[FFmpeg-devel] [PATCH] HAM6/HAM8 support for IFF demuxer/decoder

[Sebastian Vater]

Michael Niedermayer a ?crit :
> On Thu, Apr 29, 2010 at 01:51:09PM +0200, Sebastian Vater wrote:
>   
>> Michael Niedermayer a ?crit :
>>     
>>> On Thu, Apr 29, 2010 at 01:10:18PM +0200, Sebastian Vater wrote:
>>>   
>>>       
>>>> Peter Ross a ?crit :
>>>>     
>>>>         
>>>>> On Wed, Apr 28, 2010 at 11:17:12PM +0200, Sebastian Vater wrote:
>>>>>   
>>>>>       
>>>>>           
>>>>>> Sebastian Vater a ?crit :
>>>>>>     
>>>>>>         
>>>>>>             
>>>>>>> So here is it!
>>>>>>>
>>>>>>> The long awaited HAM6/8 decoding support for IFF-ILBM.
>>>>>>>       
>>>>>>>           
>>>>>>>               
>>>>> Yep, glad somebody took the time.
>>>>>
>>>>>   
>>>>>       
>>>>>           
>>>>>> +#define CAMG_EHB  0x80   // Extra HalfBrite
>>>>>>     
>>>>>>         
>>>>>>             
>>>>> Macro isn't used.
>>>>>   
>>>>>       
>>>>>           
>>>> Just added it, the next patch for EHB support will use it, should I
>>>> remove it until then and add the define when EHB is done?
>>>>
>>>>     
>>>>         
>>>>>   
>>>>>       
>>>>>           
>>>>>> @@ -152,6 +157,12 @@ static int iff_read_header(AVFormatContext *s,
>>>>>>              st->codec->channels = (get_be32(pb) < 6) ? 1 : 2;
>>>>>>              break;
>>>>>>  
>>>>>> +        case ID_CAMG:
>>>>>> +            if (data_size < 4)
>>>>>> +                return AVERROR_INVALIDDATA;
>>>>>>     
>>>>>>         
>>>>>>             
>>>>> Is this really neccessary. We dont check data_size for any of the other tags.
>>>>>   
>>>>>       
>>>>>           
>>>> The IFF standard says that chunks can be of any length, it seems that
>>>> you missed by patch which does add the checking for other tags, too.
>>>>
>>>> Not checking such stuff could also cause security issues if someone with
>>>> a bad mind prepares an attack with a malformed IFF file.
>>>>
>>>> It would not be the first case picture decoders cause buffer overflows
>>>> thanks to malformed image data/structure. ;-)
>>>>
>>>> Also chunks can always be longer than the initial definition to it, just
>>>> like you can add new data fields to a struct. ;-)
>>>>
>>>> But in one point you're correct here, it should really be evaluated if
>>>> the decoder should abort if the chunk is to small (i.e. just 2 bytes,
>>>> because the amiga display mode used for it had a zero lower word) or
>>>> just consider to small stuff to be set to 0 (in that case non-HAM and
>>>> non-EHB would always be assumed).
>>>>
>>>>     
>>>>         
>>>>>> +        if (iff->camg_display_mode & CAMG_HAM) {
>>>>>> +            switch (st->codec->bits_per_coded_sample) {
>>>>>> +            case 6: // HAM6
>>>>>> +                st->codec->bits_per_coded_sample = 12; // 4096 color HAM6 image
>>>>>> +                break;
>>>>>> +
>>>>>> +            case 8: // HAM8
>>>>>> +                st->codec->bits_per_coded_sample = 18; // 262144 color HAM8 image
>>>>>> +                break;
>>>>>>     
>>>>>>         
>>>>>>             
>>>>> My only concern here is that 'bits_per_coded_sample' is being abused to
>>>>> indicate both bpp *and* whether HAM coding is used. If somebody out there
>>>>> encodes 12-bit RGB into IFF then we will have a problem.
>>>>>   
>>>>>       
>>>>>           
>>>> I just realized that too.
>>>>
>>>> And I'm just thinking of a better solution. Is it appreciated in ffmpeg
>>>> to use codec_tag as a bitfield or split up the uint32_t internally into
>>>> 4 uint8_t or 2 2 uint8_t's?
>>>>
>>>> Right now the IFF demuxer and decoder use codec_tag to store whether
>>>> it's an PBM or ILBM file.
>>>>
>>>> I would like to split it internally into 4 bytes and use one byte for
>>>> tagging if it's EHB/HAM, one byte for transparent color index, one byte
>>>> for the compression identifier and one byte for plane number being a
>>>> mask to skip.
>>>>     
>>>>         
>>> you would like to look at AVCodecContext.extradata
>>>
>>> [...]
>>>   
>>>       
>> I know extradata, but it's used for the palette data already and moving
>> that data structures around or adding sth. at the end requires defining
>> a completely new structure (one for the palette data and one for extra
>> data).
>>
>> So I'ld use codec_tag for it, using an whole 32 bit integer just for
>> indicating if it's a PBM or ILBM (one bit would be enough for that) is
>> in my eyes, just a waste.
>>
>> Or are there any reasons for not doing it the way, I wrote my last msg?
>>     
>
> dont forget to fill AVIn/OutputFormat.codec_tag ;)
> (when you try you will see why the idea of spliting codec_tag like this
>  doesnt work)
>   

Oh ok, I got the point. So it should always be a kind of fourcc, right?

Then I have to do this with extradata...

-- 

Best regards,
                   :-) Basty/CDGS (-: