(2) 视频压缩编码
if (videoIn->formatIn != V4L2_PIX_FMT_MJPEG) {
DBG("compressing frame\n");
pglobal->size = compress_yuyv_to_jpeg(videoIn, pglobal->buf, videoIn->framesizeIn, gquality, videoIn->formatIn);
}
else {
DBG("copying frame\n");
pglobal->size = memcpy_picture(pglobal->buf, videoIn->tmpbuffer, videoIn->buf.bytesused);
}
此过程，后续文章进行详述。
4 视频的存储
视频采集的信息存储在vd->buf中，vd->buf怎么和global->buf联系起来的呢？
(1) input_init初始化全局指针
pglobal = param->global;
param->global指针指向mjpg-streamer-mini2440-read-only/mjpg_streamer.c主程序main中的global全局指针。
(2) input_run中分配空间
pglobal->buf = malloc(videoIn->framesizeIn);
if (pglobal->buf == NULL) {
fprintf(stderr, "could not allocate memory\n");
exit(EXIT_FAILURE);
}
(3) cam_thread抓取
while( !pglobal->stop ) {
/* grab a frame */
if( uvcGrab(videoIn) < 0 ) {
IPRINT("Error grabbing frames\n");
exit(EXIT_FAILURE);
}
抓取的数据存储在videoIn结构中：
int uvcGrab(struct vdIn *vd)
{
...
ret = ioctl(vd->fd, VIDIOC_DQBUF, &vd->buf);
if (ret < 0) {
perror("Unable to dequeue buffer");
goto err;
}
}
(4) global->buf和videoIn->buf的联系
if (videoIn->formatIn != V4L2_PIX_FMT_MJPEG) {
DBG("compressing frame\n");
pglobal->size = compress_yuyv_to_jpeg(videoIn, pglobal->buf, videoIn->framesizeIn, gquality, videoIn->formatIn);
}
else {
DBG("copying frame\n");
pglobal->size = memcpy_picture(pglobal->buf, videoIn->tmpbuffer, videoIn->buf.bytesused);
}
最终，通过压缩编码将videoIn->buf的数据存储在global->buf中。
【源码下载】
http://download.csdn.net/detail/tandesir/4915905
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