We're currently developing some functionality for our program that needs OpenCV. One of the ideas being tossed at the table is the use of a "buffer" which saves a minute of video data to the memory and then we need to extract like a 13-second video file from that buffer for every event trigger.
Currently we don't have enough experience with OpenCV so we don't know if it is possible or not. Looking at the documentation the only allowable function to write in memory are imencode and imdecode, but those are images. If we can find a way to write sequences of images to a video file that would be neat, but for now our idea is to use a video buffer.
We're also using OpenCV version 2 specifications.
TL;DR We want to know if it is possible to write a portion of a video to memory.
In OpenCV, every video is treated as a collection of frames(images). Depending on your cameras' FPS you can capture frames periodically and fill the buffer with them. Meanwhile you can destroy the oldest frame(taken 1 min before). So a FIFO data structure can be implemented to achieve your goal. Getting a 13 second sample is easy, just jump to a random frame and write 13*FPS frames sequentially to a video file.
But there will be some sync and timing problems AFAIK and as far as I've used OpenCV.
Here is the link of OpenCV documentation about video i/o. Especially the last chunk of code is what you will use for writing.
TL;DR : There is no video, there are sequential images with little differences. So you need to treat them as such.
Related
In order to accomplish some specific editing on some .avi files, I'd like to create an application (in C++) that is able to load, edit, and save those .avi files. But, what is the most efficient way? When first thinking about it, a simple 3D-Array containing a 2D-array of pixels for every frame seems the simplest solution; But then its size would be ENORMOUS. I mean, let's assume that a pixel only needs a color. One color would mean 3bytes (1char r, 1char b, 1char g). If I now have a 1920x1080 video format, this would mean 2MEGABYTES for only one frame! This data may or may not be smaller if using pointers for the colors, so that alreay used colors wont take more size - I don't really know, since I'm pretty new to C++ and the whole low-level stuff. (As a comparison: One of my AVI files recorded with Xvid codec is 40seconds long, 30fps, and only has 2MB.)
So how would you actually store the video data (Not even the audio, just the video) efficiently (while still being easily able to perform per-frame-changes on it)?
As you have realised, uncompressed video is enormous and it is not practical to store an entire video in this way.
Video compression is an extremely complex topic, but more-or-less, it works as follows: certain "key-frames" are compressed using fairly standard compression techniques similar or identical to still-photo compression such as JPEG. Frames following key-frames are compressed by comparing the frame with the previous one and looking for changes (such as moving blocks). Every now and again, a new key-frame is used.
You don't really have to worry much about that as you are not going to write your own video coder/decoder (codec). There are standard ones.
What will happen is that your program will decode the compressed video frame-by-frame and keep a certain number of frames in memory while you are working on them and then re-encode them when it is finished. In the uncompressed form, you will have access to the individual pixels and can work on them how you want.
You are probably not going to do that either by yourself - it is very hard. You probably need to use a framework, such as OpenCV. There are a huge number of standard filters and tools built in to these frameworks, and it may be that what you want to do is already implemented somewhere.
The OpenCV framework can return individual frames in a Mat object and you can then access the pixels. See this post Get Pixels from Mat
OpenCV
Tutorial page: Open CV Tutorial
Given that FFmpeg is the leading multimedia framework and most of the video/audio players uses it, I'm wondering somethings about audio/video players using FFmpeg as intermediate.
I'm studying and I want to know how audio/video players works and I have some questions.
I was reading the ffplay source code and I saw that ffplay handles the subtitle stream. I tried to use a mkv file with a subtitle on it and doesn't work. I tried using arguments such as -sst but nothing happened. - I was reading about subtitles and how video files uses it (or may I say containers?). I saw that there's two ways putting a subtitle: hardsubs and softsubs - roughly speaking hardsubs mode is burned and becomes part of the video, and softsubs turns a stream of subtitles (I might be wrong - please, correct me).
The question is: How does they handle this? I mean, when the subtitle is part of the video there's nothing to do, the video stream itself shows the subtitle, but what about the softsubs? how are they handled? (I heard something about text subs as well). - How does the subtitle appears on the screen and can be configured changing fonts, size, colors, without encoding everything again?
