一.前言
最近項目需要做一個工業條形讀碼器,在底層應該會適配linux v4l2框架,就自己研究了一下在應用層怎麼使用v4l2進行編程,查閱了相關資料,主要是網上的博客資料,有:
https://www.cnblogs.com/emouse/archive/2013/03/04/2943243.html
https://www.jianshu.com/p/fd5730e939e7
大家可以進行查閱,比較學習,敝人也是參考,在一些地方進行了補充,希望可以共同學習,話不多說,喝杯咖啡,開動。
二.簡介
百度這麼說v4l2
百度說,v4l2是Video4linux2的簡稱,是linux中關於視頻設備的內核驅動,在Linux中,視頻設備是設備文件,可以像訪問普通文件一樣對其進行讀寫,攝像頭設備文件位置是/dev/video0。V4L2在設計時,是要支持很多廣泛的設備的,它們之中只有一部分在本質上是真正的視頻設備。
因爲我們這篇文章不涉及內核部分攝像頭驅動的實現,大致可以簡單說下,內核部分的實現分兩部分:1.我們要根據攝像頭的種類,實現具體的攝像頭傳感器的驅動,這裏可能有一些數據和控制的通信總線的協議。2.然後這個具體的驅動需要適配這個v4l2這個框架,然後向用戶層映射成一個字符設備文件。而我們的主題就是對這個設備文件進行操作,這就是應用層該做的事,你可能會說,這能學到什麼,而我要說,學習是漸進的,先理解應用層的需求,對內核驅動的實現也會有更深的理解,你說呢?
三.使用V4L2的一般步驟
使用V4L2進行視頻採集,一般是五個步驟:
- 1.打開設備,進行初始化參數設置
- 2.申請圖像幀緩衝,並進行內存映射
- 3.把幀緩衝進行入隊操作,開始視頻流進行採集
- 4.進行出隊,然後對數據進行處理,然後入隊,如此往復
- 5.釋放資源,停止採集工作
流程如下:
視頻採集流程圖
四.V4L2編程例子
我這裏是使用vmware+ubuntu14.04的環境,筆記本自帶一個前置攝像頭。
首先先保證你的攝像頭可以在虛擬機下使用。
1.windows + r運行services.msc命令,打開服務列表,找到如下所示服務,手動開啓
image.png
2.開啓虛擬機,在 虛擬機->可移動設備 可以看到你的usb攝像頭,在虛擬機設置裏,usb控制器一欄注意兼容性,可能要設置成兼容3.0,不然有問題。
3.ubuntu下使用cheese命令,查看你的攝像頭有沒有問題
image.png
前期工作完成,下面是源碼+Makefile
/*v4l2_example.c*/
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <linux/videodev2.h>
#include <sys/mman.h>
#include <sys/time.h>
#define TRUE (1)
#define FALSE (0)
#define FILE_VIDEO "/dev/video0"
#define IMAGE "./img/demo"
#define IMAGEWIDTH 640
#define IMAGEHEIGHT 480
#define FRAME_NUM 4
int fd;
struct v4l2_buffer buf;
struct buffer
{
void * start;
unsigned int length;
long long int timestamp;
} *buffers;
int v4l2_init()
{
struct v4l2_capability cap;
struct v4l2_fmtdesc fmtdesc;
struct v4l2_format fmt;
struct v4l2_streamparm stream_para;
//打開攝像頭設備
if ((fd = open(FILE_VIDEO, O_RDWR)) == -1)
{
printf("Error opening V4L interface\n");
return FALSE;
}
//查詢設備屬性
if (ioctl(fd, VIDIOC_QUERYCAP, &cap) == -1)
{
printf("Error opening device %s: unable to query device.\n",FILE_VIDEO);
return FALSE;
}
else
{
printf("driver:\t\t%s\n",cap.driver);
printf("card:\t\t%s\n",cap.card);
printf("bus_info:\t%s\n",cap.bus_info);
printf("version:\t%d\n",cap.version);
printf("capabilities:\t%x\n",cap.capabilities);
if ((cap.capabilities & V4L2_CAP_VIDEO_CAPTURE) == V4L2_CAP_VIDEO_CAPTURE)
{
printf("Device %s: supports capture.\n",FILE_VIDEO);
}
if ((cap.capabilities & V4L2_CAP_STREAMING) == V4L2_CAP_STREAMING)
{
printf("Device %s: supports streaming.\n",FILE_VIDEO);
}
}
//顯示所有支持幀格式
fmtdesc.index=0;
fmtdesc.type=V4L2_BUF_TYPE_VIDEO_CAPTURE;
printf("Support format:\n");
while(ioctl(fd,VIDIOC_ENUM_FMT,&fmtdesc)!=-1)
{
printf("\t%d.%s\n",fmtdesc.index+1,fmtdesc.description);
fmtdesc.index++;
}
//檢查是否支持某幀格式
struct v4l2_format fmt_test;
fmt_test.type=V4L2_BUF_TYPE_VIDEO_CAPTURE;
fmt_test.fmt.pix.pixelformat=V4L2_PIX_FMT_RGB32;
if(ioctl(fd,VIDIOC_TRY_FMT,&fmt_test)==-1)
{
printf("not support format RGB32!\n");
