Sun Jun 30 15:53:09 UTC 2013

The following commit has been merged in the experimental branch:
commit eacb41b2e984dc191ecf1b4ad018bb794524ba09
Author: Michael Niedermayer <michaelni at gmx.at>
Date:   Fri Nov 10 01:41:53 2006 +0000

    mp3 header (de)compression bitstream filter
    this will make mp3 frames 4 bytes smaller, it will not give you binary identical mp3 files, but it will give you mp3 files which decode to binary identical output
    this will only work in containers providing at least packet size, sample_rate and number of channels
    bugreports about mp3 files for which this fails are welcome
    and this is experimental (dont expect compatibility and dont even expect to be able to decompress what you compressed, hell dont even expect this to work without editing the source a little)
    
    Originally committed as revision 6958 to svn://svn.ffmpeg.org/ffmpeg/trunk

diff --git a/libavcodec/allcodecs.c b/libavcodec/allcodecs.c
index cc0644e..4e6215a 100644
--- a/libavcodec/allcodecs.c
+++ b/libavcodec/allcodecs.c
@@ -869,5 +869,7 @@ void avcodec_register_all(void)
     av_register_bitstream_filter(&dump_extradata_bsf);
     av_register_bitstream_filter(&remove_extradata_bsf);
     av_register_bitstream_filter(&noise_bsf);
+    av_register_bitstream_filter(&mp3_header_compress_bsf);
+    av_register_bitstream_filter(&mp3_header_decompress_bsf);
 }
 
diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index ac128ec..daa054b 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -2662,6 +2662,8 @@ void av_bitstream_filter_close(AVBitStreamFilterContext *bsf);
 extern AVBitStreamFilter dump_extradata_bsf;
 extern AVBitStreamFilter remove_extradata_bsf;
 extern AVBitStreamFilter noise_bsf;
+extern AVBitStreamFilter mp3_header_compress_bsf;
+extern AVBitStreamFilter mp3_header_decompress_bsf;
 
 
 /* memory */
diff --git a/libavcodec/bitstream_filter.c b/libavcodec/bitstream_filter.c
index 8f878cd..ef2d741 100644
--- a/libavcodec/bitstream_filter.c
+++ b/libavcodec/bitstream_filter.c
@@ -19,6 +19,7 @@
  */
 
 #include "avcodec.h"
+#include "mpegaudio.h"
 
 AVBitStreamFilter *first_bitstream_filter= NULL;
 
@@ -124,6 +125,112 @@ static int noise(AVBitStreamFilterContext *bsfc, AVCodecContext *avctx, const ch
     return 1;
 }
 
