[FFmpeg-devel] img2dec: jpeg_probe logic

[wm4]

On Mon, 8 May 2017 16:25:43 -0400
Vadim Kalinsky <gebiet at gmail.com> wrote:

> Hey,
> 
> Trying to fix a bug #6113, I stumbled upon some strange logic in libavformat/img2dec.c:jpeg_probe. It accepts first 2048 bytes of jpeg stream, and tries to read it with some state machine. If it doesn't look like jpeg stream, it returns 0 ("it's definitely not a jpeg").
> 
> After it read through the buffer, it does this:
> 
> if (state == EOI)
>     return AVPROBE_SCORE_EXTENSION + 1;
> if (state == SOS)
>     return AVPROBE_SCORE_EXTENSION / 2;
>     
> return AVPROBE_SCORE_EXTENSION / 8;
> 
> 
> That doesn't make sense to me. All we read so far made sense for jpeg reader, it definitely looks like a jpeg image, but for some reason our confidence is AVPROBE_SCORE_EXTENSION _DIVIDED_ by 2 or 8 (in other words, "it would look more jpeg if it had .jpg extension).

These are all heuristics mostly a certain developer tuned and who
decided it'd make sense.

The problem with probing is that it uses a score system, which means
ambiguous formats get distinguished by the rather arbitrary score
across all the other probers, which makes this messy.

I'd say just send a patch.