[FFmpeg-devel] [PATCH] ALS decoder

[Thilo Borgmann]

Diego Biurrun schrieb:
> On Sun, Sep 06, 2009 at 01:52:57PM +0200, Thilo Borgmann wrote:
>> Michael Niedermayer schrieb:
>>> On Fri, Sep 04, 2009 at 09:48:46PM +0200, Thilo Borgmann wrote:
>>>> Michael Niedermayer schrieb:
>>>>> On Thu, Sep 03, 2009 at 09:44:25PM +0200, Thilo Borgmann wrote:
>>>>>> Revision 16 attached.
>>>>>> +/** Decodes an ALS frame.
>>>>>> + */
>>>>>> +static int decode_frame(AVCodecContext *avctx,
>>>>>> +                        void *data, int *data_size,
>>>>>> +                        AVPacket *avpkt)
>>>>>> +{
>>>>>> +    ALSDecContext *ctx       = avctx->priv_data;
>>>>>> +    ALSSpecificConfig *sconf = &ctx->sconf;
>>>>>> +    const uint8_t *buffer    = avpkt->data;
>>>>>> +    int buffer_size          = avpkt->size;
>>>>>> +    int invalid_frame, size;
>>>>>> +    unsigned int c, sample, ra_frame, bytes_read, shift;
>>>>>> +
>>>>>> +    init_get_bits(&ctx->gb, buffer, buffer_size * 8);
>>>>>> +    ra_frame = sconf->ra_distance && !(ctx->frame_id % sconf->ra_distance);
>>>>> is it true that if ra_distance == 0 then no frame is a ra frame?
>>>>> seems odd
>>>>>
>>>> As mentioned above, this is the specified behaviour.
>>>> And that is the reason for using av_mallocz() for the raw_buffer buffer,
>>>> to have fake 0's for prediction in the carryover samples from frame "-1".
>>> does the spec say they should be 0 ?
>> This whole case is not mentioned explicitly in the spec (as fas as I can
>> see). But this is produced by the reference encoder and can be followed
>> implicitly - before the first frame, there is silence (0). Until we
>> figured that out, we had some "randomly" broken files because of
>> non-zero memory chunks allocated for the first carryover buffer.
>>
>> If we would not implicitly assume this from the specs and treat the very
>> first frame to be a RA frame (keyframe) in all cases, decoding fails.
> 
> This is clearly something that you need to document, not mention in
> passing during the review process.

Yes, I'm sorry about that. I've almost forgotten about this conclusion.

-Thilo