[FFmpeg-devel] [Read EXIF metadata 3/3] Read EXIF metadata in JPEG input.

[Michael Niedermayer]

On Sat, Aug 10, 2013 at 01:25:23PM +0200, Thilo Borgmann wrote:
> 
> >> @@ -215,6 +218,8 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
> >>      int len, nb_components, i, width, height, pix_fmt_id;
> >>      int h_count[MAX_COMPONENTS];
> >>      int v_count[MAX_COMPONENTS];
> >> +    AVDictionary **metadata = avpriv_frame_get_metadatap(s->picture_ptr);
> >> +    AVDictionary *tmp;
> >>
> >>      s->cur_scan = 0;
> >>      s->upscale_h = s->upscale_v = 0;
> >> @@ -459,6 +464,10 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
> >>              s->avctx->pix_fmt = AV_PIX_FMT_GRAY16;
> >>      }
> >>
> >> +    // keep metadata alive by hiding AVDictioniary behind *tmp
> >> +    tmp       = *metadata;
> >> +    *metadata = NULL;
> >> +
> >>      av_frame_unref(s->picture_ptr);
> >>      if (ff_get_buffer(s->avctx, s->picture_ptr, AV_GET_BUFFER_FLAG_REF) < 0)
> >>          return -1;
> >> @@ -466,6 +475,9 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
> >>      s->picture_ptr->key_frame = 1;
> >>      s->got_picture            = 1;
> >>
> >> +    // restore metadata in current buffer
> >> +    *avpriv_frame_get_metadatap(s->picture_ptr) = tmp;
> >> +
> >>      for (i = 0; i < 3; i++)
> >>          s->linesize[i] = s->picture_ptr->linesize[i] << s->interlaced;
> >>
> >
> > this looks quite odd, metadata should only be stored in the correct
> > frame, not the old frame
> 
> Yes a hack. However, my alternative solution does somehow loose
> the metadata after _decode_frame() is finished and I don't know why.......
> attached as follow-up patch 4/3 of rev 5 (0004-Remove-hack).
> So I need another idea or a hint how to fix that alternative.
> 
> 
> >> +        // read all IFDs and store the metadata
> >> +        ifds_read = 0;
> >> +        while ((ifd_offset = ff_exif_decode_ifd(s->avctx, &gbytes, le,
> avpriv_frame_get_metadatap(s->picture_ptr))) > 0) {
> >> +            if (++ifds_read > FF_ARRAY_ELEMS(ifd_tags)) {
> >> +                av_log(s->avctx, AV_LOG_INFO, "mjpeg: found more IFD's in
> EXIF data than expected. Skipping.\n");
> >> +                break;
> >> +            }
> >> +            bytestream2_seek(&gbytes, ifd_offset, SEEK_SET);
> >> +        }
> >
> > I would assume that offsets always move forward but that theres no
> > real limit om the number of tags
> 
> TIFF 6 spec says:
> "This file offset may point anywhere in the file, even after the image data."
> So it might also point backwards again. Also, according to EXIF specs, there are
> exactly these three IFD's defined.

Theres also possibly an additional problem
if i understand the tiff spec  correctly
each IFD represents a seperate picture and each could have
exif, gps ... data. Throwing all in the same metadata is maybe not
the best thing to do. Especially if we decode just one picture

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Good people do not need laws to tell them to act responsibly, while bad
people will find a way around the laws. -- Plato
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