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• What's the point of passing address of pointer to function that needs pointer to pointer ?

  16 août 2019, par Guerlando OCs

  I’m reading an example of ffmpeg decoding and it has the address of a pointer being passed to a function :

  static AVBufferRef *hw_device_ctx = NULL;
  
  
  
  
  
  if ((err = av_hwdevice_ctx_create(&hw_device_ctx, type,
  
                                    NULL, NULL, 0)) < 0) {

  What’s the point of passing the address of a pointer as an argument ?

  I understand that when we pass the pointer itself, if the pointer has address 0x123456, then the function is going to be able to modify what’s the object that is in this address. But when I pass the address of a pointer, I’m passing the address of where this pointer number is allocated ?

  If I understood right, I’m passing the address of the variable that stores 0x123456 ? Why the function needs it ?

  Also, suppose that I want to store hw_device_ctx in a unique_ptr like this :

  std::unique_ptr<avbufferred> hw_pointer;
  
  </avbufferred>

  How can I pass it to av_hwdevice_ctx_create ? Because I get an error when I do

  av_hwdevice_ctx_create(&hw_pointer.get(),...

  It says :

  expression must be an lvalue or a function designator

• ffmpeg output images filename with time position and problem with 'not(mod(t'

  26 novembre 2020, par Дмитрий Мышков

  I'd found a such ffmpeg output images filename with time position topic, but have problem with execute that one "ffmpeg -i source -vf fps=1,select='not(mod(t,5))' -vsync 0 -frame_pts 1 z%d.jpg" :

  


  ffmpeg -i ex.avi -vf fps=1,select='not(mod(t,5))' -vsync 0 -frame_pts 1 z%d.jpg

Stream mapping:
  Stream #0:0 -> #0:0 (mpeg4 (native) -> mjpeg (native))
Press [q] to stop, [?] for help
[Parsed_select_1 @ 0x7f96ee6024c0] [Eval @ 0x7ffee72d67d0] Missing ')' or too many args in 'mod(t'
[Parsed_select_1 @ 0x7f96ee6024c0] Error while parsing expression 'not(mod(t'
[AVFilterGraph @ 0x7f96ee439cc0] Error initializing filter 'select' with args 'not(mod(t'
Error reinitializing filters!
Failed to inject frame into filter network: Invalid argument
Error while processing the decoded data for stream #0:0
Conversion failed!


  


  what do I not right ?

  


• FFmpeg Drawbox Errors ?

  12 mars 2021, par Alex Gurr

  Using ffmpeg version 4.3.2.

  


  Command :

  


  ffmpeg -y

  -i /usr/src/app/backgrounds/pink-blue.jpg
  -i files/fuzzy-octopus-5/path/file.webm
  -i files/fuzzy-octopus-5/path/file.webm
  -i /usr/src/app/logo.png
  -filter_complex "
    [0]drawbox=x=-140:y=(((H-th)/2) - 205):w=840:h=490:color=white@0.2:t=fill,
       drawbox=x=770:y=(((H-th)/2) - 205):w=840:h=490:color=white@0.2:t=fill[bg]; 
    ...


  


  However I am getting the following errors ?

  


  Error while processing the decoded data for stream #3:0
Failed to inject frame into filter network: Invalid argument
Error reinitializing filters!
[Parsed_drawbox_0 @ 0x564a87058e80] Failed to configure input pad on Parsed_drawbox_0
[Parsed_drawbox_0 @ 0x564a87058e80] Error when evaluating the expression '(((H-th)/2) - 205)'.
Last message repeated 5 times
[Parsed_drawbox_0 @ 0x564a87058e80] [Eval @ 0x7ffcf25dc650] Undefined constant or missing '(' in 'H-th)/2)-205)'


  


  I have the right amount of brackets and this definitely works on my local machine (macOS). It's only throwing this error in our deployed AWS EC2 linux instance.

  


  Any ideas ? Thanks !

  


  Update : Tried replacing th with h but get the same error