Why limit quality 84 by default?

[cyclonmaster-zz]

Please open it to any quality level. Just warn user is enough instead of redirect user to change the code in a manner of just to do testing. Since I do not has VS Studio Environment setup well yet, cannot compile the code. So far, the compression at 84 compare to libjpeg is versy imperssive. guetzli is much smaller and better quality. I just want to go lower and see the result.

[msbit]

I think it's partially due to the psychovisual model in use for the quanitisation; even if you remove the check at https://github.com/google/guetzli/blob/master/guetzli/processor.cc#L745, the minimum "quality" level is determined by the mapping at https://github.com/google/guetzli/blob/master/guetzli/quality.cc#L31.

At any rate, here are varying quality level encodings of the reference bees.png for you to check out:

70

75

80

85

90

95

[tomByrer]

Thanks @msbit; quite a bit of size differences, but seems not too much artifacts until 75%. 85% is a good default for this image.

[FossPrime]

Seems like pagespeed wants us to go down to 80, which with guetzli doesn't look half bad. Pagespeed is clearly using libjpeg or other. The minimum default for Google Guetzli should really be enough to satisfy Google Pagespeed/Search #206

[Redsandro]

Google (search) brought me here.

The limit is there for a reason I assume.

I removed the hardcoded limit just to test out lower qualities. I found that at lower quality (50 - 70) the guetzli result looks considerably worse in smooth/gradienty imagery than simple (imagemagick) jpgs. The warning did say "below 84 artefacts will be visible", so it was to be expected. I assume that whatever guetzli does, it works counter-effective at low quality rates.

However, to be fair, I did not expect the artefacts to be worse at low quality rates. It makes me wonder if the quality is actually better at the designed rate (85-100), because in all honesty, when the quality is this high, I find it hard to judge which one is better.

Assuming the quality is indeed better at same file size, I am curious what is effectively going on that causes the opposite to be true in low(er) qualities.