【发布时间】：2020-06-23 06:28:05
【问题描述】：
我正在研究计算机断层扫描仪。我使用 jetson TX2 进行图像采集和预处理。
我通过 jetson 控制转盘和摄像头。相机是 FSM-IMX304m。我需要访问原始指针。为此，我需要使用 V4L2 控制相机（我们建议不要使用 libargus 访问原始指针，因为它存储在 ISP 中并且 ISP 压缩数据..你能确认吗？）。我的第一个问题是关于 v4l2 的文档，我没有找到 C++ API 的明确文档。我需要控制：
- 曝光时间；
- 获得；
- 清除缓冲区的函数。
我在网上找到了一个示例，看看 V4L2 是如何工作的：
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <linux/ioctl.h>
#include <linux/types.h>
#include <linux/v4l2-common.h>
#include <linux/v4l2-controls.h>
#include <linux/videodev2.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <string.h>
#include <fstream>
#include <string>
using namespace std;
int main() {
// 1. Open the device
int fd; // A file descriptor to the video device
fd = open("/dev/video0",O_RDWR);
if(fd < 0){
perror("Failed to open device, OPEN");
return 1;
}
// 2. Ask the device if it can capture frames
v4l2_capability capability;
if(ioctl(fd, VIDIOC_QUERYCAP, &capability) < 0){
// something went wrong... exit
perror("Failed to get device capabilities, VIDIOC_QUERYCAP");
return 1;
}
// 3. Set Image format
v4l2_format imageFormat;
imageFormat.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
imageFormat.fmt.pix.width = 1024;
imageFormat.fmt.pix.height = 1024;
imageFormat.fmt.pix.pixelformat = V4L2_PIX_FMT_MJPEG;
imageFormat.fmt.pix.field = V4L2_FIELD_NONE;
// tell the device you are using this format
if(ioctl(fd, VIDIOC_S_FMT, &imageFormat) < 0){
perror("Device could not set format, VIDIOC_S_FMT");
return 1;
}
// 4. Request Buffers from the device
v4l2_requestbuffers requestBuffer = {0};
requestBuffer.count = 1; // one request buffer
requestBuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; // request a buffer which we can use for capturing frames
requestBuffer.memory = V4L2_MEMORY_MMAP;
if(ioctl(fd, VIDIOC_REQBUFS, &requestBuffer) < 0){
perror("Could not request buffer from device, VIDIOC_REQBUFS");
return 1;
}
// 5. Query the buffer to get raw data ie. ask for the you requested buffer
// and allocate memory for it
v4l2_buffer queryBuffer = {0};
queryBuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
queryBuffer.memory = V4L2_MEMORY_MMAP;
queryBuffer.index = 0;
if(ioctl(fd, VIDIOC_QUERYBUF, &queryBuffer) < 0){
perror("Device did not return the buffer information, VIDIOC_QUERYBUF");
return 1;
}
// use a pointer to point to the newly created buffer
// mmap() will map the memory address of the device to
// an address in memory
char* buffer = (char*)mmap(NULL, queryBuffer.length, PROT_READ | PROT_WRITE, MAP_SHARED,
fd, queryBuffer.m.offset);
memset(buffer, 0, queryBuffer.length);
// 6. Get a frame
// Create a new buffer type so the device knows which buffer we are talking about
v4l2_buffer bufferinfo;
memset(&bufferinfo, 0, sizeof(bufferinfo));
bufferinfo.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
bufferinfo.memory = V4L2_MEMORY_MMAP;
bufferinfo.index = 0;
// Activate streaming
int type = bufferinfo.type;
if(ioctl(fd, VIDIOC_STREAMON, &type) < 0){
perror("Could not start streaming, VIDIOC_STREAMON");
return 1;
}
/***************************** Begin looping here *********************/
// Queue the buffer
if(ioctl(fd, VIDIOC_QBUF, &bufferinfo) < 0){
perror("Could not queue buffer, VIDIOC_QBUF");
return 1;
}
// Dequeue the buffer
if(ioctl(fd, VIDIOC_DQBUF, &bufferinfo) < 0){
perror("Could not dequeue the buffer, VIDIOC_DQBUF");
return 1;
}
// Frames get written after dequeuing the buffer
cout << "Buffer has: " << (double)bufferinfo.bytesused / 1024
<< " KBytes of data" << endl;
// Write the data out to file
ofstream outFile;
outFile.open("webcam_output.jpeg", ios::binary| ios::app);
int bufPos = 0, outFileMemBlockSize = 0; // the position in the buffer and the amount to copy from
// the buffer
int remainingBufferSize = bufferinfo.bytesused; // the remaining buffer size, is decremented by
// memBlockSize amount on each loop so we do not overwrite the buffer
char* outFileMemBlock = NULL; // a pointer to a new memory block
int itr = 0; // counts thenumber of iterations
while(remainingBufferSize > 0) {
bufPos += outFileMemBlockSize; // increment the buffer pointer on each loop
// initialise bufPos before outFileMemBlockSize so we can start
// at the beginning of the buffer
outFileMemBlockSize = 1024; // set the output block size to a preferable size. 1024 :)
outFileMemBlock = new char[sizeof(char) * outFileMemBlockSize];
// copy 1024 bytes of data starting from buffer+bufPos
memcpy(outFileMemBlock, buffer+bufPos, outFileMemBlockSize);
outFile.write(outFileMemBlock,outFileMemBlockSize);
// calculate the amount of memory left to read
// if the memory block size is greater than the remaining
// amount of data we have to copy
if(outFileMemBlockSize > remainingBufferSize)
outFileMemBlockSize = remainingBufferSize;
// subtract the amount of data we have to copy
// from the remaining buffer size
remainingBufferSize -= outFileMemBlockSize;
// display the remaining buffer size
cout << itr++ << " Remaining bytes: "<< remainingBufferSize << endl;
delete outFileMemBlock;
}
// Close the file
outFile.close();
/******************************** end looping here **********************/
// end streaming
if(ioctl(fd, VIDIOC_STREAMOFF, &type) < 0){
perror("Could not end streaming, VIDIOC_STREAMOFF");
return 1;
}
close(fd);
return 0;
}
在 jetson 上，代码编译完美，但我无法运行代码。在这一步被阻止了：
// Dequeue the buffer
if(ioctl(fd, VIDIOC_DQBUF, &bufferinfo) < 0){
perror("Could not dequeue the buffer, VIDIOC_DQBUF");
return 1;
}
这就像代码被阻塞在一个无限循环中。我在运行 Ubuntu 18.04 的个人电脑上测试了代码，示例运行良好。
【问题讨论】：
标签： image-capture v4l2 nvidia-jetson