How to simulate CMYK decoded as RGB?

[arbingordon]

Bit of an odd request, I know, but how would I simulate CMYK incorrectly decoded as sRGB while staying in the sRGB colorspace?

I'll try and post a sample later today if that isn't clear enough
Thank you ^^

[snibgo]

I don't understand. CMYK has 4 channels. sRGB has 3 channels. So what does "incorrectly decoded" mean?

In IM, you could read the CMYK file, separate, delete a channel, combine, then "-set colorspace sRGB". This might do what you want.

[arbingordon]

Yea, I had a feeling that might be unclear.
For example:
PNG Source
https://www.dropbox.com/s/xz7310y6a4met ... a.png?dl=0

PNG Source -> CMYK JPEG
https://www.dropbox.com/s/z53m2r9zprytqpt/cmyk.jpg?dl=0

PNG Source -> CMYK JPEG, incorrectly decoded with ms paint, resaved as PNG
https://www.dropbox.com/s/ml0shmsoiqpbs ... d.png?dl=0

I'm not sure how things are getting mangled, but I expect it's something along the lines of CMY being remapped into RGB (ie C->B, M->R, Y->G), not entirely sure though.

I'll keep playing around with -set colorspace to see if I can get anything similar.

Thanks

[snibgo]

Your third link doesn't look mangled. It's a slightly tweaked version of the Lenna image, somewhat desaturated.

[fmw42]

I think the main difference between your two results is that the first one has an "sRGB IEC61966-2-1 black scaled" profile and the one you think is wrong has no profile. Look at the results from identify -verbose yourimage. Profiles can make a difference in the way it looks.

The data is slightly different (2.7%) as shown by compare

Code: Select all

compare -metric rmse cmyk.jpg cmyk_incorrectly_decoded.png null:

1761.91 (0.026885)

[arbingordon]

I think I'm still not being clear here.
Regarding the "two results", I'm not sure that's very accurate. Like I said, cmyk.jpg was derived by just converting from the PNG source using imagemagick. I understand that the wrong one has no profile - presumably because ms paint and other non-compliant decoders don't handle the profile correctly. Interesting to note however, that among the 3 images I linked, none report having a profile per say ( https://pastebin.com/tTTQrc7c )- unless by profile you meant colorspace field under the verbose output, but even that is sRGB, and not the fully qualified "sRGB IEC61966-2-1 black scaled" colorspace. That has me interested however, maybe it would be possible to achieve what I'm going for by converting to CMYK and then forcibly setting the colorspace back to sRGB, although I didn't have any luck with that earlier.


As a sidenote, I made an 8bit colormap covering from #000000-#FFFFFF (easy enough to do in a few lines of python), then imported that as RGB, converted it to CMYK colorspace, and exported as lossless TIF. Then loaded that into paint and resaved as bitmap to have a (albeit large) mapping table to map from RGB to this sort of pseudo colorspace. With that mapping table, I can just iterate over each pixel in the source, and replace each pixel according to that table. Only problem doing this is, think it could be done faster if it was done purely in imagemagick. Is there a way to remap based on a color mapping file? (not an ICC profile, but off an existing image with specific characteristics.)


The initial colormap (#000000-#FFFFFF) https://www.dropbox.com/s/b8rwiny5rn0nl ... p.png?dl=0
The "error" colormap (not sure what else to call it) https://www.dropbox.com/s/pmeiges2ev5tv ... p.png?dl=0


Let me know your thoughts on the matter.

[fmw42]

According to ImageMagick identify -verbose, it has "sRGB IEC61966-2-1 black scaled" profile.

Code: Select all

Image: cmyk.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Mime type: image/jpeg
  Class: DirectClass
  Geometry: 512x512+0+0
  Units: Undefined
  Type: TrueColor
  Endianess: Undefined
  Colorspace: sRGB
  Depth: 8-bit
  Channel depth:
    red: 8-bit
    green: 8-bit
    blue: 8-bit
  Channel statistics:
    Pixels: 262144
    Red:
      min: 60 (0.235294)
      max: 255 (1)
      mean: 184.865 (0.724961)
      standard deviation: 40.2133 (0.157699)
      kurtosis: -0.786883
      skewness: -0.613539
      entropy: 0.937151
    Green:
      min: 0 (0)
      max: 246 (0.964706)
      mean: 117.807 (0.46199)
      standard deviation: 48.613 (0.190639)
      kurtosis: -0.701391
      skewness: -0.101691
      entropy: 0.954101
    Blue:
      min: 11 (0.0431373)
      max: 218 (0.854902)
      mean: 104.063 (0.40809)
      standard deviation: 35.061 (0.137494)
      kurtosis: -0.288041
      skewness: 0.484799
      entropy: 0.919093
  Image statistics:
    Overall:
      min: 0 (0)
      max: 255 (1)
      mean: 119.421 (0.468319)
      standard deviation: 54.6132 (0.214169)
      kurtosis: -0.923633
      skewness: 0.100305
      entropy: 0.936782
  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: 512x512+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: JPEG
  Quality: 80
  Orientation: Undefined
  Properties:
    date:create: 2017-07-20T20:27:21-07:00
    date:modify: 2017-07-20T20:27:21-07:00
    icc:copyright: Dropbox, Inc.
    icc:description: sRGB IEC61966-2-1 black scaled     <------ see profile here
    icc:manufacturer: sRGB IEC61966-2-1 black scaled
    icc:model: sRGB IEC61966-2-1 black scaled
    jpeg:colorspace: 2
    jpeg:sampling-factor: 2x2,1x1,1x1
    signature: abb5d0d696e90ff59ec4db53fd5ecec59d4b02823878708aff7367a4b32452c7
  Profiles:
    Profile-icc: 564 bytes
  Artifacts:
    filename: cmyk.jpg
    verbose: true
  Tainted: False
  Filesize: 41679B
  Number pixels: 262144
  Pixels per second: 26.2144MB
  User time: 0.010u
  Elapsed time: 0:01.009
  Version: ImageMagick 6.9.9-0 Q16 x86_64 2017-07-16 http://www.imagemagick.org

For remapping according to a colortable image see -remap at http://www.imagemagick.org/Usage/quantize/#remap and use +dither or -dither none.

