Corrupt images, when capturing images from multiple cameras using the V4L2 driver

[Mácha, Karel <KMacha@xxxxxxxxxxxxxx>]

Hello, 

I would like to grab images from multiple cameras under using the V4L2
API. I followed the presentation under found on
http://linuxtv.org/downloads/presentations/summit_jun_2010/20100206-fosdem.pdf 
used the code and adapted it sightly for my purpose. It works very well
for 1 camera.

However Once I begin to grab images from multiple cameras (successively)
I get corrupt images. I uploaded an example image to
http://www.directupload.net/file/d/3733/9c4jx3pv_png.htm

Although I set the right resolution for the camera (744 x 480), the
output of buffer.bytesused, after the VIDIOC_DQBUF does not correspond
with the expected value (744x480 = 357120). This would probably explain
the corrupt images.

The more camera I use, the less buffer.bytesused I get and the more
stripes are in the image. Could you please give me a hint, what am I
doing wrong ?

Thanks, Karel

Here is the minimal C code I use for my application:


int main()
{
	/* ##################### INIT ##################### */

	int numOfCameras = 6;
	int xRes = 744;
	int yRes = 480;
	int exposure = 2000;
	unsigned int timeBetweenSnapshots = 2; // in sec
	char fileName[sizeof "./output/image 000 from camera 0.PNG"];

	static const char *devices[] = { "/dev/video0", "/dev/video1",
"/dev/video2", "/dev/video3", "/dev/video4", "/dev/video5",
"/dev/video6", "/dev/video7" };

	struct v4l2_capability cap[8];
	struct v4l2_control control[8];
	struct v4l2_format format[8];
	struct v4l2_requestbuffers req[8];
	struct v4l2_buffer buffer[8];

	int type = V4L2_BUF_TYPE_VIDEO_CAPTURE; // had to declare the type here
because of the loop

	unsigned int i;
	unsigned int j;
	unsigned int k;

	int fd[8];
	void **mem[8];
	//unsigned char **mem[8];

	/* ##################### OPEN DEVICE ##################### */

	for (j = 0; j < numOfCameras; ++j) {

		fd[j] = open(devices[j], O_RDWR);
		ioctl(fd[j], VIDIOC_QUERYCAP, &cap[j]);


		/* ##################### CAM CONTROLL ############### */

		control[j].id = V4L2_CID_EXPOSURE_AUTO;
		control[j].value = V4L2_EXPOSURE_SHUTTER_PRIORITY;
		ioctl(fd[j], VIDIOC_S_CTRL, &control[j]);

		control[j].id = V4L2_CID_EXPOSURE_ABSOLUTE;
		control[j].value = exposure;
		ioctl(fd[j], VIDIOC_S_CTRL, &control[j]);

		/* ##################### FORMAT ##################### */

		ioctl(fd[j], VIDIOC_G_FMT, &format[j]);
		format[j].type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
		format[j].fmt.pix.width = xRes;
		format[j].fmt.pix.height = yRes;
		//format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
		format[j].fmt.pix.pixelformat = V4L2_PIX_FMT_GREY;
		ioctl(fd[j], VIDIOC_S_FMT, &format[j]);

		/* ##################### REQ BUF #################### */

		req[j].type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
		req[j].count = 4;
		req[j].memory = V4L2_MEMORY_MMAP;
		ioctl(fd[j], VIDIOC_REQBUFS, &req[j]);
		mem[j] = malloc(req[j].count * sizeof(*mem));

		/* ##################### MMAP ##################### */

		for (i = 0; i < req[j].count; ++i) {
			buffer[j].type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
			buffer[j].memory = V4L2_MEMORY_MMAP;
			buffer[j].index = i;
			ioctl(fd[j], VIDIOC_QUERYBUF, &buffer[j]);
			mem[j][i] = mmap(0, buffer[j].length,
					PROT_READ|PROT_WRITE,
					MAP_SHARED, fd[j], buffer[j].m.offset);
		}

		/* ##################### CREATE QUEUE ############### */

		for (i = 0; i < req[j].count; ++i) {
			buffer[j].type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
			buffer[j].memory = V4L2_MEMORY_MMAP;
			buffer[j].index = i;
			ioctl(fd[j], VIDIOC_QBUF, &buffer[j]);
		}

	} /* ### ### end of camera init ### ### */

	/* ##################### STREAM ON ##################### */
	for (j = 0; j < numOfCameras; ++j) {

		ioctl(fd[j], VIDIOC_STREAMON, &type);
	}


	/* ##################### GET FRAME ##################### */

	k = 0;
	while (!kbhit()){
		k ++;

		for (j = 0; j < numOfCameras; j++) {

			buffer[j].type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
			buffer[j].memory = V4L2_MEMORY_MMAP;
			usleep(100000);
			ioctl(fd[j], VIDIOC_DQBUF, &buffer[j]);
			printf("\nBuffer {%p}, Buf. Index %d, Buf. bytes used %d\n",
mem[j][buffer[j].index], buffer[j].index,  buffer[j].bytesused);

			// create filename
			sprintf(fileName, "./output/image %03d from camera %d.PNG", k, j);
			// save as PNG file
			saveToPng(mem[j][buffer[j].index], fileName, xRes, yRes);

			ioctl(fd[j], VIDIOC_QBUF, &buffer[j]);

			sleep(timeBetweenSnapshots);
		}
	}

	/* ##################### STREAM OFF ##################### */
	for (j = 0; j < numOfCameras; ++j) {

		ioctl(fd[j], VIDIOC_STREAMOFF, &type);
	}

	/* ##################### CLEANUP ##################### */

	for (j = 0; j < numOfCameras; ++j) {

		close(fd[j]);
		free(mem[j]);
	}

	return (0);
}

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D-14469 Potsdam
 
Vorstand: 
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