output from `-quality 75` doesn't match default, no flag output.

[olizilla]

usage.txt suggests the default quality is 75 https://github.com/mozilla/mozjpeg/blob/master/usage.txt#L73

but running cjpeg -quality 75 foo.jpg > foo.q75.jpg produces different output to cjpeg foo.jpg > foo.default.jpg

What else does setting the quality flag change in the output?

Also... when I specify the quality flag, then gm identify verbose it doesn't match what I passed to cjpeg...

$ cjpeg -quality 75 master.732.jpg > master.732.q75.jpg
$ gm identify -verbose master.732.q75.jpg | grep Quality
  JPEG-Quality: 40

But running it without a quality flag does indeed produce images that gm thinks are at quality 75

$ gm identify -verbose master.732.default.jpg | grep Quality
  JPEG-Quality: 75

so I'm a bit confused. Can anyone explain how the quality flag works?

[kornelski]

JPEG doesn't store quality as a single number. GraphicsMagic/ImageMagick's identify function does a trick that is based on assumption that the stock libjpeg was used, and it's not compatible with other quantization tables.

That quality output is not related to actual, human-perceived quality.

I suggest opening bug against gm/ImageMagick and ask them to tune the identify function to support other quantization tables.

[olizilla]

Thanks! I guess my real question is does specifying the -quality flag change anything else? If 75 is the default then my assumption was that specifying -quality 75 is the same as not specifying it.

[olizilla]

I'm just learning, and trying to get my head around this:

cjpeg pug.jpg -quality 75 pug.mozjpeg.q75.jpg
cjpeg pug.jpg > pug.mozjpeg.default.jpg
diff pug.mozjpeg.default.jpg pug.mozjpeg.q75.jpg
Binary files pug.mozjpeg.default.jpg and pug.mozjpeg.q75.jpg differ

I've stripped the exif data, but they still differ. Does mozjpeg use the -quality to change some other settings, or am i just misunderstanding how it works?

[kornelski]

Oh, it may be a bug.

In cjpeg when you use -quality, then set_quality_ratings() is called and it calls a LOCAL jpeg_default_qtables(), which sets quantization tables based on the new JINT_BASE_QUANT_TBL_IDX option.

Otherwise jpeg_set_defaults() is used, which calls jpeg_set_quality(cinfo, 75, TRUE); and jpeg_set_linear_quality, which relies on cinfo->master->quant_tbl_master_idx being set previously.

[olizilla]

@pornel many thanks for the explanation! Is there a flag to encourage mozjpeg to use the new quantization tables even when the -quality flag is used?

[kornelski]

When you use -quality you get the new quantization tables. It's only the default quality that also unintentionally defaults to the old tables.

[olizilla]

:+1: got it.

[bdaehlie]

@pornel This issue seems resolved but earlier you wrote "Oh, it may be a bug." Is there a bug here? If so, can you write out an explanation of what it is?

[kornelski]

If I remember correctly, the code that sets the default quality and code that sets quality explicitly have quite different implementations and they find qtable to use differently.

[bdaehlie]

Should they use the same implementation? Specifically, should we just change things so that default quality is set exactly the same as if the user had passed "-quality 75"?

[kornelski]

Yes, I think qtables should be selected the same way whether you set quality or not, and it'd be nice if there was a DRY implementation for it.

[bdaehlie]

@pornel Want to do this for v3.2?

[kornelski]

I can do it, but I can't promise I'll have time to do it for v3.2

[ValZapod]

You need to use cjpeg -quant-table 0 -quality 75 to match it. See https://github.com/mozilla/mozjpeg/issues/395.