+static int mp3_header_compress(AVBitStreamFilterContext *bsfc, AVCodecContext *avctx, const char *args,
+                     uint8_t **poutbuf, int *poutbuf_size,
+                     const uint8_t *buf, int buf_size, int keyframe){
+    uint32_t header;
+    int mode_extension;
+
+    if(avctx->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL){
+        av_log(avctx, AV_LOG_ERROR, "not standards compliant\n");
+        return -1;
+    }
+
+    header = (buf[0] << 24) | (buf[1] << 16) | (buf[2] << 8) | buf[3];
+    mode_extension= (header>>4)&3;
+
+    if(ff_mpa_check_header(header) < 0 || (header&0x70000) != 0x30000){
+        *poutbuf= (uint8_t *) buf;
+        *poutbuf_size= buf_size;
+
+        av_log(avctx, AV_LOG_INFO, "cannot compress %08X\n", header);
+        return 0;
+    }
+
+    *poutbuf_size= buf_size - 4;
+    *poutbuf= av_malloc(buf_size - 4 + FF_INPUT_BUFFER_PADDING_SIZE);
+    memcpy(*poutbuf, buf + 4, buf_size - 4 + FF_INPUT_BUFFER_PADDING_SIZE);
+
+    if(avctx->channels==2){
+        if((header & (3<<19)) != 3<<19){
+            (*poutbuf)[1] &= 0x3F;
+            (*poutbuf)[1] |= mode_extension<<6;
+            FFSWAP(int, (*poutbuf)[1], (*poutbuf)[2]);
+        }else{
+            (*poutbuf)[1] &= 0x8F;
+            (*poutbuf)[1] |= mode_extension<<4;
+        }
+    }
+
+    return 1;
+}
+
+static int mp3_header_decompress(AVBitStreamFilterContext *bsfc, AVCodecContext *avctx, const char *args,
+                     uint8_t **poutbuf, int *poutbuf_size,
+                     const uint8_t *buf, int buf_size, int keyframe){
+    uint32_t header;
+    int sample_rate= avctx->sample_rate;
+    int sample_rate_index=0;
+    int lsf, mpeg25, bitrate_index, frame_size;
+
+    header = (buf[0] << 24) | (buf[1] << 16) | (buf[2] << 8) | buf[3];
+    if(ff_mpa_check_header(header) >= 0){
+        *poutbuf= (uint8_t *) buf;
+        *poutbuf_size= buf_size;
+
+        return 0;
+    }
+
+    header= 0xFFE00000 | ((4-3)<<17) | (1<<16); //FIXME simplify
+
+    lsf     = sample_rate < (24000+32000)/2;
+    mpeg25  = sample_rate < (12000+16000)/2;
+    header |= (!mpeg25)<<20;
+    header |= (!lsf   )<<19;
+    if(sample_rate<<(lsf+mpeg25) < (44100+32000)/2)
+        sample_rate_index |= 2;
+    else if(sample_rate<<(lsf+mpeg25) > (44100+48000)/2)
+        sample_rate_index |= 1;
+
+    header |= sample_rate_index<<10;
+    sample_rate= mpa_freq_tab[sample_rate_index] >> (lsf + mpeg25); //in case sample rate is a little off
+
+    for(bitrate_index=2; bitrate_index<30; bitrate_index++){
+        frame_size = mpa_bitrate_tab[lsf][2][bitrate_index>>1];
+        frame_size = (frame_size * 144000) / (sample_rate << lsf) + (bitrate_index&1);
+        if(frame_size == buf_size + 4)
+            break;
+    }
+    if(bitrate_index == 30){
+        av_log(avctx, AV_LOG_ERROR, "couldnt find bitrate_index\n");
+        return -1;
+    }
+
+    header |= (bitrate_index&1)<<9;
+    header |= (bitrate_index>>1)<<12;
+    header |= (avctx->channels==1 ? MPA_MONO : MPA_JSTEREO)<<6;
+
+    *poutbuf_size= buf_size + 4;
+    *poutbuf= av_malloc(buf_size + 4 + FF_INPUT_BUFFER_PADDING_SIZE);
+    memcpy(*poutbuf + 4, buf, buf_size + FF_INPUT_BUFFER_PADDING_SIZE);
+
+    if(avctx->channels==2){
+        if(lsf){
+            FFSWAP(int, (*poutbuf)[5], (*poutbuf)[6]);
+            header |= ((*poutbuf)[5] & 0xC0)>>2;
+        }else{
+            header |= (*poutbuf)[5] & 0x30;
+        }
+    }
+
+    (*poutbuf)[0]= header>>24;
+    (*poutbuf)[1]= header>>16;
+    (*poutbuf)[2]= header>> 8;
+    (*poutbuf)[3]= header    ;
+
+    return 1;
+}
+
 AVBitStreamFilter dump_extradata_bsf={
     "dump_extra",
     0,
@@ -141,3 +248,15 @@ AVBitStreamFilter noise_bsf={
     sizeof(int),
     noise,
 };
+
+AVBitStreamFilter mp3_header_compress_bsf={
+    "mp3comp",
+    0,
+    mp3_header_compress,
+};
+
+AVBitStreamFilter mp3_header_decompress_bsf={
+    "mp3decomp",
+    0,
+    mp3_header_decompress,
+};

-- 
Libav/FFmpeg packaging