}
else
{
printf("support format RGB32\n");
}
//查看及設置當前格式
printf("set fmt...\n");
fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_RGB32; //jpg格式
//fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;//yuv格式
fmt.fmt.pix.height = IMAGEHEIGHT;
fmt.fmt.pix.width = IMAGEWIDTH;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
printf("fmt.type:\t\t%d\n",fmt.type);
printf("pix.pixelformat:\t%c%c%c%c\n",fmt.fmt.pix.pixelformat & 0xFF, (fmt.fmt.pix.pixelformat >> 8) & 0xFF,(fmt.fmt.pix.pixelformat >> 16) & 0xFF, (fmt.fmt.pix.pixelformat >> 24) & 0xFF);
printf("pix.height:\t\t%d\n",fmt.fmt.pix.height);
printf("pix.width:\t\t%d\n",fmt.fmt.pix.width);
printf("pix.field:\t\t%d\n",fmt.fmt.pix.field);
if(ioctl(fd, VIDIOC_S_FMT, &fmt) == -1)
{
printf("Unable to set format\n");
return FALSE;
}
printf("get fmt...\n");
if(ioctl(fd, VIDIOC_G_FMT, &fmt) == -1)
{
printf("Unable to get format\n");
return FALSE;
}
{
printf("fmt.type:\t\t%d\n",fmt.type);
printf("pix.pixelformat:\t%c%c%c%c\n",fmt.fmt.pix.pixelformat & 0xFF, (fmt.fmt.pix.pixelformat >> 8) & 0xFF,(fmt.fmt.pix.pixelformat >> 16) & 0xFF, (fmt.fmt.pix.pixelformat >> 24) & 0xFF);
printf("pix.height:\t\t%d\n",fmt.fmt.pix.height);
printf("pix.width:\t\t%d\n",fmt.fmt.pix.width);
printf("pix.field:\t\t%d\n",fmt.fmt.pix.field);
}
//設置及查看幀速率,這裏只能是30幀,就是1秒採集30張圖
memset(&stream_para, 0, sizeof(struct v4l2_streamparm));
stream_para.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
stream_para.parm.capture.timeperframe.denominator = 30;
stream_para.parm.capture.timeperframe.numerator = 1;
if(ioctl(fd, VIDIOC_S_PARM, &stream_para) == -1)
{
printf("Unable to set frame rate\n");
return FALSE;
}
if(ioctl(fd, VIDIOC_G_PARM, &stream_para) == -1)
{
printf("Unable to get frame rate\n");
return FALSE;
}
{
printf("numerator:%d\ndenominator:%d\n",stream_para.parm.capture.timeperframe.numerator,stream_para.parm.capture.timeperframe.denominator);
}
return TRUE;
}
int v4l2_mem_ops()
{
unsigned int n_buffers;
struct v4l2_requestbuffers req;
//申請幀緩衝
req.count=FRAME_NUM;
req.type=V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory=V4L2_MEMORY_MMAP;
if(ioctl(fd,VIDIOC_REQBUFS,&req)==-1)
{
printf("request for buffers error\n");
return FALSE;
}
// 申請用戶空間的地址列
buffers = malloc(req.count*sizeof (*buffers));
if (!buffers)
{
printf ("out of memory!\n");
return FALSE;
}
// 進行內存映射
for (n_buffers = 0; n_buffers < FRAME_NUM; n_buffers++)
{
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = n_buffers;
//查詢
if (ioctl (fd, VIDIOC_QUERYBUF, &buf) == -1)
{
printf("query buffer error\n");
return FALSE;
}
//映射
buffers[n_buffers].length = buf.length;
buffers[n_buffers].start = mmap(NULL,buf.length,PROT_READ|PROT_WRITE, MAP_SHARED, fd, buf.m.offset);
if (buffers[n_buffers].start == MAP_FAILED)
{
