2023-04-30：用go語言重寫ffmpeg的resampling_audio.c示例，它實現了音頻重采樣的功能。

答案2023-04-30：

resampling_audio.c?是 FFmpeg 中的一個源文件，其主要功能是實現音頻重采樣。

音頻重采樣是指將一段音頻數據從一個采樣率、聲道數或樣本格式轉換為另一種采樣率、聲道數或樣本格式。在實際應用中，不同的設備和系統(tǒng)可能需要不同的音頻格式，因此進行音頻重采樣是非常常見的操作。

resampling_audio.c?中實現了多種音頻重采樣算法，包括最近鄰插值法、線性插值法、升采樣過濾器、降采樣過濾器等等。這些算法可以針對不同的輸入和輸出音頻格式進行選擇，以達到最佳效果。

使用?resampling_audio.c?可以方便地完成音頻重采樣操作，并在保證音質的同時提高處理效率。因此，它是 FFmpeg 中非常重要的一個模塊。

代碼見github/moonfdd/ffmpeg-go庫。

這段代碼是一個使用 FFmpeg 中的 libswresample 庫進行音頻重采樣的示例程序。大體過程如下：

--1. 初始化輸入和輸出音頻參數，包括聲道數、采樣率、樣本格式等。

--3. 創(chuàng)建 libswresample 的上下文（SwrContext）。

--5. 通過?AvOptSetXXX?函數設置輸入輸出參數。

--7. 調用?SwrInit?函數初始化 resampler 上下文。

--9. 申請輸入和輸出音頻數據緩沖區(qū)。

--11. 循環(huán)讀取輸入音頻數據，重采樣并保存為輸出音頻數據。每次循環(huán)中：

----a. 填充源音頻數據緩沖區(qū)（即生成或從文件中讀取音頻數據）。

----b. 計算重采樣后的目標音頻數據大小。

----c. 申請足夠的輸出音頻數據緩沖區(qū)空間。

----d. 調用?SwrConvert?函數將源音頻數據轉換為目標音頻數據。

----e. 將重采樣后的目標音頻數據寫入輸出文件。

--13. 釋放資源并退出程序。

需要注意的是，在實際使用中需要根據具體情況調整輸入輸出音頻參數以及重采樣算法等設置。

命令如下：

go?run?./examples/internalexamples/resampling_audio/main.go?./out/res.aac

./lib/ffplay?-f?s16le?-channel_layout?7?-channels?3?-ar?44100?./out/res.aac

golang代碼如下：

package?main

import?(
????"fmt"
????"math"
????"os"
????"unsafe"

????"github.com/moonfdd/ffmpeg-go/ffcommon"
????"github.com/moonfdd/ffmpeg-go/libavutil"
????"github.com/moonfdd/ffmpeg-go/libswresample"
)

func?main0()?(ret?ffcommon.FInt)?{
????var?src_ch_layout?ffcommon.FInt64T?=?libavutil.AV_CH_LAYOUT_STEREO
????var?dst_ch_layout?ffcommon.FInt64T?=?libavutil.AV_CH_LAYOUT_SURROUND
????var?src_rate?ffcommon.FInt?=?48000
????var?dst_rate?ffcommon.FInt?=?44100
????var?src_data,?dst_data?**ffcommon.FUint8T
????var?src_nb_channels,?dst_nb_channels?ffcommon.FInt
????var?src_linesize,?dst_linesize?ffcommon.FInt
????var?src_nb_samples?ffcommon.FInt?=?1024
????var?dst_nb_samples?ffcommon.FInt
????var?max_dst_nb_samples?ffcommon.FInt
????var?src_sample_fmt?libavutil.AVSampleFormat?=?libavutil.AV_SAMPLE_FMT_DBL
????var?dst_sample_fmt?libavutil.AVSampleFormat?=?libavutil.AV_SAMPLE_FMT_S16
????var?dst_filename?string
????var?dst_file?*os.File
????var?dst_bufsize?ffcommon.FInt
????var?fmt0?string
????var?swr_ctx?*libswresample.SwrContext
????var?t?ffcommon.FDouble

????if?len(os.Args)?!=?2?{
????????fmt.Printf("Usage:?%s?output_file\nAPI?example?program?to?show?how?to?resample?an?audio?stream?with?libswresample.\nThis?program?generates?a?series?of?audio?frames,?resamples?them?to?a?specified?output?format?and?rate?and?saves?them?to?an?output?file?named?output_file.\n",
????????????os.Args[0])
????????os.Exit(1)
????}
????dst_filename?=?os.Args[1]
????dst_file,?_?=?os.Create(dst_filename)
????if?dst_file?==?nil?{
????????fmt.Printf("Could?not?open?destination?file?%s\n",?dst_filename)
????????os.Exit(1)
????}

