[FFmpeg-devel] [PATCH] aac: Clip large (legal) PNS/IS values

[Alex Converse]

On Tue, Oct 13, 2009 at 2:58 PM, Robert Swain <robert.swain at gmail.com> wrote:
> 2009/10/10 Alex Converse <alex.converse at gmail.com>:
>> On Fri, Oct 9, 2009 at 7:34 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>>> On Fri, Oct 09, 2009 at 12:38:17PM -0400, Alex Converse wrote:
>>>> On Fri, Oct 9, 2009 at 9:37 AM, Robert Swain <robert.swain at gmail.com> wrote:
>>>> > 2009/10/6 Alex Converse <alex.converse at gmail.com>:
>>>> >> Large intensity stereo and PNS indices are legal. Clip them instead of
>>>> >> erroring out. A magnitude of 100 corresponds to 2^25 so the will most
>>>> >> likely result in clipped output anyway.
>>>> >>
>>>> >> None of the conformance streams fall in the range that need to be clipped.
>>>> >
>>>> > Looks OK to me as long as it isn't arbitrary clipping but rather
>>>> > something needed/allowed within and specified by the specification.
>>>> >
>>>>
>>>> The clipping is semi-arbitrary. It was designed such that after the
>>>> third patch the ranges are the same as regular scalefactors. However
>>>> these large vales (even when clipped) should cause cause output
>>>> clipping on any streams that aren't purely academic exercises.
>>>
>>> IMHO, either
>>> A. it can be in a valid stream -> we should support it
>>> B. it can not be in a valid stream -> error concealment (silence /repeat
>>> ? last block / something better) should be applied
>>> cliping seems always wrong (to me) unless mandated by a spec.
>>> switching between A and B, in the sense of "it could be valid but most
>>> likely is not, lets ask the user" can be done by using
>>> AVCodecContext.error_recognition
>>>
>>
>> It's a mix between that (the error recognition case) and the fact that
>> even if you decode it properly there isn't going to be a difference
>> between clipping and not.
>>
>> For instance let's look at intensity stereo. If the IS scalefactor is
>> 100, associated then the gain is 2^-25 The only way for the channel
>> receiving the intensity stereo to get something that doesn't round to
>> zero is for the source channel to be clipped beyond belief.
>>
>> If that gain of 2**-25 seems reasonable to you, then consider another
>> legal IS scalefactor, 512. 512 corresponds to a gain of 2**128. That
>> is larger than the maximum finite float.
>>
>> IS scalefactors are coded differentially from 0 with a maximum step of
>> +/-60 which is another reason why very large values are unlikely.
>
> As long as the scalefactor values are valid within the specification,
> I think we should only clip them if it is provable that doing so does
> not alter the output of any valid stream, academic exercise or not.
>

Well right now we are throwing out valid streams rather than clipping
them which is worse IMHO.