printf("buffer map error\n");
return FALSE;
}
}
return TRUE;
}
int v4l2_frame_process()
{
unsigned int n_buffers;
enum v4l2_buf_type type;
char file_name[100];
char index_str[10];
long long int extra_time = 0;
long long int cur_time = 0;
long long int last_time = 0;
//入隊和開啓採集
for (n_buffers = 0; n_buffers < FRAME_NUM; n_buffers++)
{
buf.index = n_buffers;
ioctl(fd, VIDIOC_QBUF, &buf);
}
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
ioctl(fd, VIDIOC_STREAMON, &type);
//出隊,處理,寫入yuv文件,入隊,循環進行
int loop = 0;
while(loop < 15)
{
for(n_buffers = 0; n_buffers < FRAME_NUM; n_buffers++)
{
//出隊
buf.index = n_buffers;
ioctl(fd, VIDIOC_DQBUF, &buf);
//查看採集數據的時間戳之差,單位爲微妙
buffers[n_buffers].timestamp = buf.timestamp.tv_sec*1000000+buf.timestamp.tv_usec;
cur_time = buffers[n_buffers].timestamp;
extra_time = cur_time - last_time;
last_time = cur_time;
printf("time_deta:%lld\n\n",extra_time);
printf("buf_len:%d\n",buffers[n_buffers].length);
//處理數據只是簡單寫入文件,名字以loop的次數和幀緩衝數目有關
printf("grab image data OK\n");
memset(file_name,0,sizeof(file_name));
memset(index_str,0,sizeof(index_str));
sprintf(index_str,"%d",loop*4+n_buffers);
strcpy(file_name,IMAGE);
strcat(file_name,index_str);
strcat(file_name,".jpg");
//strcat(file_name,".yuv");
FILE *fp2 = fopen(file_name, "wb");
if(!fp2)
{
printf("open %s error\n",file_name);
return(FALSE);
}
fwrite(buffers[n_buffers].start, IMAGEHEIGHT*IMAGEWIDTH*2,1,fp2);
fclose(fp2);
printf("save %s OK\n",file_name);
//入隊循環
ioctl(fd, VIDIOC_QBUF, &buf);
}
loop++;
}
return TRUE;
}
int v4l2_release()
{
unsigned int n_buffers;
enum v4l2_buf_type type;
//關閉流
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
ioctl(fd, VIDIOC_STREAMON, &type);
//關閉內存映射
for(n_buffers=0;n_buffers<FRAME_NUM;n_buffers++)
{
munmap(buffers[n_buffers].start,buffers[n_buffers].length);
}
//釋放自己申請的內存
free(buffers);
//關閉設備
close(fd);
return TRUE;
}
/*int v4l2_video_input_output()
{
struct v4l2_input input;
struct v4l2_standard standard;
//首先獲得當前輸入的index,注意只是index,要獲得具體的信息,就的調用列舉操作
memset (&input,0,sizeof(input));
if (-1 == ioctl (fd, VIDIOC_G_INPUT, &input.index)) {
printf("VIDIOC_G_INPUT\n");
return FALSE;
}
//調用列舉操作,獲得 input.index 對應的輸入的具體信息
if (-1 == ioctl (fd, VIDIOC_ENUMINPUT, &input)) {
printf("VIDIOC_ENUM_INPUT \n");
return FALSE;
}
printf("Current input %s supports:\n", input.name);
//列舉所有的所支持的 standard,如果 standard.id 與當前 input 的 input.std 有共同的
//bit flag,意味着當前的輸入支持這個 standard,這樣將所有驅動所支持的 standard 列舉一個
//遍,就可以找到該輸入所支持的所有 standard 了。
memset(&standard,0,sizeof (standard));
standard.index = 0;
while(0 == ioctl(fd, VIDIOC_ENUMSTD, &standard)) {
if (standard.id & input.std){
printf ("%s\n", standard.name);
}
standard.index++;
}
// EINVAL indicates the end of the enumeration, which cannot be empty unless this device falls under the USB exception.