????/*?create?resampler?context?*/
????swr_ctx?=?libswresample.SwrAlloc()
????if?swr_ctx?==?nil?{
????????fmt.Printf("Could?not?allocate?resampler?context\n")
????????ret?=?-libavutil.ENOMEM
????????goto?end
????}

????/*?set?options?*/
????libavutil.AvOptSetInt(uintptr(unsafe.Pointer(swr_ctx)),?"in_channel_layout",?src_ch_layout,?0)
????libavutil.AvOptSetInt(uintptr(unsafe.Pointer(swr_ctx)),?"in_sample_rate",?int64(src_rate),?0)
????libavutil.AvOptSetSampleFmt(uintptr(unsafe.Pointer(swr_ctx)),?"in_sample_fmt",?src_sample_fmt,?0)

????libavutil.AvOptSetInt(uintptr(unsafe.Pointer(swr_ctx)),?"out_channel_layout",?dst_ch_layout,?0)
????libavutil.AvOptSetInt(uintptr(unsafe.Pointer(swr_ctx)),?"out_sample_rate",?int64(src_rate),?0)
????libavutil.AvOptSetSampleFmt(uintptr(unsafe.Pointer(swr_ctx)),?"out_sample_fmt",?dst_sample_fmt,?0)

????/*?initialize?the?resampling?context?*/
????ret?=?swr_ctx.SwrInit()
????if?ret?<?0?{
????????fmt.Printf("Failed?to?initialize?the?resampling?context\n")
????????goto?end
????}

????/*?allocate?source?and?destination?samples?buffers?*/

????src_nb_channels?=?libavutil.AvGetChannelLayoutNbChannels(uint64(src_ch_layout))
????ret?=?libavutil.AvSamplesAllocArrayAndSamples(&src_data,?&src_linesize,?src_nb_channels,
????????src_nb_samples,?src_sample_fmt,?0)
????if?ret?<?0?{
????????fmt.Printf("Could?not?allocate?source?samples\n")
????????goto?end
????}

????/*?compute?the?number?of?converted?samples:?buffering?is?avoided
?????*?ensuring?that?the?output?buffer?will?contain?at?least?all?the
?????*?converted?input?samples?*/
????dst_nb_samples?=?int32(libavutil.AvRescaleRnd(int64(src_nb_samples),?int64(dst_rate),?int64(src_rate),?libavutil.AV_ROUND_UP))
????max_dst_nb_samples?=?dst_nb_samples

????/*?buffer?is?going?to?be?directly?written?to?a?rawaudio?file,?no?alignment?*/
????dst_nb_channels?=?libavutil.AvGetChannelLayoutNbChannels(uint64(dst_ch_layout))
????ret?=?libavutil.AvSamplesAllocArrayAndSamples(&dst_data,?&dst_linesize,?dst_nb_channels,
????????dst_nb_samples,?dst_sample_fmt,?0)
????if?ret?<?0?{
????????fmt.Printf("Could?not?allocate?destination?samples\n")
????????goto?end
????}

????t?=?0
????for?{
????????/*?generate?synthetic?audio?*/
????????fill_samples((*float64)(unsafe.Pointer(*src_data)),?src_nb_samples,?src_nb_channels,?src_rate,?&t)

????????/*?compute?destination?number?of?samples?*/
????????dst_nb_samples?=?int32(libavutil.AvRescaleRnd(swr_ctx.SwrGetDelay(int64(src_rate))+
????????????int64(src_nb_samples),?int64(dst_rate),?int64(src_rate),?libavutil.AV_ROUND_UP))
????????if?dst_nb_samples?>?max_dst_nb_samples?{
????????????libavutil.AvFreep(uintptr(unsafe.Pointer(dst_data)))
????????????ret?=?libavutil.AvSamplesAlloc(dst_data,?&dst_linesize,?dst_nb_channels,
????????????????dst_nb_samples,?dst_sample_fmt,?1)
????????????if?ret?<?0?{
????????????????break
????????????}
????????????max_dst_nb_samples?=?dst_nb_samples
????????}

????????/*?convert?to?destination?format?*/
????????ret?=?swr_ctx.SwrConvert(dst_data,?dst_nb_samples,?src_data,?src_nb_samples)
????????if?ret?<?0?{
????????????fmt.Printf("Error?while?converting\n")
????????????goto?end
????????}
????????dst_bufsize?=?libavutil.AvSamplesGetBufferSize(&dst_linesize,?dst_nb_channels,
????????????ret,?dst_sample_fmt,?1)
????????if?dst_bufsize?<?0?{
????????????fmt.Printf("Could?not?get?sample?buffer?size\n")
????????????goto?end
????????}
????????fmt.Printf("t:%f?in:%d?out:%d\n",?t,?src_nb_samples,?ret)
????????dst_file.Write(ffcommon.ByteSliceFromByteP(*dst_data,?int(dst_bufsize)))
????????if?t?<?10?{

