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• Why are Cb and Cr planes displaced differently from lum by the displace complex filter in ffmpeg ?

  10 janvier 2024, par Neb

  I have a video encoded with the yuv420p pixel format and I want to displace its pixels. I'm using ffmpeg and its new displace filter. The filter takes as inputs (the video to be displaced and) two displacement maps respectively for X and Y axis. I decided to create the displacement maps directly into ffmpeg using the nullsrc video source filter and the geq filter to specify the value of the three planes : lum, Cb, Cr. The script is the following :

  



  ffmpeg INPUT.mp4 -f lavfi -i nullsrc=size=${WIDTH}x${HEIGHT}:d=0.1,geq='lum=128+30*sin(2*PI*X/400):Cb=128+30*sin(2*PI*X/400):Cr='128+30*sin(2*PI*X/400)' -f lavfi -i nullsrc=size=${WIDTH}x${HEIGHT}:d=0.1,geq='lum=128+30*sin(2*PI*X/400):Cb=128+30*sin(2*PI*X/400):Cr=128+30*sin(2*PI*X/400)' -lavfi '[0][1][2]displace' OUTPUT.mp4


  



  I used the example provided in the documentation of ffmpeg, since the expression used in geq is irrelevant for the purposes of the problem.

  



  At the and of the computation, I get the pixels of the input video not properly displaced, meaning that I can clearly see a sort of ghost carrying-color-information video under a displaced but b/w one.
After some tests, I noticed that the displacemnt map created had only the luma plane displaced correctly while the chrominance planes were displaced, but differently from luma, which is the origin of the planes disalignment in the intput video as you can see in the following extract frames :

  



  

  



  I also noticed that the video describing the Cb and Cr planes of the displacement maps have half resolution of the luma plane.

  



  My question is : how can i setup correctly the Cr and Cb planes in the geq definition so that they are exactly identical to the luma plane ?

  



  It would be also great if someone could explain me why ffmpeg gives me an output so much different for luma and Cb, Cr planes even if the function provided is the same.

  



  If, it can help, i'm using ffmpeg 3.3-static build.

  



  Thanks for your time.

  


• Open cv.VideoCapture(index) - ffmpeg list camera names - How to match ?

  15 novembre 2024, par Chris P

      def fetch_camera_input_settings(self):
        try:
            self.database_functions = database_functions

            self.camera_input_device_name = database_functions.read_setting("camera_input_device_name")["value"]
            self.camera_input_device_number = int(self.database_functions.read_setting("camera_input_device_number")["value"])

            self.camera_input_devices = [[0,-1,"Καμία συσκευή κάμερας"]]
            self.available_cameras = [{"device_index":-1,"device_name":"Καμία συσκευή κάμερας"}]

            # FFmpeg command to list video capture devices on Windows
            cmd = ["ffmpeg", "-list_devices", "true", "-f", "dshow", "-i", "dummy"]
            result = subprocess.run(cmd, stderr=subprocess.PIPE, stdout=subprocess.PIPE, text=True)
            output = result.stderr  # FFmpeg sends device listing to stderr

            # Updated regular expression to capture both video and audio devices
            device_pattern = re.compile(r'\[dshow @ .+?\] "(.*?)" \(video\)')
            cameras = device_pattern.findall(output)
            counter = 0
            for camera in cameras:
                counter += 1
                self.camera_input_devices.append([counter,counter-1,camera])
                self.available_cameras.append({"device_index": counter-1, "device_name": camera})

            self.to_emitter.send({"type":"available_devices","devices":self.camera_input_devices,"device_index":self.camera_input_device_number})
        except:
            error_message = traceback.format_exc()
            self.to_emitter.send({"type":"error","error_message":error_message})



  


  How to match ffmpeg camera device names output with cv2.VideoCapture which wants camera index as input ?

  


• i want to convert images(3 4ea) and mp3 to mp4 file

  15 juillet 2015, par John smith

  i am convert images(3 4) and mp3 to video with ffmpeg

  i use this command

  ffmpeg -loop 1 -i %05d.jpg -i sample.mp3 -c:v libx264 -tune stillimage -c:a aac -strict experimental -b:a 192k -pix_fmt yuv420p -shortest out.mp4

  converting is well done. but, Images are changing very short period.

  i want to set images changing period once every 10 15 seconds.

  How would I do that using ffmpeg ?