isom: convert mp4 dvdsub extradata to vobsub format

mp4 files embedding DVD subtitles do not use the same extradata format
as the rest of Libav expects. The subtitle decoder in libavcodec in
particular does not understand this format.

Convert the extradata to the vobsub .idx format. mp4 stores the palette
as binary 32 bit ints in YUV. The subtitle resolution is stored
separately in the track header, which we access through AVStream.
Signed-off-by: NAnton Khirnov <anton@khirnov.net>

libavformat/isom.c

@@ -26,6 +26,7 @@
 #include "isom.h"
 #include "libavcodec/mpeg4audio.h"
 #include "libavcodec/mpegaudiodata.h"
+#include "libavutil/intreadwrite.h"
 
 /* http://www.mp4ra.org */
 /* ordered by muxing preference */
@@ -419,8 +420,60 @@ static const AVCodecTag mp4_audio_types[] = {
     { AV_CODEC_ID_NONE,   AOT_NULL },
 };
 
+static uint32_t yuv_to_rgba(uint32_t ycbcr)
+{
+    uint8_t r, g, b;
+    int y, cb, cr;
+
+    y  = (ycbcr >> 16) & 0xFF;
+    cr = (ycbcr >> 8)  & 0xFF;
+    cb =  ycbcr        & 0xFF;
+
+    b = av_clip_uint8(1.164 * (y - 16)                      + 2.018 * (cb - 128));
+    g = av_clip_uint8(1.164 * (y - 16) - 0.813 * (cr - 128) - 0.391 * (cb - 128));
+    r = av_clip_uint8(1.164 * (y - 16) + 1.596 * (cr - 128));
+
+    return (r << 16) | (g << 8) | b;
+}
+
+static int mov_rewrite_dvd_sub_extradata(AVStream *st)
+{
+    char pal_s[256];
+    char buf[256];
+    int pal_s_pos = 0;
+    uint8_t *src = st->codec->extradata;
+
+    if (st->codec->extradata_size != 64)
+        return 0;
+
+    for (int i = 0; i < 16; i++) {
+        uint32_t yuv = AV_RB32(src + i * 4);
+        uint32_t rgba = yuv_to_rgba(yuv);
+
+        snprintf(pal_s + pal_s_pos, sizeof(pal_s) - pal_s_pos, "%06x%s", rgba,
+                 i != 15 ? ", " : "");
+        pal_s_pos = strlen(pal_s);
+        if (pal_s_pos >= sizeof(pal_s))
+            return 0;
+    }
+
+    snprintf(buf, sizeof(buf), "size: %dx%d\npalette: %s\n",
+             st->codec->width, st->codec->height, pal_s);
+
+    av_freep(&st->codec->extradata);
+    st->codec->extradata_size = 0;
+    st->codec->extradata = av_mallocz(strlen(buf) + FF_INPUT_BUFFER_PADDING_SIZE);
+    if (!st->codec->extradata)
+        return AVERROR(ENOMEM);
+    st->codec->extradata_size = strlen(buf);
+    memcpy(st->codec->extradata, buf, st->codec->extradata_size);
+
+    return 0;
+}
+
 int ff_mp4_read_dec_config_descr(AVFormatContext *fc, AVStream *st, AVIOContext *pb)
 {
+    int err;
     int len, tag;
     int object_type_id = avio_r8(pb);
     avio_r8(pb); /* stream type */
@@ -460,6 +513,10 @@ int ff_mp4_read_dec_config_descr(AVFormatContext *fc, AVStream *st, AVIOContext
                                                         cfg.object_type)))
                 st->codec->codec_id = AV_CODEC_ID_AAC;
         }
+        if (st->codec->codec_id == AV_CODEC_ID_DVD_SUBTITLE) {
+            if ((err = mov_rewrite_dvd_sub_extradata(st)) < 0)
+                return err;
+        }
     }
     return 0;
 }