if (errno != EINVAL || standard.index == 0) {
printf("VIDIOC_ENUMSTD\n");
return FALSE;
}
}*/
int main(int argc, char const *argv[])
{
printf("begin....\n");
sleep(10);
v4l2_init();
printf("init....\n");
sleep(10);
v4l2_mem_ops();
printf("malloc....\n");
sleep(10);
v4l2_frame_process();
printf("process....\n");
sleep(10);
v4l2_release();
printf("release\n");
sleep(20);
return TRUE;
}
#Makefile
CC = gcc
CFLAGS = -g -Wall -O2
TARGET = v4l2_example.bin
SRCS = v4l2_example.c
C_OBJS = v4l2_example.o
all:$(TARGET)
$(TARGET):$(C_OBJS)
$(CC) $(CFLAGS) -o $@ $^
%.o:%.c
$(CC) $(CFLAGS) -c -o $@ $<
.PHONY:clean
clean:
rm -rf *.o $(TARGET) $(CXX_OBJS) $(C_OBJS)
rm ./img/*
v4l2_example.c,Makefile所在目錄新建img目錄用於存放圖像文件,可以編譯運行,如下:
begin....
driver: uvcvideo
card: HP Wide Vision HD Camera
bus_info: usb-0000:03:00.0-2
version: 201480
capabilities: 84200001
Device /dev/video0: supports capture.
Device /dev/video0: supports streaming.
Support format:
1.MJPEG
2.YUV 4:2:2 (YUYV)
support format RGB32
set fmt...
fmt.type: 1
pix.pixelformat: RGB4
pix.height: 480
pix.width: 640
pix.field: 4
get fmt...
fmt.type: 1
pix.pixelformat: MJPG
pix.height: 480
pix.width: 640
pix.field: 1
numerator:1
denominator:30
init....
malloc....
time_deta:17741459080
buf_len:614400
grab image data OK
save ./img/demo0.jpg OK
time_deta:33239
buf_len:614400
grab image data OK
save ./img/demo1.jpg OK
time_deta:33466
buf_len:614400
grab image data OK
save ./img/demo2.jpg OK
time_deta:33139
buf_len:614400
grab image data OK
save ./img/demo3.jpg OK
time_deta:33239
buf_len:614400
grab image data OK
save ./img/demo4.jpg OK
time_deta:33382
buf_len:614400
grab image data OK
save ./img/demo5.jpg OK
time_deta:33044
buf_len:614400
grab image data OK
save ./img/demo6.jpg OK
time_deta:33225
buf_len:614400
grab image data OK
save ./img/demo7.jpg OK
time_deta:33369
buf_len:614400
grab image data OK
save ./img/demo8.jpg OK
time_deta:33091
buf_len:614400
grab image data OK
save ./img/demo9.jpg OK
time_deta:32418
buf_len:614400
grab image data OK
save ./img/demo10.jpg OK
time_deta:34051
buf_len:614400
grab image data OK
save ./img/demo11.jpg OK
time_deta:33189
buf_len:614400
grab image data OK
save ./img/demo12.jpg OK
time_deta:32841
buf_len:614400
grab image data OK
save ./img/demo13.jpg OK
time_deta:33599
buf_len:614400
grab image data OK
save ./img/demo14.jpg OK
time_deta:32628
buf_len:614400
grab image data OK
save ./img/demo15.jpg OK
time_deta:33783
buf_len:614400
grab image data OK
save ./img/demo16.jpg OK
time_deta:32628
buf_len:614400
grab image data OK
save ./img/demo17.jpg OK
time_deta:33105
buf_len:614400
grab image data OK
save ./img/demo18.jpg OK
time_deta:34140
buf_len:614400
grab image data OK
save ./img/demo19.jpg OK
time_deta:33219
buf_len:614400
grab image data OK
save ./img/demo20.jpg OK
time_deta:33334
buf_len:614400
grab image data OK
save ./img/demo21.jpg OK
time_deta:33245
buf_len:614400
grab image data OK
save ./img/demo22.jpg OK
time_deta:33276
buf_len:614400
grab image data OK
save ./img/demo23.jpg OK
time_deta:33133
buf_len:614400
grab image data OK
save ./img/demo24.jpg OK
time_deta:32533
buf_len:614400
grab image data OK
save ./img/demo25.jpg OK
time_deta:34036
