Save Images to disk from a vector of images

[HanzDieter]

Hello,

I have the following code:

#include <opencv2/opencv.hpp>
#include <librealsense2/rs.hpp>

int main() {
	// Declare the RealSense pipeline and configure it for IR stream
	rs2::pipeline pipeline;
	rs2::config cfg;
	cfg.enable_stream(RS2_STREAM_INFRARED, 1, 1280, 720, RS2_FORMAT_Y8, 30);
	cfg.enable_stream(RS2_STREAM_INFRARED, 2, 1280, 720, RS2_FORMAT_Y8, 30);

	// Start the pipeline
	pipeline.start(cfg);

	// Declare vectors to store the left and right images
	std::vector<cv::Mat> left_images;
	std::vector<cv::Mat> right_images;

	// Capture and save 100 images from the left and right IR streams
	for (int i = 0; i < 100; i++) {
		// Wait for a new frame from the camera
		rs2::frameset frames = pipeline.wait_for_frames();

		// Get the left and right IR images from the frameset
		rs2::frame left_frame = frames.get_infrared_frame(1);
		rs2::frame right_frame = frames.get_infrared_frame(2);

		// Convert the left and right IR images to OpenCV Mat objects
		cv::Mat left_image(cv::Size(1280, 720), CV_8UC1, (void*)left_frame.get_data());
		cv::Mat right_image(cv::Size(1280, 720), CV_8UC1, (void*)right_frame.get_data());

		// Save the left and right images to the vectors
		left_images.emplace_back(left_image);
		right_images.emplace_back(right_image);
		cv::imshow("left_i", left_images[i]);
		cv::waitKey(100);
		
	}
	
	for (int a = 0; a < left_images.size(); a++)
	{
		std::cout << left_images.size() << std::endl;
		// Save the left and right images to the hard disk
		std::string left_filename, right_filename;
		left_filename = "left_image_" + std::to_string(a) + ".png";
		right_filename = "right_image_" + std::to_string(a) + ".png";
cv::imwrite(left_filename, left_images[a]);
		cv::imwrite(right_filename, right_images[a]);
		cv::imshow("left", left_images[a]);
		cv::waitKey(100);
	}

	// Stop the pipeline
	pipeline.stop();

	return 0;
}

My idea was to capture 100 frames and save them into two vectors. After the 100 frames there is a second for loop for saving the images from two vectors of the left and right stream. But this is not working! As you can see in the second for loop I have cv::imshow("left", left_images[a]); So I can see what is actually been saved. To my surprise not the images from the first for loop where saved, but actual frames of my D405 are showed to me and saved, but I´m not in the first for loop where the images from my D405 are recieved. I am in the second for loop and just want to show and save my images, which where recieved in the first for loop. Can anyone explain this to me?
Another strange thing is: If i put the codeline pipeline.stop(); before the second for loop, then I cant save my two vectors of images to disk (2nd for loop) and I cant see them with cv::imshow("left", left_images[a]);. It seems the vector is empty or sth. else, but the left_images.size() which is called in the second for loop gives me the 100.
I dont know whats going on there.

Another strange behaviour is, that in the second for loop the left and right images where mixed. Means that I have images which belongs to the right imager, and they where saved as left image. The left images where saved in the following way: two frames from the left imager, two frames of the right imager, two frames of the left, then two frames of the right and so on, with ascending numeration.
In the first for loop I also watch the actual frames with cv::imshow("left_i", left_images[i]);. But here the frames are correct, means that only frames from the left imager is shown.
Can somebody help me please?

[Nir-Az]

@HanzDieter Hi,
According to cv::Mat docs

data. Matrix constructors that take data and step parameters do not allocate matrix data. Instead, they just initialize the matrix header that points to the specified data, which means that no data is copied

This means that when you loop you drop the frames managed by the SDK.
The SDK see its out of scope so he releases this memory.
You have 2 options:

1. explicitaly copy the frames from the SDK
2. Use other SDK API that has the keep method.
   this will tell the SDK not to free this memory

Good luck

[HanzDieter]

Hi Nir-Az,

that was the problem. Now all problems are gone. Thank you very very much!