Re: [FFmpeg-devel] [PATCH] avcodec/photocd: allow decoding vlc with errors

From: Paul B Mahol <onemda@gmail.com>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH] avcodec/photocd: allow decoding vlc with errors
Date: Thu, 28 Sep 2023 11:48:14 +0200
Message-ID: <CAPYw7P4mLHKGnSf6GjNg2-nADy=OB-yL_XNegii-YCf7qG9Ljg@mail.gmail.com> (raw)
In-Reply-To: <169589387341.6638.15012517934493438253@lain.khirnov.net>

On 9/28/23, Anton Khirnov <anton@khirnov.net> wrote:
> Quoting Paul B Mahol (2023-09-27 21:53:54)
>> Attached.
>>
>> From 6b7e51e2b4facead9c45fb94647c85b58e6fdf22 Mon Sep 17 00:00:00 2001
>> From: Paul B Mahol <onemda@gmail.com>
>> Date: Wed, 27 Sep 2023 21:48:49 +0200
>> Subject: [PATCH] avcodec/photocd: allow decoding vlc with errors
>>
>> Also rename option that no longer works.
>> Also make tables finding more robust.
>
> Why do all these things in a single patch?
>

I can boost my commits if you prefer that way.

> And why are there no tests for this decoder?

Its complicated.

>
>> Signed-off-by: Paul B Mahol <onemda@gmail.com>
>> ---
>>  libavcodec/photocd.c | 7 +++----
>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/libavcodec/photocd.c b/libavcodec/photocd.c
>> index 07e8d460bd..a9e42bb86a 100644
>> --- a/libavcodec/photocd.c
>> +++ b/libavcodec/photocd.c
>> @@ -277,8 +277,6 @@ static av_noinline int decode_huff(AVCodecContext
>> *avctx, AVFrame *frame,
>>              if (get_bits_left(&g) <= 0)
>>                  return AVERROR_INVALIDDATA;
>>              m = get_vlc2(&g, s->vlc[idx].table, s->vlc[idx].bits, 2);
>> -            if (m < 0)
>> -                return AVERROR_INVALIDDATA;
>
> Shouldn't this error be at least logged? Perhaps it should still fail on
> AV_EF_EXPLODE.
>
>>              m = sign_extend(m, 8);
>>              data[x] = av_clip_uint8(data[x] + m);
>>          }
>> @@ -389,6 +387,7 @@ static int photocd_decode_frame(AVCodecContext *avctx,
>> AVFrame *p,
>>              return AVERROR_INVALIDDATA;
>>
>>          if (s->resolution == 4) {
>> +            s->streampos = (AV_RB16(avpkt->data + 2048 + 1536 + 3) + 12)
>> * 2048;
>
> Where do these magic numbers come from? Same below.

From offset from internal image file bitstream table structure.

>
> --
> Anton Khirnov
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