buf_len:614400
grab image data OK
save ./img/demo26.jpg OK
time_deta:33093
buf_len:614400
grab image data OK
save ./img/demo27.jpg OK
time_deta:33374
buf_len:614400
grab image data OK
save ./img/demo28.jpg OK
time_deta:33432
buf_len:614400
grab image data OK
save ./img/demo29.jpg OK
time_deta:32969
buf_len:614400
grab image data OK
save ./img/demo30.jpg OK
time_deta:32412
buf_len:614400
grab image data OK
save ./img/demo31.jpg OK
time_deta:34113
buf_len:614400
grab image data OK
save ./img/demo32.jpg OK
time_deta:33193
buf_len:614400
grab image data OK
save ./img/demo33.jpg OK
time_deta:33654
buf_len:614400
grab image data OK
save ./img/demo34.jpg OK
time_deta:33330
buf_len:614400
grab image data OK
save ./img/demo35.jpg OK
time_deta:32732
buf_len:614400
grab image data OK
save ./img/demo36.jpg OK
time_deta:33140
buf_len:614400
grab image data OK
save ./img/demo37.jpg OK
time_deta:33969
buf_len:614400
grab image data OK
save ./img/demo38.jpg OK
time_deta:33044
buf_len:614400
grab image data OK
save ./img/demo39.jpg OK
time_deta:32326
buf_len:614400
grab image data OK
save ./img/demo40.jpg OK
time_deta:333137
buf_len:614400
grab image data OK
save ./img/demo41.jpg OK
time_deta:31840
buf_len:614400
grab image data OK
save ./img/demo42.jpg OK
time_deta:33712
buf_len:614400
grab image data OK
save ./img/demo43.jpg OK
time_deta:32597
buf_len:614400
grab image data OK
save ./img/demo44.jpg OK
time_deta:67859
buf_len:614400
grab image data OK
save ./img/demo45.jpg OK
time_deta:33813
buf_len:614400
grab image data OK
save ./img/demo46.jpg OK
time_deta:33024
buf_len:614400
grab image data OK
save ./img/demo47.jpg OK
time_deta:33146
buf_len:614400
grab image data OK
save ./img/demo48.jpg OK
time_deta:33227
buf_len:614400
grab image data OK
save ./img/demo49.jpg OK
time_deta:33007
buf_len:614400
grab image data OK
save ./img/demo50.jpg OK
time_deta:33680
buf_len:614400
grab image data OK
save ./img/demo51.jpg OK
time_deta:32909
buf_len:614400
grab image data OK
save ./img/demo52.jpg OK
time_deta:32553
buf_len:614400
grab image data OK
save ./img/demo53.jpg OK
time_deta:33578
buf_len:614400
grab image data OK
save ./img/demo54.jpg OK
time_deta:33308
buf_len:614400
grab image data OK
save ./img/demo55.jpg OK
time_deta:33421
buf_len:614400
grab image data OK
save ./img/demo56.jpg OK
time_deta:32596
buf_len:614400
grab image data OK
save ./img/demo57.jpg OK
time_deta:33463
buf_len:614400
grab image data OK
save ./img/demo58.jpg OK
time_deta:33100
buf_len:614400
grab image data OK
save ./img/demo59.jpg OK
process....
release
相關的說明註釋已經很明顯了,關於被註釋的函數int v4l2_video_input_output()
目前有些問題,你也可以研究一下,其他應該沒問題。
通過結果,可以看出,圖片大小640*480,格式是MJPG,幀率爲30,採集了60張圖片,每次緩衝區的時間戳之差爲33463us左右,1s/33463us = 29.88,約爲30。主函數中設置一些sleep 爲了在運行中新開終端去測試此程序所佔內存,結果爲
begin ->4204Kbyte
init->4204Kbyte
malloc ->6736Kbyte
process->6736Kbyte
release->4336Kbyte
申請的一個緩衝區爲614400byte,4個一共是2457.6Kbyte,而6736-4204=2532,還要考慮程序自己申請的堆空間及一些其他情況,可以接受
這邊是使用ps -aux | grep 程序名,進行分析的,可能會有更好的工具,更細緻的分析,希望你可以評論。
另外你可以把格式改成yuv的,需要專門yuv的查看器,去看採集的圖片
,然後還可以把文件打開的方式改成追加方式,把數據寫成一個yuv文件,可以進行圖像幀的播放,感覺就像是動畫。
作者:爲瞬間停留
鏈接:https://www.jianshu.com/p/0ac427d267d4
來源:簡書
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