304 lines
10 KiB
Markdown
304 lines
10 KiB
Markdown
## Initialization
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```cpp
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av_register_all();
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avformat_network_init();
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```
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## Command line
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利用FFMpeg來轉檔
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```
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ffmpeg -i <SOURCE.mp4> -t 10 -s <W>x<H> -pix_fmt <OUTPUT_FORMAT> <OUTPUT_FILENAME>
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```
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- `-i`：來源檔名
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- `-t`：影片長度（秒）
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- `-s`：影片大小，長x寬
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- `-pix_fmt`：輸出的影像格式
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example:
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```
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ffmpeg -i test.mp4 -t 10 -s 240x128 -pix_fmt yuv420p out240x128.yuv
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```
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## Open file
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使用`avformat_open_input()`來打開檔案。
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```cpp
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AVFormatContext* pFormatContext = NULL;
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char* path = "E:\\download\\Incredible Patagonia_1920x1080.mp4";
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AVDictionary* pDict = NULL;
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int result = avformat_open_input(&pFormatContext, path, NULL, &pDict);
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```
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若是開啟失敗`result`會是一個非0的值。可以用`av_strerror()`來印出詳細的錯誤訊息。例：
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```cpp
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char errorMsg[1024] = { 0 };
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av_strerror(result, errorMsg, sizeof(errorMsg) - 1);
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printf("Open file %s error, code = %d, message = %s\n", path, result, errorMsg);
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```
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### Get file information
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用`avformat_find_stream_info()`來把檔案的video/audio相關訊息放到`AVFormatContext`裡面。例：
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```cpp
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result = avformat_find_stream_info(pFormatContext, NULL);
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```
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也可以用
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```cpp
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av_dump_format(pFormatContext, 0, path, 0);
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```
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來直接印出相關訊息，不過這方法是Ffmpeg內定的，如果要印出自己想要的內容，就必須將`AVFormatContext`裡面的`AVStream`一一撈出來：
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```cpp
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for (int i = 0; i < pFormatContext->nb_streams; i++) {
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AVStream* avStream = pFormatContext->streams[i];
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if (avStream->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
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printf("Stream[%d] is a audio stream.\n", i);
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printf(" Sample rate: %d\n", avStream->codecpar->sample_rate);
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printf(" Format : %d\n", avStream->codecpar->format);
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printf(" Channels : %d\n", avStream->codecpar->channels);
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printf(" FPS : %d/%d\n", avStream->avg_frame_rate.num, avStream->avg_frame_rate.den);
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printf(" Frame Size : %d\n", avStream->codecpar->frame_size);
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printf(" Codec ID : %d\n", avStream->codecpar->codec_id);
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} else if (avStream->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
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printf("Stream[%d] is a video stream.\n", i);
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printf(" Width : %d\n", avStream->codecpar->width);
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printf(" Height: %d\n", avStream->codecpar->height);
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printf(" FPS : %d/%d\n", avStream->avg_frame_rate.num, avStream->avg_frame_rate.den);
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printf(" Codec ID: %d\n", avStream->codecpar->codec_id);
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}
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}
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```
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這個for-loop可以找出檔案裡面的video stream index與audio stream index。
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另一個找出video stream index與audio stream index的方法：
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```cpp
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int videoStream = av_find_best_stream(pFormatContext, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
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int audioStream = av_find_best_stream(pFormatContext, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
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printf("Found best video stream index: %d\n", videoStream);
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printf("Found best audio stream index: %d\n", audioStream);
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```
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## Demux
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### 重要結構
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#### AVPackt
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##### 重要的member：
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- `AVBufferRef *buf`：buffer的reference count，基本上不會用到。
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- `int64_t pts`：顯示順序
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- `int64_t dts`：decode順序
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- `uint8_t *data`：實際存data的buffer
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- `int size`：上面那個buffer的大小
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##### 操作function
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- `AVPacket* av_packet_alloc()`：產生一個兂的AVPacket
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- `AVPacket* av_packet_clone(const AVPacket*)`：複製一個AVPacket
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- `int av_packet_ref(AVPacket*, const AVPacket*)`：增加reference count
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- `av_packet_unref(AVPacket*)`：減少reference count
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- `void av_packet_free(AVPacket**)`：釋放AVPacket
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- `void av_init_packet(AVPacket*)`：初始化AVPacket
