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To change generated image quality use the following code:
add_filter('jpeg_quality', function($arg){return 80;});
If a problem with minifying – also using of plugins like https://wordpress.org/plugins/wp-smushit/ is recomended
For detecting image quality ImageMagick could be used:
# yum install ImageMagick.x86_64
# identify -verbose CA0002-1-204x160.jpg
Image: CA0002-1-204x160.jpg
Format: JPEG (Joint Photographic Experts Group JFIF format)
Class: DirectClass
Geometry: 204x160+0+0
Resolution: 72x72
Print size: 2.83333x2.22222
Units: PixelsPerInch
Type: TrueColor
Endianess: Undefined
Colorspace: RGB
Depth: 8-bit
Channel depth:
red: 8-bit
green: 8-bit
blue: 8-bit
Channel statistics:
Red:
min: 14 (0.054902)
max: 255 (1)
mean: 218.905 (0.858452)
standard deviation: 49.2991 (0.19333)
kurtosis: -0.115958
skewness: -1.00308
Green:
min: 13 (0.0509804)
max: 255 (1)
mean: 218.695 (0.857629)
standard deviation: 49.4865 (0.194065)
kurtosis: -0.142898
skewness: -0.993778
Blue:
min: 11 (0.0431373)
max: 255 (1)
mean: 219.786 (0.861904)
standard deviation: 47.917 (0.18791)
kurtosis: -0.132297
skewness: -0.988068
Image statistics:
Overall:
min: 11 (0.0431373)
max: 255 (1)
mean: 219.129 (0.859328)
standard deviation: 48.9059 (0.191788)
kurtosis: -0.12572
skewness: -0.996289
Rendering intent: Undefined
Interlace: None
Background color: white
Border color: rgb(223,223,223)
Matte color: grey74
Transparent color: black
Compose: Over
Page geometry: 204x160+0+0
Dispose: Undefined
Iterations: 0
Compression: JPEG
Quality: 90
Orientation: Undefined
Properties:
date:create: 2015-11-08T02:29:20+02:00
date:modify: 2015-11-08T02:29:20+02:00
jpeg:colorspace: 2
jpeg:sampling-factor: 1x1,1x1,1x1
signature: a940ce5a3f82fc8ae13a1eec334389466c2052dd14384b761646c12d1bda8f0c
Artifacts:
verbose: true
Tainted: False
Filesize: 5.63KBB
Number pixels: 32.6KB
Pixels per second: 326.4TB
User time: 0.000u
Elapsed time: 0:01.000
Version: ImageMagick 6.7.2-7 2015-07-23 Q16 http://www.imagemagick.org
Quality: 90 is the answer.