[FFmpeg-devel] img2dec: jpeg_probe logic

[Vadim Kalinsky]

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).

Compare it with png, for example:

static int png_probe(AVProbeData *p)
{
    const uint8_t *b = p->buf;

    if (AV_RB64(b) == 0x89504e470d0a1a0a)
        return AVPROBE_SCORE_MAX - 1;
    return 0;
}

"Return max confidence, if it has proper signature, 0 otherwise"

Shouldn't it be something like this?

if (state == EOI)
    return AVPROBE_SCORE_EXTENSION + 3;
if (state == SOS)
    return AVPROBE_SCORE_EXTENSION + 2;
    
return AVPROBE_SCORE_EXTENSION + 1;


I'll send a patch if there're no objections.

Thanks!
V. Kalinsky