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- `int av_packet_from_data(AVPacket*, uint8_t*, int size)`：從自己的data
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- `av_seek_frame`：
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- `AVFormatContext*`
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- `int stream_index`：要seek的stream
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- `int64_t timestamp`：timestamp
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- `int flag`：seek的方法
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- `#define AVSEEK_FLAG_BACKWARD 1`：往後找
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- `#define AVSEEK_FLAG_BYTE 2`：以byte為單位，跳到timestamp相對應的byte
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- `#define AVSEEK_FLAG_ANY 4`：跳到任意frame，不一定要key frame
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- `#define AVSEEK_FLAG_FRAME 8`：依照frame number來找
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## Decode
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### 重要結構
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#### AVCodecContext
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##### 重要的member
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- `int thread_count`：執行續的數量
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- `AVRational time_base`
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##### 操作function
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- `avcodec_register_all()`：註冊所有的decoder。
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- `AVCodec* avcodec_find_decoder(enum AVCodecID)`：用codec ID來找到相對應的codec instance
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- `AVCodec* avcodec_find_decoder_by_name(const char* name)`：用codec的名子來找到相對應的codec instance
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- `AVCodecContext* avcodec_alloc_context3(const AVCodec*)`：用codec來建立`AVCodecContext`，codec只是用來解碼，解碼之後的相關資料要放在`AVCodecContext`裡面。
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- `void avcodec_free_context(AVCodecContext**)`：釋放`AVCodecContext`。
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- `int avcodec_open2(AVCodecContext*, const AVCodec*, AVDictionary*)`
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- 開始解碼
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- `AVDictionary`的選項請看`libavcodec/options_table.h`
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#### AVFrame
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##### 重要的member
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- `uint8_t* data[AV_NUM_DATA_POINTERS]`：存放這個frame的資料
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- `int linesize[AV_NUM_DATA_POINTERS]`：存放指向各個plane的pointer
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- Plane mode
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- For video
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- 0：Y
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- 1：U
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- 2：V
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- 3：NULL
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- For audio
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- 0：Left channel
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- 1：Right channel
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- 2：NULL
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- Non plane mode
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- For video
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- 0：RGB RGB RGB
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- 1：NULL
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- `int width`：For video, frame width
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- `int height`：For video, frame height
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- `int nb_samples`：For audio
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- `int64_t pts`：這個frame的pts
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- `int64_t pkt_dts`：Packet的dts
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- `int sample_rate`：For audio，sampling rate
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- `uint64_t channel_layout`：For audio
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- `int channels`：For audio
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- `int format`：For video it's AVPixelFormat, for audio it's AVSampleFormat
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##### 操作function
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- `AVFrame* av_frame_alloc()`：
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- `void av_frame_free(AVFrame*)`：
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- `AVFrame* av_frame_clone(const AVFrame*)`：
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- `int av_frame_ref(AVFrame* dst, const AVFrame* src)`：
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- `void av_frame_unref(AVFrame*)`：
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- `int avcodec_send_packet(AVCodecContext*, const AVPacket*)`
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- `int avcodec_receive_frame(AVCodecContext*, AVFrame*)`
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## Frame scaling
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##### 操作function
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- `sws_getContext`
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- `sws_getCachedContext`
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```
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SwsContext* sws_getCachedContext(
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SwsContext*,
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int srcW, int srcH, enum AVPixelFormat srcFormat,
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int dstW, int dstH, enum AVPixelFormat dstFormat,
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int flags,
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SwsFilter srcFilter,
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SwsFilter dstFilter,
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const double* param)
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```
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- `sws_scale`
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```
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int sws_scale(
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SwsContext*,
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const uint8_t* const srcSlice[], // 來源buffer的pointer, 如果是plane mode，要丟srcSlice[3]
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const int srcStride[], // linesize
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int srcSliceY,
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int srcSliceH, // image height
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uint8_t* const dst[], // 目標buffer的pointer, 如果是plane mode，要丟dst[3]
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const int dstStride[]) // linesize
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```
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- `void sws_freeContext(SwsContext*)`
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## Audio resampling
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- `SwrContext* swr_alloc()`
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- `swr_alloc_set_opts`
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```cpp
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SwrContext* swr_alloc_set_opts(
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SwrContext*,
