v4l - USB Web カメラ (640x480) 用の v4L キャプチャ プログラムの書き方

Question

Linux 2.6 バージョンに USB ウェブカメラ ドライバをインストールした後、V4L を使用してキャプチャしたいので、ウェブカメラを使用してキャプチャするプログラムの書き方を教えてください。

Answer 1

linuxTV のリファレンス ( http://linuxtv.org/downloads/v4l-dvb-apis/ ) を使用して V4l を操作します。V4l に関するドキュメントといくつかの例があります。次の例を見てください（私はlinuxTVから入手しました）：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <linux/videodev2.h>
#include "../libv4l/include/libv4l2.h"

#define CLEAR(x) memset(&(x), 0, sizeof(x))

struct buffer {
        void   *start;
        size_t length;
};

static void xioctl(int fh, int request, void *arg)
{
        int r;

        do {
                r = v4l2_ioctl(fh, request, arg);
        } while (r == -1 && ((errno == EINTR) || (errno == EAGAIN)));

        if (r == -1) {
                fprintf(stderr, "error %d, %s\n", errno, strerror(errno));
                exit(EXIT_FAILURE);
        }
}

int main(int argc, char **argv)
{
        struct v4l2_format              fmt;
        struct v4l2_buffer              buf;
        struct v4l2_requestbuffers      req;
        enum v4l2_buf_type              type;
        fd_set                          fds;
        struct timeval                  tv;
        int                             r, fd = -1;
        unsigned int                    i, n_buffers;
        char                            *dev_name = "/dev/video0";
        char                            out_name[256];
        FILE                            *fout;
        struct buffer                   *buffers;

        fd = v4l2_open(dev_name, O_RDWR | O_NONBLOCK, 0);
        if (fd < 0) {
                perror("Cannot open device");
                exit(EXIT_FAILURE);
        }

        CLEAR(fmt);
        fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        fmt.fmt.pix.width       = 640;
        fmt.fmt.pix.height      = 480;
        fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_RGB24;
        fmt.fmt.pix.field       = V4L2_FIELD_INTERLACED;
        xioctl(fd, VIDIOC_S_FMT, &fmt);
        if (fmt.fmt.pix.pixelformat != V4L2_PIX_FMT_RGB24) {
                printf("Libv4l didn't accept RGB24 format. Can't proceed.\n");
                exit(EXIT_FAILURE);
        }
        if ((fmt.fmt.pix.width != 640) || (fmt.fmt.pix.height != 480))
                printf("Warning: driver is sending image at %dx%d\n",
                        fmt.fmt.pix.width, fmt.fmt.pix.height);

        CLEAR(req);
        req.count = 2;
        req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        req.memory = V4L2_MEMORY_MMAP;
        xioctl(fd, VIDIOC_REQBUFS, &req);

        buffers = calloc(req.count, sizeof(*buffers));
        for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {
                CLEAR(buf);

                buf.type        = V4L2_BUF_TYPE_VIDEO_CAPTURE;
                buf.memory      = V4L2_MEMORY_MMAP;
                buf.index       = n_buffers;

                xioctl(fd, VIDIOC_QUERYBUF, &buf);

                buffers[n_buffers].length = buf.length;
                buffers[n_buffers].start = v4l2_mmap(NULL, buf.length,
                              PROT_READ | PROT_WRITE, MAP_SHARED,
                              fd, buf.m.offset);

                if (MAP_FAILED == buffers[n_buffers].start) {
                        perror("mmap");
                        exit(EXIT_FAILURE);
                }
        }

        for (i = 0; i < n_buffers; ++i) {
                CLEAR(buf);
                buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
                buf.memory = V4L2_MEMORY_MMAP;
                buf.index = i;
                xioctl(fd, VIDIOC_QBUF, &buf);
        }
        type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

        xioctl(fd, VIDIOC_STREAMON, &type);
        for (i = 0; i < 20; i++) {
                do {
                        FD_ZERO(&fds);
                        FD_SET(fd, &fds);

                        /* Timeout. */
                        tv.tv_sec = 2;
                        tv.tv_usec = 0;

                        r = select(fd + 1, &fds, NULL, NULL, &tv);
                } while ((r == -1 && (errno = EINTR)));
                if (r == -1) {
                        perror("select");
                        return errno;
                }

                CLEAR(buf);
                buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
                buf.memory = V4L2_MEMORY_MMAP;
                xioctl(fd, VIDIOC_DQBUF, &buf);

                sprintf(out_name, "out%03d.ppm", i);
                fout = fopen(out_name, "w");
                if (!fout) {
                        perror("Cannot open image");
                        exit(EXIT_FAILURE);
                }
                fprintf(fout, "P6\n%d %d 255\n",
                        fmt.fmt.pix.width, fmt.fmt.pix.height);
                fwrite(buffers[buf.index].start, buf.bytesused, 1, fout);
                fclose(fout);

                xioctl(fd, VIDIOC_QBUF, &buf);
        }

        type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        xioctl(fd, VIDIOC_STREAMOFF, &type);
        for (i = 0; i < n_buffers; ++i)
                v4l2_munmap(buffers[i].start, buffers[i].length);
        v4l2_close(fd);

        return 0;
}