Can I get compressed data from cvCapture, which is stored into buffer after cvGrabFrame, before using cvRetrieveFrame? I need to keep them in the an archive of compressed frames.
I use cvCaptureFromFile for getting rtsp stream from Axis IP camera.
As far as I see it it's not possible. CvCapture is a black box structure. There is not really a possibility to access the data inside.
There is the function cv::imencode to compress images. It is not optimal to compress the image again but it should work.
By the way, you're using the old C API and there is a newer API for C++ that makes capturing more elegant: cv::VideoCapture.
Related
I'm using ffmpeg to encode H263 and MPEG4 videos.
I take frames from a camera by opencv.
I'd like to know if it's possible to store in a memory buffer the frame I would write on the file with the av_write_frame() function I use now.
And if it's possible, how do you do it? (which functions of the API do you use, and how?)
Due to some reasons I need to access directly to image buffer from a camera through OpenCV's VideoCapture but I cannot find a way. To make it more clear, I want to access to the data from cv::VideoCapture::grab() before retrieving it to a cv::Mat.
I check the OpenCV source code here
and it seems OpenCV decodes it automatically before outputing the frame. Intuitionally I am thinking about "encoding" the frame to obtain the original data, however, cv::imencode requires a specific file extension.
Is there a way to access the camera buffer data without tweaking the source code?
Best,
Eric
Has anyone tried to modify the CISCO openh264 library to take JPEG images as input and compress them into P and I frames (output as frames, NOT video) and similarly to modify decoder to take compressed P and I frames and generate uncompressed-frames ?
I have a camera looking at a static scene and taking pictures (1280x720p) every 30 second. The scene is almost static. Currenlty I am using JPEG compression to compress each frame individually and it is resulting in an image size of ~270KB. This compressed frame is transferred via internet to a storage server. Since there is very little motion in the scene, the 'I' frame size will be very small (I think it should be ~20-50KB). So it will be very cost effective to transmit I frames over internet instead of JPEG images.
Can anyone guide me to some project or about how to proceed with this task ?
You are describing exactly what a codec does. It takes images, and compresses them. There relationship in time is irrelevant to the compression step. The decoder than decides how to display or just write them to disk. You don't need to modify open264, what you want to do is exactly what it is designed to do.
I want to read in an .avi video file for a program that I am making. I have the file location saved as a string. Is there any good tutorials on using .avi files in c++ or does anyone know who to read one in? Is it the same as normal files?
I have a previously asked SO question that goes into better detail but here is what I want to do:
I am making a program that will detect faces (though OpenCV) As of now I have been given a video processor program that will detect each face on a frame, and return the frame as a image and the CvRec of the faces. I want to take these faces and test them to validate that they are all actually faces.
After I have all the faces (tested) I want to then take the images and test them together. I test the faces on each frame for size and distance changes. If the faces pass this for a frame length of two seconds, then I want to crop the face and make it the subject of each frame.
After each frame is cropped I then want to save the new video file for the user.
Hopefully that helps. If anyone needs a better explanation please let me know.
First of all, a little background.
What is AVI?
AVI stands for Audio Video Interleave. It is a special case of the RIFF (Resource Interchange File Format). AVI is defined by Microsoft and it is the most common format for audio/video data.
I assume you would want to read a avi file and decode the compressed video frames. AVI file is just like any other normal file and you can use fread()(in C) or iostream(in C++) to open an avi file and read it contents. But the contents of an avi file are video frames in a compressed format. The compression allows video content of bigger sizes to be efficiently packed in less memory space.To make any sense of this compressed data you would have to decode the encoded data format.You will have to study the standard which describes how AVI encoding is done and then extract and decode the frames. this raw video data now when fed to a video device will be displayed in video format.
It seems you are staying within OpenCV so things are easy. If OpenCV is compiled properly it is capable of delegating io/coding/decoding to other libraries. Quicktime and others for example, but best is to use ffmpeg. You open, read and decode everything using the OpenCV API which gives you the video frame by frame.
Make sure your OpenCV is compiled with ffmpeg support and then read the OpenCV tutorial on how to read/write AVI files. It's really easy.
Getting OpenCV to be built with ffmpeg support might be hard though. You might want to switch to an older version of OpenCV if you can't get ffmpeg running with the current one.
Personally i would not spent time trying to read the video by yourself and delegate the task to OpenCV. That's how it is supposed to be used.
I am writing some video files under Windows from a camera.
I need the data unaltered - not MP4's 'uncompressed' ie. no YUV, no color interpolation - just the raw camera sensor bytestream.
At the moment I am writing this direct to disk and re-reading it later to recode into a usable video. But with no header I have to keep track of image size, frame rate, color balance etc separately.
I could add a custom header but even if the actual video data is unreadable by anything other than my app, using an AVI file would at least give me a relatively standard header to store all the camera parameters and also means that resolution, length etc would show up in explorer.
Is there an easy way of generating an AVI header/footer without sending all the data through directshow or vfw? The data is coming in at >250MB/s and I can't lose any frames so I don't have time to do much more than dump each frame to disk.
edit: Perhaps MP4 would be better I have a lot of metadata about the camera config that isn't in the AVI standard
Well, after figuring out what 'reasonable' AVI headers would be for your stream (e.g. if you use a custom codec fourcc, no application would probably be able to do useful things with it -- so why bother with AVI?)
you could just write a prebuild RIFF-AVI header at the beginning of your file. It's not too hard to figure out the values.
Each frame then has to be enclosed in its own RIFF chunk (4 Byte type: "00db" + 4 byte length + your data).
After the fact you have to fix the num_frames and some length fields in the header. And for files >2GB don't forget the OpenDML extension for the header.
Martin, since you are proficient in OpenCV, couldn't you just use cvCreateVideoWriter() for creating an uncompressed .avi?
CvVideoWriter* cvCreateVideoWriter(const char* filename, int fourcc, double fps, CvSize frame_size, int is_color=1)
Regarding the foucc param, the documentation states:
Under Win32 if 0 is passed while using an avi filename it will create a video writer that creates an uncompressed avi file.
It sounds like you could really benefit from using opencv, it could probably handle alot of this nicely for you. Take a look and see if it suits your needs: http://opencv.willowgarage.com/documentation/cpp/reading_and_writing_images_and_video.html#videocapture
You can use OpenCV to read and write avi file.
see http://opencv.willowgarage.com/documentation/cpp/reading_and_writing_images_and_video.html
Note that OpenCV can also be used to grab images from a camera.