[FFmpeg-devel] [PATCH] png parser

[Peter Holik]

Michael Niedermayer schrieb:
> On Sat, Jun 13, 2009 at 01:54:46PM +0200, Peter Holik wrote:
> [...]
>> +static int png_parse(AVCodecParserContext *s, AVCodecContext *avctx,
>> +                     const uint8_t **poutbuf, int *poutbuf_size,
>> +                     const uint8_t *buf, int buf_size)
>> +{
>> +    ParseContext *pc = s->priv_data;
>> +    uint32_t *state_ptr = &pc->state;
>
>> +    uint32_t *chunk_length_ptr = (uint32_t *)&pc->state64;
>
> this looks like you misuse the field

yes i use unused fields, and to be able to read the code i do not use the
unused variables directly - i use this construct to have a better understanding
and not to have waste memory.


>> +    uint32_t *remaining_size_ptr = &pc->overread_index;
>
> thats a very obfuscated way o access a field from the struct

i think it is better to do it this way than have to do something like

---
uint32_t index = pc->overread_index;

use index ....

pc->overread_index = index;
---

in my version i do

---
uint32_t *index = &pc->overread_index;

use *index
---


but if you won't like coding like this than i make an extra struct.


>> +    int next = END_NOT_FOUND;
>> +    int i = 0;
>> +
>> +    *poutbuf_size = 0;
>> +    if (buf_size == 0)
>> +        return 0;
>> +
>> +    if (!pc->frame_start_found) {
>> +        uint64_t *state64_ptr = &pc->state64;
>> +
>> +        for (; i < buf_size; i++) {
>> +            *state64_ptr = (*state64_ptr << 8) | buf[i];
>> +            if (*state64_ptr == PNGSIG || *state64_ptr == MNGSIG) {
>> +                pc->frame_start_found = 1;
>> +                if (++i == 8)
>> +                    break;
>
>> +                /* workaround to start packet with a signature */
>> +                buf_size = 8;
>> +                *state64_ptr = be2me_64(*state64_ptr);
>> +                if (ff_combine_frame(pc, next, (const uint8_t **)&state64_ptr, &buf_size) !=
>> -1) {
>
> whatever this is supposed to do, it looks very hackish

my idea of a parser is to take good content out of a garbaged stream.

well in this case if there were some bytes of the header in the last packet
i form a packet starting with a signature.

because most of ffmpeg decoders do not search for a begin of a signature, they drop.

>> +                    next = buf_size;
>> +                    goto fail;
>> +                }
>> +                return i;
>> +            }
>> +        }
>> +    } else
>> +        if (*remaining_size_ptr) {
>> +            i = FFMIN(*remaining_size_ptr, buf_size);
>> +            *remaining_size_ptr -= i;
>> +            if (*remaining_size_ptr)
>> +                goto flush;
>> +            if (pc->frame_start_found == -1) {
>> +                next = i;
>> +                goto flush;
>> +            }
>> +        }
>> +
>> +    for (;pc->frame_start_found && i < buf_size; i++) {
>> +        *state_ptr = (*state_ptr << 8) | buf[i];
>> +        if (pc->frame_start_found == 4) {
>> +                *chunk_length_ptr = AV_RL32(state_ptr) + 4;
>> +                if (*chunk_length_ptr > 0x7fffffff) {
>> +                    next = buf_size;
>> +                    goto fail;
>> +                }
>
> i think this discards data?
> if so, thats wrong, parsers should not discard any data, just split it the
> best they can.
> a decoder might very well be able to decode the correct parts of a frame ...

if taken this check out of the decoder, whats wrong here?

cu Peter