I'm using OpenCV4 to read from a camera. Similar to a webcam. Works great, code is somewhat like this:
cv::VideoCapture cap(0);
cap.set(cv::CAP_PROP_FRAME_WIDTH , 1600);
cap.set(cv::CAP_PROP_FRAME_HEIGHT, 1200);
while (true)
{
cv::Mat mat;
// wait for some external event here so I know it is time to take a picture...
cap >> mat;
process_image(mat);
}
Problem is, this gives many video frames, not a single image. This is important because in my case I don't want nor need to be processing 30 FPS. I actually have specific physical events that trigger reading the image from the camera at certain times. Because OpenCV is expecting the caller to want video -- not surprising considering the class is called cv::VideoCapture -- it has buffered many seconds of frames.
What I see in the image is always from several seconds ago.
So my questions:
Is there a way to flush the OpenCV buffer?
Or to tell OpenCV to discard the input until I tell it to take another image?
Or to get the most recent image instead of the oldest one?
The other option I'm thinking of investigating is using V4L2 directly instead of OpenCV. Will that let me take individual pictures or only stream video like OpenCV?
Related
I am using Opencv for video decoding now. I can play the video with no problem. However, I could not find a way to skip frame, get total number of frames and current frame number. Is there a way to use VideoReader like VideoCapture? I have gone through their api and could not get anything.
OpenCV is Open Source library. If you use VideoCapture then the object of this class has any methods:
To skip a frame you should read 2 times a frame.
To get total number of frames as follows:
cv::VideoCapture m_capture;
....
int totalFrames = m_capture.get(cv::CAP_PROP_FRAME_COUNT)
To get a current frame number:
int currentFrame = m_capture.get(cv::CAP_PROP_POS_FRAMES)
For detailed information visit the OpenCV documentation https://docs.opencv.org/3.1.0/d8/dfe/classcv_1_1VideoCapture.html.
I'm trying to save a video to analyse later with OpenCV algorithms.
I'm using a C++ library of the camera to obtain the frames.
IplImage *iplImageInput = QueryFrame(); //runs every 30 ms
std::vector <cv::Mat> splittedVector;
cv::split(cv::Mat(iplImageInput), splittedVector); //stereo vision camera
// splittedVector buffer is used by the algorithms
So I would like to replace the function "QueryFrame()" with data saved before.
I have already tried some things with cv::videowriter/cv::videocapture but with no luck.
Do you have any hints to eliminate the need for the camera while testing algorithms?
How should I implement a writer and reader to save like 150 frames?
Thanks a lot.
The steps I follow for writing a video file in openCV are as follows:
CvVideoWriter *writer =cvCreateVideoWriter(fileName, Codec ID, frameRate, frameSize); // Create Video Writer
cvWriteFrame(writer, frame); // Write frame
cvReleaseVideoWriter(&writer); // Release video writer
The above code snippet writes at a fixed frame rate. I need to write out variable frame rate videos. The approach I had used earlier with libx264 involved writing individual timestamps to each frame.
So, the question is how do I write timestamps to a frame in openCV - what is the specific API ? More generally, how do I create variable frame rate videos ?
I don't think it is possible to do this with OpenCV directly without modifying the code to give access under the hood. You would need to use a different library like libvlc to do so using the imem to get your raw RGB frames in OpenCV into a file. This link provides an example using imem with raw images loaded from OpenCV. You would just need to change the :sout options to save to the file you want using your preferred codec.
I'm trying to capture the raw data of the logitech pro 9000 (eg. the so called Bayer pattern). This can be achieved by using the so called bayer application, that can be found floating over the internet. It should return a 8 bit bayer pattern, but the results are quite obviously not such a pattern.
However; The image that is being streamed seems to be quite off. As can be seen in the image below, I get 2 images of the scene in a 3 channel image (meaning 6 channels in total). Each image is 1/4th of the total capture area, so it would seem that there is some kind of YUV data being streamed.
I was unable to convert this data into anything meaningful using the conversions provided by openCV. Any ideas what kind of data is being sent and (more importantly) how to convert this into RGB?
EDIT
As requested; the codesnippet that is used to generate the image.
system("Bayer.exe 1 8"); //Sets the camera to raw mode
// set up camera
VideoCapture capture(0);
if(!capture.isOpened()){
waitKey();
exit(0);
}
Mat capturedFrame;
while(true){
capture>>capturedFrame;
imshow("Raw",capturedFrame);
waitKey(25);
}
How did you get frames from stream using openCV? Can you share some code snippets? There are too many video formats in openCV for getting correct color channel and compressed data.
I think you should be able to obtain correct image frames as mentioned here :
http://forum.openrobotino.org/archive/index.php/t-295.html?s=c33acb1fb91f5916080f8dfd687598ec
This is most likely to happen if the out put data format ( width, height, bit depth, no of channels...) of camera and the data format your program expect is different.
However i could capture of logitec pro cam, simply by using
Mat img;
VideoCapture cap(0);
cap >> img;
To track object on video frame, first of all I extract image frames from video and save those images to a folder. Then I am supposed to process those images to find an object. Actually I do not know if this is a practical thing, because all the algorithm did this for one step. Is this correct?
Well, your approach will consume a lot of space on your disk depending on the size of the video and the size of the frames, plus you will spend a considerable amount of time reading frames from the disk.
Have you tried to perform real-time video processing instead? If your algorithm is not too slow, there are some posts that show the things that you need to do:
This post demonstrates how to use the C interface of OpenCV to execute a function to convert frames captured by the webcam (on-the-fly) to grayscale and displays them on the screen;
This post shows a simple way to detect a square in an image using the C++ interface;
This post is a slight variation of the one above, and shows how to detect a paper sheet;
This thread shows several different ways to perform advanced square detection.
I trust you are capable of converting code from the C interface to the C++ interface.
There is no point in storing frames of a video if you're using OpenCV, as it has really handy methods for capturing frames from a camera/stored video real-time.
In this post you have an example code for capturing frames from a video.
Then, if you want to detect objects on those frames, you need to process each frame using a detection algorithm. OpenCV brings some sample code related to the topic. You can try to use SIFT algorithm, to detect a picture, for example.