[FFmpeg-devel] [PATCH] png parser

[Peter Holik]

Peter Holik schrieb:
> Michael Niedermayer schrieb:
>> 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)
>
> now with 64bits

ping, pleas apply

cu Peter