The Value of retval is always False, which means the video is not read by the command. It must be True to read frames. I don't know what to do. However when I use my default webcam it turns to be True. I tried many videos and the same problem appears. Note: I have installed the ffmpeg correctly.
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Note: This is not the full code, in this step I am only validating cap.read() either True or False
2 Answers
This method is guaranteed 100%
first of all check your version of OpenCV, say for instance 2.4.11. you can check it by typing the following commands in your Python Shell:
Then go to C:opencvbuildx86vc12bin and copy opencv_ffmpeg2411.dll.Finally go to root directory of Python ex: C:Python27 and paste opencv_ffmpeg2411.dll in it
check the name of the file opencv_ffmpeg2411.dll, whether the version of opencv is written or not, if not do the following opencv_ffmpeg(version of your opencv without dots).dll
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After that create a new Python file and copy this code and paste it loading your own video
you will have an output video for example like this:Result
Tes3awyTes3awyFinding the root directory of Python can be a little tricky. I am using an Enthought distribution and, at first, pasted the opencv_ffmpeg file into the wrong Python directory.
WRONG:
RIGHT:
Long story short, make sure you find the right Python directory.
Leonardo Alves MachadoNot the answer you're looking for? Browse other questions tagged python-2.7opencvvideo-capturevideo-processing or ask your own question.
Goal¶
- Learn to read video, display video and save video.
- Learn to capture from Camera and display it.
- You will learn these functions : cv2.VideoCapture(), cv2.VideoWriter()
Capture Video from Camera¶
Often, we have to capture live stream with camera. OpenCV provides a very simple interface to this. Let’s capture a video from the camera (I am using the in-built webcam of my laptop), convert it into grayscale video and display it. Just a simple task to get started.
To capture a video, you need to create a VideoCapture object. Its argument can be either the device index or the name of a video file. Device index is just the number to specify which camera. Normally one camera will be connected (as in my case). So I simply pass 0 (or -1). You can select the second camera by passing 1 and so on. After that, you can capture frame-by-frame. But at the end, don’t forget to release the capture.
cap.read() returns a bool (True/False). If frame is read correctly, it will be True. So you can check end of the video by checking this return value.
Sometimes, cap may not have initialized the capture. In that case, this code shows error. You can check whether it is initialized or not by the method cap.isOpened(). If it is True, OK. Otherwise open it using cap.open().
Install Video Reader Opencv For Mac Download
You can also access some of the features of this video using cap.get(propId) method where propId is a number from 0 to 18. Each number denotes a property of the video (if it is applicable to that video) and full details can be seen here: Property Identifier. Some of these values can be modified using cap.set(propId, value). Value is the new value you want.
For example, I can check the frame width and height by cap.get(3) and cap.get(4). It gives me 640x480 by default. But I want to modify it to 320x240. Just use ret=cap.set(3,320) and ret=cap.set(4,240).
Note
If you are getting error, make sure camera is working fine using any other camera application (like Cheese in Linux).
Playing Video from file¶
It is same as capturing from Camera, just change camera index with video file name. Also while displaying the frame, use appropriate time for cv2.waitKey(). If it is too less, video will be very fast and if it is too high, video will be slow (Well, that is how you can display videos in slow motion). 25 milliseconds will be OK in normal cases.
Note
Mac Python Opencv
Make sure proper versions of ffmpeg or gstreamer is installed. Sometimes, it is a headache to work with Video Capture mostly due to wrong installation of ffmpeg/gstreamer.
Saving a Video¶
So we capture a video, process it frame-by-frame and we want to save that video. For images, it is very simple, just use cv2.imwrite(). Here a little more work is required.
This time we create a VideoWriter object. We should specify the output file name (eg: output.avi). Then we should specify the FourCC code (details in next paragraph). Then number of frames per second (fps) and frame size should be passed. And last one is isColor flag. If it is True, encoder expect color frame, otherwise it works with grayscale frame.
Install Opencv Mac Os
FourCC is a 4-byte code used to specify the video codec. The list of available codes can be found in fourcc.org. It is platform dependent. Following codecs works fine for me.
- In Fedora: DIVX, XVID, MJPG, X264, WMV1, WMV2. (XVID is more preferable. MJPG results in high size video. X264 gives very small size video)
- In Windows: DIVX (More to be tested and added)
- In OSX : (I don’t have access to OSX. Can some one fill this?)
FourCC code is passed as cv2.VideoWriter_fourcc('M','J','P','G') or cv2.VideoWriter_fourcc(*'MJPG) for MJPG.
Below code capture from a Camera, flip every frame in vertical direction and saves it.