從格式上下文中讀取

格式包含一個或多個編碼和多路複用流。我們通常以塊的形式讀取它們，塊通常稱為幀（儘管在某些情況下，FFmpeg 僅將解碼的原始媒體塊稱為幀，將編碼塊稱為資料包，這可能會造成混淆）。要從格式中讀取單個幀，請使用以下命令：

// A Format Context - see other examples on how to create it
AVFormatContext *formatContext;

// Initialize the AVPacket manually
AVPacket avPacket;
av_init_packet(&avPacket); // set fields of avPacket to default.
avPacket.data = NULL;
avPacket.size = 0;

// Read from the context into the packet
if(av_read_frame(formatContext, &avPacket) == 0){
    // nothing read
}

// Use the packet (such as decoding it and playing it)

// Free packet
av_packet_unref(&avPacket);