ffmpeg crop detect python using subprocess

Question

I want to use python subprocess to call ffmpeg and use crop detect to find all black in a video. The crop detect return I want to put into a string variable and put in a database. At the moment I can get the process running in terminal and but I am unsure about how to grab the specific part of the terminal (stdout) output:

the script:

def cropDetect():
    p = subprocess.Popen(["ffmpeg", "-i", "/Desktop/ffprobe_instance/Crop_detect/video_file.mpg", "-vf", "cropdetect=24:16:0", "-vframes", "10", "dummy.mp4"], stdout=subprocess.PIPE)
    result = p.communicate()[0]
    print result


# SCRIPT
cropDetect()

Result in terminal: [Parsed_cropdetect_0 @ 0x7fa1d840cb80] x1:719 x2:0 y1:575 y2:0 w:-704 h:-560 x:714 y:570 pos:432142 pts:44102 t:0.490022 crop=-704:-560:714:570

How do I take "crop=-704:-560:714:570" and put it into a variable that I can store in a database?

As per update:

def cropDetect1():
    p = subprocess.check_output(["ffmpeg", "-i", "/Desktop/ffprobe_instance/Crop_detect/video_file.mpg", "-vf", "cropdetect=24:16:0", "-vframes", "10", "dummy.mp4"])
    match = re.search("crop\S+", p)
    crop_result = None
    if match is not None:
        crop_result = match.group()
        print "hello %s" % crop_result

I can't seem to print out the "crop_result" - I am presuming that means that the variable is empty?

UPDATE: Found it:

def detectCropFile(localPath):
    fpath = "/xxx/xx/Desktop/Crop_detect/videos/USUV.mp4"
    print "File to detect crop: %s " % fpath
    p = subprocess.Popen(["ffmpeg", "-i", fpath, "-vf", "cropdetect=24:16:0", "-vframes", "500", "-f", "rawvideo", "-y", "/dev/null"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    infos = p.stderr.read()
    print infos
    allCrops = re.findall(CROP_DETECT_LINE + ".*", infos)
    print allCrops 
    mostCommonCrop = Counter(allCrops).most_common(1)
    print "most common crop: %s" % mostCommonCrop
    print mostCommonCrop[0][0]
    global crop
    crop = mostCommonCrop[0][0]
    video_rename()

Use: p = subprocess.Popen(["ffmpeg", "-i", fpath, "-vf", "cropdetect=24:16:0", "-vframes", "500", "-f", "rawvideo", "-y", "/dev/null"], stdout=subprocess.PIPE, stderr=subprocess.PIPE) to pipe it out

Do you see the 'hello' string? I'd either step through this in a debugger, or throw in print statements to see. For instance, print p, and then print match. I suspect you won't see anything for match. — jwilner
– jwilner, Commented Apr 8, 2015 at 16:12
Hey - no there is no hello - but if I do "print 'hello p %s' % p" and "print 'hello match %s' % match" I do get "hello p" and "hello match" — JRM
– JRM, Commented Apr 8, 2015 at 16:17
Well, you shouldn't be seeing just 'hello p' in that situation. You should be seeing 'hello Parsed_cropdetect_0 etc p', no? Same with the match -- you should be seeing something like `hello <_sre.SRE_Match at 0xmemoryaddress> match'. It looks like your ffmpeg isn't outputting to stdout. — jwilner
– jwilner, Commented Apr 8, 2015 at 16:19
running the script I get full result in the terminal window: [Parsed_cropdetect_0 @ 0x7f96cb40cb80] x1:719 x2:0 y1:575 y2:0 w:-704 h:-560 x:714 y:570 pos:-1 pts:8102 t:0.090022 crop=-704:-560:714:570 [Parsed_cropdetect_0 @ 0x7f96cb40cb80] x1:719 x2:0 y1:575 y2:0 w:-704 h:-560 x:714 y:570 pos:10254 pts:11702 t:0.130022 crop=-704:-560:714:570 - I just can't seem to figure out how to take all that and parse/write out/pipe out that information — JRM
– JRM, Commented Apr 8, 2015 at 16:21
What about when you redirect the command? Run the command but redirect stderr to /dev/null like so: command 2> /dev/null. I think that terminal output is probably being sent to stderr instead of stdout, in which case the Python would need to listen on stderr instead. — jwilner
– jwilner, Commented Apr 8, 2015 at 16:23

jwilner · Accepted Answer · 2015-04-08 16:29:50Z

1

It looks like you don't need to be using the lower level subprocess.Popen interface; I'd just call subprocess.check_output, which will return the value of the os call as a string. From there, just do string processing to get your value.

result = subprocess.check_output(["ffmpeg", "-i" ... ])
# this regex matches the string crop followed by one or more non-whitespace characters 
match = re.search("crop\S+", result) 
crop_result = None
if match is not None:
     crop_result = match.group()

If ffmpeg output is instead to stderr:

result = subprocess.check_output(["ffmpeg", ...], stderr=subprocess.STDOUT)

edited Apr 8, 2015 at 16:29

answered Apr 8, 2015 at 15:49

jwilner

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3 Comments

JRM Over a year ago

getting: match = re.search("crop\S+", result) NameError: global name 'result' is not defined

jwilner Over a year ago

That means the variable result isn't defined yet, which means you probably named the returned value from subprocess.check_output something different.

JRM Over a year ago

Can't get it to work it seems - I updated the code above

JRM · Accepted Answer · 2015-05-12 18:15:25Z

0

Ok found it

def detectCropFile(localPath):
    fpath = "/xxx/xx/Desktop/Crop_detect/videos/USUV.mp4"
    print "File to detect crop: %s " % fpath
    p = subprocess.Popen(["ffmpeg", "-i", fpath, "-vf", "cropdetect=24:16:0", "-vframes", "500", "-f", "rawvideo", "-y", "/dev/null"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    infos = p.stderr.read()
    print infos
    allCrops = re.findall(CROP_DETECT_LINE + ".*", infos)
    print allCrops 
    mostCommonCrop = Counter(allCrops).most_common(1)
    print "most common crop: %s" % mostCommonCrop
    print mostCommonCrop[0][0]
    global crop
    crop = mostCommonCrop[0][0]
    video_rename()

So use this in the subprocess call to grab the values from crop detect:

p = subprocess.Popen(["ffmpeg", "-i", fpath, "-vf", "cropdetect=24:16:0", "-vframes", "500", "-f", "rawvideo", "-y", "/dev/null"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)

answered May 12, 2015 at 18:15

JRM

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1 Comment

jfs Over a year ago

you should use stdout_value, stderr_value = p.communicate() or merged_value = check_ouptut(..., stderr=STDOUT) as @jwilner suggested

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