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

[Michael Niedermayer]

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)

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Everything should be made as simple as possible, but not simpler.
-- Albert Einstein
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