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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.

Jonathan Hall
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Soatl
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    What do you want to do with the AVI file? You might want to have a look at FFmpeg: http://ffmpeg.org I also see an OpenCV tag. Could you give us a bit more of an explanation what you're trying to do? – Bart May 05 '11 at 14:33
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    Use fread() to read any file in you want, even .avi files :) – codymanix May 05 '11 at 14:33
  • Or ifstream, which is more C++y than fread(), but both work just as well as the other. If you want video frames, you will need something that can parse AVI files and decode the compressed frames, like ffmpeg/libavfile/libavcodec as suggested above. – onitake May 05 '11 at 14:40
  • @Bart - Check the edit for an explanation of the program – Soatl May 05 '11 at 15:00
  • So you have a program that (based on a video) gives you a set of images. And once you're done with those images, you want to make them into a new video file? – Bart May 05 '11 at 15:04
  • Yes. I guess the program is supposed to take a video, and make it so that there is always at least one face that is the subject of the frame (my lingo may be off). It is a frame by frame comparison of the faces (for cropping) and in the end we want to take each frame and use them to make a video file, – Soatl May 05 '11 at 15:13

2 Answers2

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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.

Alok Save
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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.

count0
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  • I think opencv 2.1 ffmpeg support is quite broken as of now (api change in ffmpeg). I had no opportunity to check on latest release so im curious if it has been fixed yet. – count0 May 05 '11 at 16:14