I was studying some video players source codes and some or most of them uses OpenGL as renderer of the frame and others uses (such as Qt's QWidget) (kind of or for sure) canvas. - What is the most used and which one is fastest and better? OpenGL with shaders and stuffs? Handling YUV or RGB and so on? How does that work?
It might be a dump question but what is the format that AVFrame returns? For example, when we want to save frames as images first we need the frame and then we convert, from which format we are converting from? Does it change according with the video codec or it's always the same?
Most of the videos I've been trying to handle is using YUV720P, I tried to save the frames as png and I need to convert to RGB first. I did a test with the players and I put at the same frame and I took also screenshots and compared. The video players shows the frames more colorful. I tried the same with ffplay that uses SDL (OpenGL) and the colors (quality) of the frames seems to be really low. What might be? What they do? Is it shaders (or a kind of magic? haha).
Well, I think that is it for now. I hope you help me with that.
If this isn't the correct place, please let me know where. I haven't found another place in Stack Exchange communities.
There are a lot of question in one post:
How are 'soft subtitles' handled
The same way as any other stream :
read packets from a stream to the container
Give the packet to a decoder
Use the decoded frame as you wish. Here with most containers supporting subtitles the presentation time will be present. All you need at this time is get the text and burn it onto the image at the same presentation time. There are a lot of ways to print the text on the video, with ffmpeg or another library
What is the most used renderer and which one is fastest and better?
most used depend on the underlying system. For instance Qt only wrap native renderers, and even has a openGL version
You can only be as fast as the underlying system allows. Does it support ouble-buffering? Can it render in your decoded pixel format or do you have to perform color conversion before? This topic is too broad
Better only depend on the use case. this is too broad
what is the format that AVFrame returns?
It is a raw format (enum AVPixelFormat), and depends on the codec. There is a list of YUV and RGB FOURCCs which cover most formats in ffmpeg. Programmatically you can access the table AVCodec::pix_fmts to obtain the pixel format a specific codec support.
I'm trying to write an application that records and saves the screen in C++ on the windows platform. I'm not sure where to start with this. I assume I need some sort of API, (FFMPEG, maybe OpenGL?). Could someone point me in the right direction?
You could start by looking at Windows remote desktop protocol, maybe some programming libraries are provided for that.
I know of a product that intercepts calls into the Windows GDI dll and uses that to store the screen drawing activities.
A far more simpler approach would be to do screenshots as often as possible and somehow minimize redundant data (parts of the screen that didn't change between frames).
If the desired output of your app is a video file (like mpeg) you are probably better off just grabbing frames and feeding them into a video encoder. I don't know how fast the encoders are these days. Ffmpeg would be a good place to start.
If the encoder turns out not fast enough, you can try storing the frames and encoding the video file afterwards. Consecutive frames should have many matching pixels, so you could use that to reduce the amount of data stored.
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.
I am looking for a fast way to load in a video file and to create images from them at certain intervals ( every second, every minute, every hour, etc.).
I tried using DirectShow, but it just ran too slow for me to start the video file and move to a certain location to get data and to save it out to an image. Even if I disabled the reference clock. Tried OpenCV, but it has trouble opening the AVI file unless I know the exact codec information. So if I know a way to get the codec information out from OpenCV I may give it another shot. I tried to use FFMPEG, but I don't have as much control over it as well as I would wish.
Any advice would be greatly appreciated. This is being developed on a Windows box since it has to be hosted on a Windows box.
MPEG-4 format is not an intra-coded format, so you can't just jump to a random frame and decode it on its own, as most frames only encode the differences from one or more other frames. I suspect your decoding is slow because when you land on a frame for which several other dependent frames to be decoded first.
One way to improve performance would be to determine which frames are keyframes (or sometimes also called 'sync' points) and limit your decoding to those frames, since these can be decoded on their own.
I'm not very familiar with DirectShow capabilities, but I would expect it has some API to expose sync points.
Also, I should mention that the QuickTime SDK on Windows is possibly another good option that you have for decoding frames from movies. You should first test that your AVI movies are played correctly in the QuickTime Player. And the QT SDK does expose sync points, see the section Finding Interesting Times in the QT SDK documentation.
ffmpeg's libavformat might work for ya...