[FFmpeg-devel] [PATCH] png parser

[Michael Niedermayer]

On Thu, Jun 25, 2009 at 02:14:14PM +0200, Peter Holik wrote:
> Michael Niedermayer schrieb:
> > On Thu, Jun 25, 2009 at 12:59:07PM +0200, Peter Holik wrote:
> >> Michael Niedermayer schrieb:
> >> > On Wed, Jun 24, 2009 at 05:54:17PM +0200, Peter Holik wrote:
> >> >> Michael Niedermayer schrieb:
> >> > [...]
> >> >> >> +            if (*state64_ptr == PNGSIG || *state64_ptr == MNGSIG) {
> >> >> >> +                i++;
> >> >> >> +                if (ppc->pc.index) {
> >> >> >> +                    i -= 8;
> >> >> >> +                    next = i;
> >> >> >> +                } else {
> >> >> >> +                    ppc->pc.frame_start_found = 1;
> >> >> >> +                    ppc->chunk_index = 1;
> >> >> >> +                    if (i == 8)
> >> >> >> +                        break;
> >> >> >> +                    else if (i > 8) {
> >> >> >> +                        buf += i - 8;
> >> >> >> +                        buf_size = 8;
> >> >> >> +                    } else /* if (i < 8) */
> >> >> >> +                        buf_size = i;
> >> >> >> +                }
> >> >> >> +                ff_combine_frame(&ppc->pc, next, &buf, &buf_size);
> >> >> >> +                return i;
> >> >> >
> >> >> > could you explain why this is not just looking like lets say mpeg4:
> >> >> >
> >> >> >     if(!vop_found){
> >> >> >         for(i=0; i<buf_size; i++){
> >> >> >             state= (state<<8) | buf[i];
> >> >> >             if(state == 0x1B6){
> >> >> >                 i++;
> >> >> >                 vop_found=1;
> >> >> >                 break;
> >> >> >             }
> >> >> >         }
> >> >> >     }
> >> >>
> >> >> png signature is 8 Bytes
> >> >
> >> > s/state/state64/ in the example, the question stays the same
> >> >
> >> >
> >> >>
> >> >>
> >> >> > what is the point of all the special cases?
> >> >>
> >> >>
> >> >> Because this png parser should filter out good png images.
> >> >
> >> > thats not the job of a parser, besides good is a fuzzy term, a
> >> > png with a bit fliped in the signature is pretty good and easy to decode
> >> > your code would drop it i think.
> >>
> >> Following parser also drops:
> >>
> >> mpegaudio_parser.c
> >> mlp_parser.c
> >> dvbsub_parser.c
> >> pnm_parser.c
> >>
> >> why may my parser not drop?
> >
> > i have explained it already
> > because it drops things that can still be decoded easily in principle
> > the others you quote may very well be buggy
> >
> >
> >>
> >> Anyway fighting against windmills is meaningless.
> >>
> >> Here now a version without dropping anything and using 32 Bits for
> >> signature check instead of 64 Bits.
> >
> > why 32bit?
> > isnt 64bit simpler for 64bit sigs?
> 
> suggested by "Kostya" <kostya.shishkov at gmail.com>
> 
> I also would prefere 64bits
> 
> What shall i do now?

i cant see a reason or advantage to use 32bit so use 64bit
unless someome, kostya? can explain what advantage 32bit would
have
(and no it shouldnt matter speewise as normally these loops run only
an insignificant number of iterations)

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Into a blind darkness they enter who follow after the Ignorance,
they as if into a greater darkness enter who devote themselves
to the Knowledge alone. -- Isha Upanishad
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