なるほどわからん

$ wget https://solar.lowtechmagazine.com/dithers/sps_close.png
$ identify ./sps_close.png
$ identify -verbose ./sps_close.png
./sps_close.png PNG 800x533 800x533+0+0 8-bit sRGB 16c 43415B 0.000u 0:00.000
Image: ./sps_close.png
Format: PNG (Portable Network Graphics)
Mime type: image/png
Class: PseudoClass
Geometry: 800x533+0+0
Units: Undefined
Colorspace: sRGB
Type: Grayscale
Base type: Undefined
Endianess: Undefined
Depth: 8-bit
Channel depth:
red: 8-bit
green: 8-bit
blue: 8-bit
Channel statistics:
Pixels: 426400
Red:
min: 25 (0.0980392)
max: 250 (0.980392)
mean: 188.277 (0.73834)
standard deviation: 64.4151 (0.252608)
kurtosis: -0.438785
skewness: -0.844747
entropy: 0.907696
Green:
min: 25 (0.0980392)
max: 250 (0.980392)
mean: 188.277 (0.73834)
standard deviation: 64.4151 (0.252608)
kurtosis: -0.438785
skewness: -0.844747
entropy: 0.907696
Blue:
min: 25 (0.0980392)
max: 250 (0.980392)
mean: 188.277 (0.73834)
standard deviation: 64.4151 (0.252608)
kurtosis: -0.438785
skewness: -0.844747
entropy: 0.907696
Image statistics:
Overall:
min: 25 (0.0980392)
max: 250 (0.980392)
mean: 188.277 (0.73834)
standard deviation: 64.4151 (0.252608)
kurtosis: -0.438777
skewness: -0.844749
entropy: 0.907696
Colors: 6
Histogram:
11896: ( 25, 25, 25) #191919 srgb(25,25,25)
44611: ( 75, 75, 75) #4B4B4B srgb(75,75,75)
58764: (125,125,125) #7D7D7D grey49
78631: (175,175,175) #AFAFAF srgb(175,175,175)
103699: (225,225,225) #E1E1E1 srgb(225,225,225)
128799: (250,250,250) #FAFAFA grey98
Colormap entries: 16
Colormap:
0: ( 25, 25, 25) #191919 srgb(25,25,25)
1: ( 75, 75, 75) #4B4B4B srgb(75,75,75)
2: (125,125,125) #7D7D7D grey49
3: (175,175,175) #AFAFAF srgb(175,175,175)
4: (225,225,225) #E1E1E1 srgb(225,225,225)
5: (250,250,250) #FAFAFA grey98
6: ( 6, 6, 6) #060606 srgb(6,6,6)
7: ( 7, 7, 7) #070707 srgb(7,7,7)
8: ( 8, 8, 8) #080808 grey3
9: ( 9, 9, 9) #090909 srgb(9,9,9)
10: ( 10, 10, 10) #0A0A0A grey4
11: ( 11, 11, 11) #0B0B0B srgb(11,11,11)
12: ( 12, 12, 12) #0C0C0C srgb(12,12,12)
13: ( 13, 13, 13) #0D0D0D grey5
14: ( 14, 14, 14) #0E0E0E srgb(14,14,14)
15: ( 15, 15, 15) #0F0F0F grey6
Rendering intent: Perceptual
Gamma: 0.454545
Chromaticity:
red primary: (0.64,0.33)
green primary: (0.3,0.6)
blue primary: (0.15,0.06)
white point: (0.3127,0.329)
Background color: white
Border color: srgb(223,223,223)
Matte color: grey74
Transparent color: black
Interlace: None
Intensity: Undefined
Compose: Over
Page geometry: 800x533+0+0
Dispose: Undefined
Iterations: 0
Compression: Zip
Orientation: Undefined
Properties:
date:create: 2018-10-01T18:08:35+09:00
date:modify: 2018-10-01T12:55:39+09:00
png:IHDR.bit-depth-orig: 4
png:IHDR.bit_depth: 4
png:IHDR.color-type-orig: 3
png:IHDR.color_type: 3 (Indexed)
png:IHDR.interlace_method: 0 (Not interlaced)
png:IHDR.width,height: 800, 533
png:PLTE.number_colors: 16
png:sRGB: intent=0 (Perceptual Intent)
signature: 2186c5c608b90bfd2086fc9211ccd41ccc8c98ac65c0e01e25afc49e29961f6a
Artifacts:
filename: ./sps_close.png
verbose: true
Tainted: False
Filesize: 43415B
Number pixels: 426400
User time: 0.010u
Elapsed time: 0:01.000
Version: ImageMagick 6.9.10-8 Q16 x86_64 20180723 https://www.imagemagick.org