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int64_t out_ch_layout, // Output
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AVSampleFormat outSampleFormat, // Output
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int outSampleRate, // Output
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int64_t in_ch_layout, // Input
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AVSampleFormat inSampleFormat, // Input
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int inSampleRate, // Input
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int log_offset, // 一般丟0
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void* log_ctx) // 一般丟0
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```
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- `int swr_init(SwrContext*)`
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- `void swr_free(SwrContext**)`
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- `swr_convert`
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```cpp
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int swt_convert(
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SwrContext*,
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uint8_t** outData,
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int outCount, // Sample count for one channel
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uint8_t** inData,
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int inCount) // Sample count for one channel
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```
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## Audio 播放
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### 會用到的class
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#### QAudioFormat
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- `setSampleRate`
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- `setSampleSize`
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- `setChannelCount`
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- `setCodec("audio/pcm")`
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- `setByteOrder(QAudioFormat::LittleEndian)`
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- `setSampleType(QAudioFormat::UnSignedInt)`
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#### QAudioOutput
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- Constuctor參數就是一個`QAudioFormat`
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- `QIODevice* start()`
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- `suspend()`
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- `resume()`
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- `bufferSize()`
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- `byteFree()`
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- `periodSize()`
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#### QIODevice
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- `qint64 write(const char* data, qint64 len)`
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#### QBuffer
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```cpp
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QBuffer buffer;
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if (buffer.open(QIODevice::ReadWrite)) {
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qInfo() << "Buffer Opened!";
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QByteArray data("Hello World");
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for (int i = 0; i < 5; i++) {
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buffer.write(data);
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buffer.write("\r\n");
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}
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//File and device access you may need to flush the data to the device
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//buffer.flush()
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//Move to the first position
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buffer.seek(0);
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qInfo() << buffer.readAll(); // retuen a QVariant
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qInfo() << "closing the buffer";
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//ALWAYS close your device!
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buffer.close();
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}
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```
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### Example
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```cpp
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QAudioFormat audioFormat;
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audioFormat.setSampleRate(44100);
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audioFormat.setSampleSize(16);
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audioFormat.setChannelCount(2);
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audioFormat.setCodec("audio/pcm");
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audioFormat.setByteOrder(QAudioFormat::LittleEndian);
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audioFormat.setSampleType(QAudioFormat::UnSignedInt);
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QAudioOutput* audioOutput = new QAudioOutput(audioFormat);
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QIODevice* ioDevice = audioOutput->start();
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int size = audioOutput->periodSize();
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char* buffer = new char[size];
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FILE *pFile = fopen("E:\\download\\out.pcm", "rb");
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while (!feof(pFile)) {
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if (audioOutput->bytesFree() < size) {
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QThread::msleep(1);
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}
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qint64 len = fread(buffer, 1, size, pFile);
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if (len <= 0) {
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break;
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}
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ioDevice->write(buffer, len);
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}
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fclose(pFile);
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delete buffer;
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buffer = NULL;
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```
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### 使用FFmpeg decode audio
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- [音视频开发之旅（60) -调试分析FFmpeg (解封装部分的）常用结构体](https://zhuanlan.zhihu.com/p/441055685)
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## 參考資料
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- [mpv.io](https://mpv.io/)
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- [《FFmpeg基础知识》](https://opensourcelibs.com/lib/zhangfangtaozft-ffmpeg)
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- [ffmpeg - 专题 - 简书](https://www.jianshu.com/c/21a6fafc8ee3)
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- [FFmpeg打开输入文件 · FFmpeg原理](https://ffmpeg.xianwaizhiyin.net/api-ffmpeg/input.html)
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- [FFplay视频同步分析 - 掘金](https://juejin.cn/post/7156903872789741581)
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