[FFmpeg-devel] [PATCH 2/2] img2dec: Add mime_type to image formats

[Derek Buitenhuis]

On 2/26/2016 2:44 PM, Carl Eugen Hoyos wrote:
> Derek Buitenhuis <derek.buitenhuis <at> gmail.com> writes:
> 
>> On 2/25/2016 4:30 PM, Carl Eugen Hoyos wrote:
> You misunderstand:
> Most "imageauto" demuxers return a score of 51. In all cases 
> when 51 is returned, the detection is nearly certain. Your 
> patch disables the detection for all these cases (although 
> it is nearly certain) in favour of the mime type.
> 
> (That is how I understand the patch, please correct me if 
> setting mime_type actually has another effect than 
> returning 75 in case of conformance.)
> 
> I don't think the solution is to increase the score for 
> these probe functions: mpeg streams may start with any 
> bytes and 32 bit accordance should not return maximum 
> (or nearly maximum) score.

See below. The scoring should be fixed.

> My argument is: Let's please improve the jpeg auto-detection 
> (the only one that shows issues) instead of disabling it.

Forgot what this particular patch does for a second.

I'm not saying it should be disabled. I'm saying scoring should be
as such:

1) Probing the file file itself: Highest score.
2) Mime-type: Medium score.
3) File extension: Lower score.

Mime-type is a *much* better mechanism for detection than file
extension over HTTP(S), and it will not affect detection in local
files at all. Mime-type should be tried before e.g. mp3 probing,
which seems to have a lot of misdirections.

This can only be a net improvement, IMO.

> (You should be slightly more careful with your arguments: 
> I originally thought that there might be one or two probe 
> functions that should sometimes return a score instead of 
> 0 because the decoders may accept invalid files but that 
> is not the case except for the case without mime_type.)
> 
> There are 14 "imageauto" demuxers with image type 
> auto-detection. Three (png, dds, webp) return a score of 99, 
> I don't know if they would be effected by the patch in 
> question or not but all three decoders do not work if the 
> respective probe function fails.
> 
> The qdraw decoder fails for samples that fail auto-detection.
> 
> Of the nine image format types with probe functions that 
> return 51, the following fail decoding if the probe 
> function returns 0:
> bmp, dpx, exr, j2k, sgi, sunrast, tiff
> 
> The pictor decoder fails for images that are not 
> auto-detected.
> 
> That leaves only the jpegls autodetection that may not 
> return a score for images that can be decoded: Since the 
> patch you provided does not add a mime_type for jpegls, 
> I don't think it can be used as an argument here.
> 
> So let's improve the jpeg auto-detection, I know it isn't 
> perfect yet.

See above. It's not meant to replace, it's meant to augment.

> I am sorry if I misunderstand this (who does "the push"?):
> Are you arguing user-set mime_types are (except for jpeg) 
> more plausible than an autodetection that never returns 0 
> for images that can be decoded?

Again, see above where I have laid out the scoring I think
would be appropriate.

However, mime-type is far, far more likely to be accurate
than the file extension. I am arguing that it's better than
that, for URLs.

- Derek