The problem is the following:
There is a CIF video file that is supposed to be processed by the OpenCV.
Unfortunately, I am not able to read frames from this video. The following code
cv::VideoCapture cap = cv::VideoCapture("foreman.cif");
if(!cap.isOpened()) {
std::cout << "Open file error" << std::endl;
return -1;
}
gives Open file error in console.
Is there any way to grab frames from CIF video using OpenCV?
I don't think openCV can read raw YUV streams directly.
You can use memcoder to convert it to an avi, compressed or not.
Or you can use regular c/c++ to read the data blocks, copy them into an image and use cvCvtColor() to convert YUV->BGR
Related
Reading video from disk is simple - CvCapture* capture = cvCreateFileCapture(argv[1]);, but I need to read mp4 file which is already in std::string VideoBuffer.
How to do that?
I want to detect motion in already existing video, The video is stored in the webm format. I have seen some demo of opencv but those samples is capturing the motion of the live webcam streaming.
Is there any library or api which capture the motion of the webm video file in c++?
please help me.
If you have the code that run with the webcam input you only have to change the input type to accept the video file as input.
Basically, you can accomplish it using the VideoCapture object.
cv::VideoCapture cap("path/for/file.fileextension")
and then, putting this input into a Mat datatype (separating by frame):
Mat frame;
cap >> frame;
I am currently work on a project that capture video from webcam and send the encoded stream via UDP to do a real time streaming.
#include "opencv2/highgui/highgui.hpp"
#include <iostream>
using namespace cv;
using namespace std;
int main(int argc, char* argv[])
{
VideoCapture cap(0); // open the video camera no. 0
double dWidth = cap.get(CV_CAP_PROP_FRAME_WIDTH); //get the width of frames of the video
double dHeight = cap.get(CV_CAP_PROP_FRAME_HEIGHT); //get the height of frames of the video
while (1)
{
Mat frame;
bool bSuccess = cap.read(frame); // read a new frame from video
if (!bSuccess) //if not success, break loop
{
cout << "Cannot read a frame from video stream" << endl;
break;
}
return 0;
}
Some people say that the frame get from cap.read(frame) is already the decoded frame,I have no idea how and when does that happen. And what I want is the encoded frame or stream. What should I do to get it? Should I encoded it back again?
According to the docs, calling VideoCapture::read() is equivalent to calling VideoCapture::grab() then VideoCapture::retrieve().
The docs for the Retrieve function say it does indeed decode the frame.
Why not just use the decoded frame; presumably you'd be decoding it at the far end in any case?
OpenCV API does not give access to the encoded frames.
You will have to use a more low-level library, probably device and platform dependent. If your OS is Linux, Video4Linux2 may be an option, there must be equivalent libraries for Windows/MacOS. You may also have a look at mjpg-streamer, which does something very similar to what you want to achieve (on linux only).
Note that the exact encoding of the image will depend on your webcam, some usb webcam support mjpeg compression (or even h264), but other are only able to send raw data (usually in yuv colorspace).
Another option is to grab the decoded image wit Opencv, and reencode it, for example with imencode. It has the advantages of simplicity and portability, but image reencoding will use more resource.
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 want to read and show a video using opencv. I've recorded with Direct-show, the Video has UYVY (4:2:2) codec, since opencv can't read that format, I want to convert the codec to an RGB color model, I readed about ffmpeg and I want to know if it's possible to get this done with it ? if not if you a suggestion I'll be thankful.
As I explained to you before, OpenCV can read some formats of YUV, including UYVY (thanks to FFmpeg/GStreamer). So I believe the cv::Mat you get from the camera is already converted to the BGR color space which is what OpenCV uses by default.
I modified my previous program to store the first frame of the video as PNG:
cv::Mat frame;
if (!cap.read(frame))
{
return -1;
}
cv::imwrite("mat.png", frame);
for(;;)
{
// ...
And the image is perfect. Executing the command file on mat.png reveals:
mat.png: PNG image data, 1920 x 1080, 8-bit/color RGB, non-interlaced
A more accurate test would be to dump the entire frame.data() to the disk and open it with an image editor. If you do that keep in mind that the R and B channels will be switched.