Recherche avancée

Sur d’autres sites (2266)

• Meaning of "ffprobe -v trace demo.mp4" output

  17 mai 2019, par bluesky

  The ’mdat box’ of Mp4 file may at the last of file. I want to know the position of ’mdat’ box using ’ffmpeg’ or ’ffprobe’.

  Mp4 consists of ’ftyp’, ’moov’ and ’mdat’ BOX. each BOX consists of "BoxHeader" and "BoxData". "BoxHeader" consists of "BoxSize(4Byte)", "BoxType(4Byte)", "BoxLargesize(8Byte, only have when box size exceeding the range of 4Byte expression, then the value of BoxSize is 1)".

  In program, you could first read 8 Byte and know the size of ’ftyp box’, then seek the size and read 8 Byte to know if the next box is ’moov box’. If not ’moov’, it shoud be ’mdat box’, then seek cross ’mdat box’ to find ’mdat box’...

  But I want to use ’ffprobe’ to find the position of ’moov’. I use ’ffprobe -v trace demo.mp4’, and output is like below

  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc8fd000e00] Format mov,mp4,m4a,3gp,3g2,mj2 probed with size=2048 and score=100
  
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc8fd000e00] type:'ftyp' parent:'root' sz: 28 8 41044500
  
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc8fd000e00] ISO: File Type Major Brand: mp42
  
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc8fd000e00] type:'moov' parent:'root' sz: 17943 36 41044500
  
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc8fd000e00] type:'mvhd' parent:'moov' sz: 108 8 17935

  I want to know the meaning of type:'ftyp' parent:'root' sz: 28 8 41044500 :
  type:'ftyp' parent:'root'is easy to know, sz: 28 8 41044500 is really make me confused, I guess 28 is size of ftyp box,but the meaning of 8 41044500 is what ?

  Could you explain the meaning of sz: 28 8 41044500, and where could find the doc ?

• Meaning of Timestamp Retrieved by AudioQueueGetCurrentTime() in AudioQueue Callback

  30 juillet 2024, par White0930

  I'm working on an audio project and have a question regarding the meaning of the timestamp retrieved by AudioQueueGetCurrentTime().

  


  According to the Apple Developer Documentation, the following calculation gives the audio time being played (since AudioQueueStart) :

  


  - (Float64) GetCurrentTime {
    AudioTimeStamp c;       
    AudioQueueGetCurrentTime(playState.queue, NULL, &c, NULL);  
    return c.mSampleTime / _av->audio.sample_rate;
}


  


  However, in a project I'm working on, I noticed the following code inside the fillAudioBuffer callback function of AudioQueue :

  


  
static void fillAudioBuffer(AudioQueueRef queue, AudioQueueBufferRef buffer){
    
    int lengthCopied = INT32_MAX;
    int dts= 0;
    int isDone = 0;

    buffer->mAudioDataByteSize = 0;
    buffer->mPacketDescriptionCount = 0;
    
    OSStatus err = 0;
    AudioTimeStamp bufferStartTime;

    AudioQueueGetCurrentTime(queue, NULL, &bufferStartTime, NULL);
    

    
    while(buffer->mPacketDescriptionCount < numPacketsToRead && lengthCopied > 0){
        if (buffer->mAudioDataByteSize) {
            break;
        }
        
        lengthCopied = getNextAudio(_av,buffer->mAudioDataBytesCapacity-buffer->mAudioDataByteSize, (uint8_t*)buffer->mAudioData+buffer->mAudioDataByteSize,&dts,&isDone);
        if(!lengthCopied || isDone) break;
      
        if(aqStartDts < 0) aqStartDts = dts;
        if (dts>0) currentDts = dts;
        if(buffer->mPacketDescriptionCount ==0){
            bufferStartTime.mFlags = kAudioTimeStampSampleTimeValid;
            bufferStartTime.mSampleTime = (Float64)(dts-aqStartDts) * _av->audio.frame_size;
            
            if (bufferStartTime.mSampleTime <0 ) bufferStartTime.mSampleTime = 0;
            PMSG2("AQHandler.m fillAudioBuffer: DTS for %x: %lf time base: %lf StartDTS: %d\n", (unsigned int)buffer, bufferStartTime.mSampleTime, _av->audio.time_base, aqStartDts);
            
        }
        buffer->mPacketDescriptions[buffer->mPacketDescriptionCount].mStartOffset = buffer->mAudioDataByteSize;
        buffer->mPacketDescriptions[buffer->mPacketDescriptionCount].mDataByteSize = lengthCopied;
        buffer->mPacketDescriptions[buffer->mPacketDescriptionCount].mVariableFramesInPacket = _av->audio.frame_size;
        
        buffer->mPacketDescriptionCount++;
        buffer->mAudioDataByteSize += lengthCopied;
        
    }
    
#ifdef DEBUG
    int audioBufferCount, audioBufferTotal,  videoBufferCount, videoBufferTotal;
    bufferCheck(_av,&videoBufferCount, &videoBufferTotal, &audioBufferCount, &audioBufferTotal);
    
    PMSG2("AQHandler.m fillAudioBuffer: Video Buffer: %d/%d Audio Buffer: %d/%d\n", videoBufferCount, videoBufferTotal, audioBufferCount, audioBufferTotal);
    
    PMSG2("AQHandler.m fillAudioBuffer: Bytes copied for buffer 0x%x: %d\n",(unsigned int)buffer, (int)buffer->mAudioDataByteSize );
#endif  
    if(buffer->mAudioDataByteSize){
        
        if(err=AudioQueueEnqueueBufferWithParameters(queue, buffer, 0, NULL, 0, 0, 0, NULL, &bufferStartTime, NULL))
        {
#ifdef DEBUG
            char sErr[10];

            PMSG2(@"AQHandler.m fillAudioBuffer: Could not enqueue buffer 0x%x: %d %s.", buffer, err, FormatError(sErr, err));
#endif
        }
    }

}


  


  Based on the documentation for AudioQueueEnqueueBufferWithParameters and the variable naming used by the author, bufferStartTime seems to represent the time when the newly filled audio buffer will start playing, i.e., the time when all current audio in the queue has finished playing and the new audio starts. This interpretation suggests bufferStartTime is not the same as the time of the audio currently being played.

  


  I have browsed through many related questions, but I still have some doubts.. I'm currently fixing an audio-video synchronization issue in my project, and there isn't much detailed information in the Apple Developer Documentation (or maybe my search skills are lacking).

  


  Can someone clarify the exact meaning of the timestamp returned by AudioQueueGetCurrentTime() in this context ? Is it the time when the current audio will finish playing, or is it the time when the new audio will start playing ? Any additional resources or documentation that explain this in detail would also be appreciated.

  


• avutil/pixfmt : Clarify the meaning of the "alpha" bit in rgb555/bgr555

  6 février 2015, par Michael Niedermayer

  avutil/pixfmt : Clarify the meaning of the "alpha" bit in rgb555/bgr555
  Found-by : wm4
  
  Reviewed-by : wm4 <nfxjfg@googlemail.com>
  
  Signed-off-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavutil/pixfmt.h