????????}?else?{
????????????break
????????}
????}

????ret?=?get_format_from_sample_fmt(&fmt0,?dst_sample_fmt)
????if?ret?<?0?{
????????goto?end
????}
????fmt.Printf("Resampling?succeeded.?Play?the?output?file?with?the?command:\nffplay?-f?%s?-channel_layout?%d?-channels?%d?-ar?%d?%s\n",
????????fmt0,?dst_ch_layout,?dst_nb_channels,?dst_rate,?dst_filename)

end:
????dst_file.Close()

????if?src_data?!=?nil?{
????????libavutil.AvFreep(uintptr(unsafe.Pointer(src_data)))
????}
????libavutil.AvFreep(uintptr(unsafe.Pointer(&src_data)))

????if?dst_data?!=?nil?{
????????libavutil.AvFreep(uintptr(unsafe.Pointer(dst_data)))
????}
????libavutil.AvFreep(uintptr(unsafe.Pointer(&dst_data)))

????libswresample.SwrFree(&swr_ctx)
????if?ret?<?0?{
????????return?1
????}?else?{
????????return?0
????}
}

func?get_format_from_sample_fmt(fmt0?*string,?sample_fmt?libavutil.AVSampleFormat)?(ret?ffcommon.FInt)?{
????switch?sample_fmt?{
????case?libavutil.AV_SAMPLE_FMT_U8:
????????*fmt0?=?"u8"
????case?libavutil.AV_SAMPLE_FMT_S16:
????????*fmt0?=?"s16le"
????case?libavutil.AV_SAMPLE_FMT_S32:
????????*fmt0?=?"s32le"
????case?libavutil.AV_SAMPLE_FMT_FLT:
????????*fmt0?=?"f32le"
????case?libavutil.AV_SAMPLE_FMT_DBL:
????????*fmt0?=?"f64le"
????default:
????????fmt.Printf("sample?format?%s?is?not?supported?as?output?format\n",
????????????libavutil.AvGetSampleFmtName(sample_fmt))
????????ret?=?-1
????}
????return
}

/**
*?Fill?dst?buffer?with?nb_samples,?generated?starting?from?t.
?*/
func?fill_samples(dst?*ffcommon.FDouble,?nb_samples,?nb_channels,?sample_rate?ffcommon.FInt,?t?*ffcommon.FDouble)?{
????var?i,?j?ffcommon.FInt
????tincr?:=?1.0?/?float64(sample_rate)
????dstp?:=?dst
????c?:=?2?*?libavutil.M_PI?*?440.0

????/*?generate?sin?tone?with?440Hz?frequency?and?duplicated?channels?*/
????for?i?=?0;?i?<?nb_samples;?i++?{
????????*dstp?=?math.Sin(c?*?*t)
????????for?j?=?1;?j?<?nb_channels;?j++?{
????????????*(*float64)(unsafe.Pointer(uintptr(unsafe.Pointer(dstp))?+?uintptr(8*j)))?=?*dstp
????????}
????????dstp?=?(*ffcommon.FDouble)(unsafe.Pointer(uintptr(unsafe.Pointer(dstp))?+?uintptr(8*nb_channels)))
????????*t?+=?tincr
????}
}

func?main()?{

????os.Setenv("Path",?os.Getenv("Path")+";./lib")
????ffcommon.SetAvutilPath("./lib/avutil-56.dll")
????ffcommon.SetAvcodecPath("./lib/avcodec-58.dll")
????ffcommon.SetAvdevicePath("./lib/avdevice-58.dll")
????ffcommon.SetAvfilterPath("./lib/avfilter-56.dll")
????ffcommon.SetAvformatPath("./lib/avformat-58.dll")
????ffcommon.SetAvpostprocPath("./lib/postproc-55.dll")
????ffcommon.SetAvswresamplePath("./lib/swresample-3.dll")
????ffcommon.SetAvswscalePath("./lib/swscale-5.dll")

????genDir?:=?"./out"
????_,?err?:=?os.Stat(genDir)
????if?err?!=?nil?{
????????if?os.IsNotExist(err)?{
????????????os.Mkdir(genDir,?0777)?//??Everyone?can?read?write?and?execute
????????}
????}

????main0()
}