[FFmpeg-devel] [PATCH] gifdec: resync support

[Paul B Mahol]

On 1/21/13, Don Moir <donmoir at comcast.net> wrote:
>
> ----- Original Message -----
> From: "Michael Niedermayer" <michaelni at gmx.at>
> To: "FFmpeg development discussions and patches" <ffmpeg-devel at ffmpeg.org>
> Sent: Monday, January 21, 2013 12:56 PM
> Subject: [FFmpeg-devel] [PATCH] gifdec: resync support
>
>
>> This allows decoding gifs that have junk at the begin
>> and also allows byte wise seeking
>>
>> Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
>> ---
>> libavformat/gifdec.c |   32 ++++++++++++++++++++++++++------
>> 1 file changed, 26 insertions(+), 6 deletions(-)
>>
>> diff --git a/libavformat/gifdec.c b/libavformat/gifdec.c
>> index 4f4ce2c..357d698 100644
>> --- a/libavformat/gifdec.c
>> +++ b/libavformat/gifdec.c
>> @@ -70,6 +70,19 @@ static int gif_probe(AVProbeData *p)
>>     return AVPROBE_SCORE_MAX;
>> }
>>
>> +static int resync(AVIOContext *pb)
>> +{
>> +    int i;
>> +    for (i=0; i<6; i++) {
>> +        int b = avio_r8(pb);
>> +        if(b != gif87a_sig[i] && b != gif89a_sig[i])
>> +            i = -(b != 'G');
>> +        if(url_feof(pb))
>> +            return AVERROR_EOF;
>> +    }
>> +    return 0;
>> +}
>> +
>> static int gif_read_header(AVFormatContext *s)
>> {
>>     GIFDemuxContext *gdc = s->priv_data;
>> @@ -77,8 +90,7 @@ static int gif_read_header(AVFormatContext *s)
>>     AVStream        *st;
>>     int width, height, ret;
>>
>> -    /* skip 6-byte magick */
>> -    if ((ret = avio_skip(pb, 6)) < 0)
>> +    if ((ret = resync(pb)) < 0)
>>         return ret;
>>
>>     gdc->delay  = gdc->default_delay;
>> @@ -171,6 +183,7 @@ static int gif_read_packet(AVFormatContext *s,
>> AVPacket *pkt)
>>     }
>>
>>     if (keyframe) {
>> +parse_keyframe:
>>         /* skip 2 bytes of width and 2 of height */
>>         if ((ret = avio_skip(pb, 4)) < 0)
>>             return ret;
>> @@ -196,7 +209,7 @@ static int gif_read_packet(AVFormatContext *s,
>> AVPacket *pkt)
>>     while (GIF_TRAILER != (block_label = avio_r8(pb)) && !url_feof(pb)) {
>>         if (block_label == GIF_EXTENSION_INTRODUCER) {
>>             if ((ret = gif_read_ext (s)) < 0 )
>> -                return ret;
>> +                goto resync;
>>         } else if (block_label == GIF_IMAGE_SEPARATOR) {
>>             /* skip to last byte of Image Descriptor header */
>>             if ((ret = avio_skip(pb, 8)) < 0)
>> @@ -215,11 +228,11 @@ static int gif_read_packet(AVFormatContext *s,
>> AVPacket *pkt)
>>             /* read LZW Minimum Code Size */
>>             if (avio_r8(pb) < 1) {
>>                 av_log(s, AV_LOG_ERROR, "lzw minimum code size must be >=
>> 1\n");
>> -                return AVERROR_INVALIDDATA;
>> +                goto resync;
>>             }
>>
>>             if ((ret = gif_skip_subblocks(pb)) < 0)
>> -                return ret;
>> +                goto resync;
>>
>>             frame_end = avio_tell(pb);
>>
>> @@ -244,7 +257,14 @@ static int gif_read_packet(AVFormatContext *s,
>> AVPacket *pkt)
>>             break;
>>         } else {
>>             av_log(s, AV_LOG_ERROR, "invalid block label\n");
>> -            return AVERROR_INVALIDDATA;
>> +resync:
>> +            if (!keyframe)
>> +                avio_seek(pb, frame_start, SEEK_SET);
>> +            if((ret = resync(pb)) < 0)
>> +                return ret;
>> +            frame_start = avio_tell(pb) - 6;
>> +            keyframe = 1;
>> +            goto parse_keyframe;
>>         }
>>     }
>
> There is also a av_log (s,AV_LOG_FATAL,...) in gif_read_ext in same file.
> Should be AV_LOG_ERROR I think.
>

Fixed long ago, you should really not hijack thread.
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