Re: [FFmpeg-devel] [PATCH 1/1] avcodec/dolby_e: Add error recovery when parse_mantissas run out of bits

[Nicolas Gaullier <nicolas.gaullier@cji.paris>]

>fre 2023-03-10 klockan 10:17 +0100 skrev Nicolas Gaullier:
>> +                if (i == c->nb_groups - 1
>> +                    && count * size1 > get_bits_left(&s->gb)
>> +                    && get_bits_left(&s->gb) >= 0
>> +                    && (int)(mnt - c->mantissas) >= MIN_MANTISSAS) {
>> +                    av_log(s->avctx, AV_LOG_WARNING, "Truncated
>> mantissas @%d, "
>> +                        "highest frequencies not recoverable\n",
>> (int)(mnt - c->mantissas));
>> +                    break;
>> +                }
>
>Surely there's a proper way to fix this rather than having an arbitrary threshold. At the very least the get_bits_left() check could be moved to before parse_mantissas(). If get_bits_left() is < 0 after
>parse_mantissas() then a warning could be issued instead of erroring out, which should have an effect similar to this.
>
>Is there a spec saying what to do with truncated packets? Since this is Dolby-E I suspect the answer is "no".
>
>/Tomas

Thank you for inspecting the patch.
Concerning the arbitrary threshold: it may be understood as a very high level safeguard, but my intend was mostly to make the code more easily understandable.
Actually, the fix is restricted to the last audio band (see "i == c->nb_groups - 1") and, at the end, the MIN_MANTISSAS condition could be simply removed with no audible risk, I guess.
But to get this precision, the fix cannot be moved elsewhere. The idea here is to still be able to detect harsh packet truncation with full errors and keep current code behaviour in that case.

Of course, this patch looks as a work-around, but a work-around that make it possible to decode the official reference bitstream implementation (dp600 at least, but maybe they are no different ones).
So, this is a somewhat special scenario. Sadly, I don't see any another way to handle this.

To conclude, I don't have any other idea than the complete removal of the arbitrary threshold. Would it make the patch more acceptable ?

Nicolas
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