OpenCV 2.4 in python - Video processing

Question

The Project: Add a running date/time stamp on each and every frame of a video. (The result of digital video camera, and my father asked me how can he add the timestamp(to the mi

Answer 1

Your task should be relatively easy to accomplish using OpenCV and Python. It seems that you are new to OpenCV, so I will try and keep my explanation thorough, but feel free to ask if you need any clarity.

I am not sure if you are getting your data from a live camera video feed, or are post-processing recorded footage. Either way...

Get data. If using a live feed:

capture = cv2.VideoCapture(0)

If using recorded footage:

capture = cv2.VideoCapture("your_reading_file.avi")

Initialise video writer. Look at this question for help with codecs- finding a working codec is not trivial. I am also using Windows 7 x64, and the below-mentioned codec was the only one that worked for me. Also, set the variable fps as close to the actual incoming video framerate as possible- you can not change it once you have started writing frames.

flag, frame = capture.read() # **EDIT:** to get frame size
width = np.size(frame, 1) #here is why you need numpy!  (remember to "import numpy as np")
height = np.size(frame, 0)
writer = cv2.VideoWriter(filename="your_writing_file.avi", 
fourcc=cv2.cv.CV_FOURCC('I', 'Y', 'U', 'V'), #this is the codec that works for me
fps=15, #frames per second, I suggest 15 as a rough initial estimate
frameSize=(width, height))

Process this data and add your text. Lastly, write the edited frame to a video file.

while True:
    flag, frame = capture.read() #Flag returns 1 for success, 0 for failure. Frame is the currently processed frame

    if flag == 0: #Something is wrong with your data, or the end of the video file was reached
        break 
    x = width/2
    y = height/2 #change to the desired coordinates
    text_color = (255,0,0) #color as (B,G,R)
    cv2.putText(frame, "your_string", (x,y), cv2.FONT_HERSHEY_PLAIN, 1.0, text_color, thickness=1, lineType=cv2.CV_AA)

    writer.write(frame) #write to the video file

As simple as that! I use the above code to write text to video files almost daily, so it definitely works. The only potential problems I can foresee are with the codecs, which I unfortunately do not know a lot about. I hope that this may solve your problem, feel free to ask more questions.

EDIT: Answers to your comment's questions.

1.) As far as I know, you can only use .avi because you have to use an uncompressed format with OpenCV. I am afraid I have no knowledge of using other (compressed) formats. Maybe you could use a third-party program to do pre/post-conversion? The reason for the frame exception was my mistake, I have edited the answer to include the missing line.

2.) I'm afraid I have no idea how to read metadata. If I find out I will let you know. My own hackish solution for finding video framerate is to let OpenCV run through the video once, using the Time module to calculate the average framerate. This estimate can then be used when writing the video file.

3.) I have found that the size of the resulting video may differ significantly from the original depending on several factors, the most important being how close the chosen fps was to the actual original framerate.

4.) As for other fonts, there are several available. I can refer you to this question for a quick overview. Here is the relevant documentation:

fontFace – Font type. One of FONT_HERSHEY_SIMPLEX, 
FONT_HERSHEY_PLAIN, 
FONT_HERSHEY_DUPLEX, 
FONT_HERSHEY_COMPLEX, 
FONT_HERSHEY_TRIPLEX, 
FONT_HERSHEY_COMPLEX_SMALL, 
FONT_HERSHEY_SCRIPT_SIMPLEX, or 
FONT_HERSHEY_SCRIPT_COMPLEX, 
where each of the font ID’s can be combined with FONT_HERSHEY_ITALIC to get the slanted letters.