I made a small image using imagemagick, this image has only 2 colors (alpha + some other color) so I want to turn in into index-color mode.
Once I finish the image, I run the command
convert file.png +dither -colors 2 file.png
The resulting image is 169 bytes. Now when I open the image with gimp, I make no change, I just export it as .png, over the old image, on the window that says "Export image as PNG" I untick everything, and keep compression level at 9.
The resulting image is 109 bytes. What makes this difference ? At first I thought it would be meta-data or something like that, I tried from imagemagick to use the -strip command, but it kept the image at 169 bytes. So, it it because of the compression algorithm? What compression algorithm does gimp use ? How can I replicate that with imagemagick?
The reason I want to do it with imagemagick is so I can automate the process, as I plan to do this on 100+ images.
EDIT:
Output of identify -verbose imagemagickfile.png
Image: ic_stage.png
Format: PNG (Portable Network Graphics)
Mime type: image/png
Class: DirectClass
Geometry: 176x176+0+0
Units: Undefined
Type: PaletteAlpha
Endianess: Undefined
Colorspace: sRGB
Depth: 8-bit
Channel depth:
red: 4-bit
green: 8-bit
blue: 1-bit
alpha: 1-bit
Channel statistics:
Pixels: 30976
Red:
min: 0 (0)
max: 102 (0.4)
mean: 60.0126 (0.235343)
standard deviation: 50.1973 (0.196852)
kurtosis: -1.87106
skewness: -0.359086
entropy: 0.977354
Green:
min: 0 (0)
max: 150 (0.588235)
mean: 88.2538 (0.346093)
standard deviation: 73.8196 (0.289489)
kurtosis: -1.87106
skewness: -0.359086
entropy: 0.977354
Blue:
min: 0 (0)
max: 255 (1)
mean: 150.031 (0.588359)
standard deviation: 125.493 (0.492131)
kurtosis: -1.87106
skewness: -0.359086
entropy: 0.977354
Alpha:
min: 0 (0)
max: 255 (1)
mean: 150.031 (0.588359)
standard deviation: 125.493 (0.492131)
kurtosis: -1.87106
skewness: 0.359086
entropy: 0.977354
Image statistics:
Overall:
min: 0 (0)
max: 255 (1)
mean: 100.817 (0.395359)
standard deviation: 99.3306 (0.389532)
kurtosis: -1.05614
skewness: 0.476254
entropy: 0.977354
Alpha: none #00000000
Colors: 2
Histogram:
12751: ( 0, 0, 0, 0) #00000000 none
18225: (102,150,255,255) #6696FFFF srgba(102,150,255,1)
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: srgba(255,255,255,1)
Border color: srgba(223,223,223,1)
Matte color: grey74
Transparent color: none
Interlace: None
Intensity: Undefined
Compose: Over
Page geometry: 176x176+0+0
Dispose: Undefined
Iterations: 0
Compression: Zip
Orientation: Undefined
Properties:
date:create: 2015-10-26T11:27:21+02:00
date:modify: 2015-10-26T11:27:21+02:00
png:bKGD: chunk was found (see Background color, above)
png:cHRM: chunk was found (see Chromaticity, above)
png:IHDR.bit-depth-orig: 2
png:IHDR.bit_depth: 2
png:IHDR.color-type-orig: 3
png:IHDR.color_type: 3 (Indexed)
png:IHDR.interlace_method: 0 (Not interlaced)
png:IHDR.width,height: 176, 176
png:PLTE.number_colors: 3
png:sRGB: intent=0 (Perceptual Intent)
png:tRNS: chunk was found
signature: 219cc9a10e56ed2940dc1d92e37bec98d49d12c1bd5f2adfd2bcc91fd7b56f85
Artifacts:
filename: ic_stage.png
verbose: true
Tainted: False
Filesize: 218B
Number pixels: 31K
Pixels per second: 0B
User time: 0.000u
Elapsed time: 0:01.000
Version: ImageMagick 6.9.2-0 Q16 x86_64 2015-08-18 http://www.imagemagick.org
Output of identify -verbose gimpfile.png
Image: ic_stage_2.png
Format: PNG (Portable Network Graphics)
Mime type: image/png
Class: DirectClass
Geometry: 176x176+0+0
Units: Undefined
Type: PaletteAlpha
Endianess: Undefined
Colorspace: sRGB
Depth: 8-bit
Channel depth:
red: 4-bit
green: 8-bit
blue: 1-bit
alpha: 1-bit
Channel statistics:
Pixels: 30976
Red:
min: 102 (0.4)
max: 255 (1)
mean: 164.981 (0.646985)
standard deviation: 75.296 (0.295278)
kurtosis: -1.87106
skewness: 0.359086
entropy: 0.977354
Green:
min: 150 (0.588235)
max: 255 (1)
mean: 193.222 (0.757735)
standard deviation: 51.6737 (0.202642)
kurtosis: -1.87106
skewness: 0.359086
entropy: 0.977354
Blue:
min: 255 (1)
max: 255 (1)
mean: 255 (1)
standard deviation: 0 (0)
kurtosis: 0
skewness: 0
entropy: -nan
Alpha:
min: 0 (0)
max: 255 (1)
mean: 150.031 (0.588359)
standard deviation: 125.493 (0.492131)
kurtosis: -1.87106
skewness: 0.359086
entropy: 0.977354
Image statistics:
Overall:
min: 0 (0)
max: 255 (1)
mean: 179.543 (0.70409)
standard deviation: 77.6019 (0.304321)
kurtosis: 1.86389
skewness: -1.46287
entropy: -nan
Alpha: srgba(255,255,255,0) #FFFFFF00
Colors: 2
Histogram:
18225: (102,150,255,255) #6696FFFF srgba(102,150,255,1)
12751: (255,255,255, 0) #FFFFFF00 srgba(255,255,255,0)
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: srgba(223,223,223,1)
Matte color: grey74
Transparent color: none
Interlace: None
Intensity: Undefined
Compose: Over
Page geometry: 176x176+0+0
Dispose: Undefined
Iterations: 0
Compression: Zip
Orientation: Undefined
Properties:
date:create: 2015-10-26T11:27:52+02:00
date:modify: 2015-10-26T11:27:52+02:00
png:IHDR.bit-depth-orig: 1
png:IHDR.bit_depth: 1
png:IHDR.color-type-orig: 3
png:IHDR.color_type: 3 (Indexed)
png:IHDR.interlace_method: 0 (Not interlaced)
png:IHDR.width,height: 176, 176
png:PLTE.number_colors: 2
png:sRGB: intent=0 (Perceptual Intent)
png:tRNS: chunk was found
signature: a22844c38c7ef3a612d94c2d3b9d1be29bb9d5e2f897f87c92947946fe6bc868
Artifacts:
filename: ic_stage_2.png
verbose: true
Tainted: False
Filesize: 138B
Number pixels: 31K
Pixels per second: 30.976GB
User time: 0.000u
Elapsed time: 0:01.000
Version: ImageMagick 6.9.2-0 Q16 x86_64 2015-08-18 http://www.imagemagick.org