[arbingordon]

As you can see from the pastebin earlier, identify wasn't reporting any of that information. I guess it has to do with me being a few months out of date on my installed copy of imagemagick (ImageMagick 6.9.8-0 Q16 x64 2017-03-11).
I'll try updating my 7.X install (trying to keep 6.X where it's at to avoid breaking python wand bindings), and see if that gives me the additional profile info.

[fmw42]

check your libpng and update it to the latest.

convert -list format

PNG* PNG rw- Portable Network Graphics (libpng 1.6.29,1.6.30)

Not sure why listing is showing two versions. MacPorts only has one listed as active.

libpng @1.6.29_0
libpng @1.6.30_0 (active)

This is weird. I have not seen that before.

[arbingordon]

I updated to ImageMagick 7.0.6-1 Q16 x64 2017-07-15, but it's interesting that I cannot reproduce your verbose options.
The verbose output is as follows:

Code: Select all

Image: cmyk.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Mime type: image/jpeg
  Class: DirectClass
  Geometry: 512x512+0+0
  Units: Undefined
  Type: ColorSeparation
  Endianess: Undefined
  Colorspace: CMYK
  Depth: 8-bit
  Channel depth:
    Cyan: 8-bit
    Magenta: 8-bit
    Yellow: 8-bit
    Black: 8-bit
  Channel statistics:
    Pixels: 262144
    Cyan:
      min: 0 (0)
      max: 72 (0.282353)
      mean: 1.79726 (0.00704806)
      standard deviation: 3.72136 (0.0145936)
      kurtosis: 47.416
      skewness: 5.43316
      entropy: 0.399016
    Magenta:
      min: 0 (0)
      max: 253 (0.992157)
      mean: 125.014 (0.490251)
      standard deviation: 47.6473 (0.186852)
      kurtosis: -0.652454
      skewness: -0.148786
      entropy: 0.944862
    Yellow:
      min: 0 (0)
      max: 251 (0.984314)
      mean: 102.496 (0.401944)
      standard deviation: 37.1756 (0.145787)
      kurtosis: -0.059145
      skewness: -0.787878
      entropy: 0.923729
    Black:
      min: 0 (0)
      max: 200 (0.784314)
      mean: 74.5686 (0.292426)
      standard deviation: 48.9337 (0.191897)
      kurtosis: -0.757828
      skewness: 0.713265
      entropy: 0.94823
  Image statistics:
    Overall:
      min: 0 (0)
      max: 253 (0.992157)
      mean: 75.9689 (0.297917)
      standard deviation: 60.5663 (0.237515)
      kurtosis: -1.17537
      skewness: 0.210133
      entropy: 0.803959
  Total ink density: 232.157%
  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)
  Matte color: grey74
  Background color: white
  Border color: srgb(223,223,223)
  Transparent color: none
  Interlace: None
  Intensity: Undefined
  Compose: Over
  Page geometry: 512x512+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: JPEG
  Quality: 92
  Orientation: Undefined
  Properties:
    date:create: 2017-07-21T02:09:04-04:00
    date:modify: 2017-07-21T02:09:04-04:00
    jpeg:colorspace: 4
    jpeg:sampling-factor: 1x1,1x1,1x1,1x1
    signature: 2f2a7182c3f2f90d774e99f66475a1e3af04007d49c1cc05507865b700eb6592
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 195767B
  Number pixels: 262144
  Pixels per second: 37.4493MB
  User time: 0.016u
  Elapsed time: 0:01.006
  Version: ImageMagick 7.0.6-1 Q16 x64 2017-07-15 http://www.imagemagick.org

As for the libpng version:

Code: Select all

      PNG* PNG       rw-   Portable Network Graphics (libpng 1.6.29)
           See http://www.libpng.org/ for details about the PNG format.
    PNG00* PNG       rw-   PNG inheriting bit-depth, color-type from original, if possible
    PNG24* PNG       rw-   opaque or binary transparent 24-bit RGB (zlib 1.2.9)
    PNG32* PNG       rw-   opaque or transparent 32-bit RGBA
    PNG48* PNG       rw-   opaque or binary transparent 48-bit RGB
    PNG64* PNG       rw-   opaque or transparent 64-bit RGBA
     PNG8* PNG       rw-   8-bit indexed with optional binary transparency

I'm using the prebuilt windows builds by the way, not sure if you're assuming that I've built from source or anything.

[arbingordon]

Can't believe I didn't read your post more clearly - yea it looks like dropbox is fudging things. Depending on how you save the image from their site, it will add the profile or give you the original untouched source. Here's a zip with the original files, hopefully dropbox can't malform them then
https://www.dropbox.com/s/vp7pzgyt6vb8vd3/cmyk.zip?dl=0

[fmw42]

Your two files are quite different. The "correct" one is CMYK. The "bad" one is sRGB.

Code: Select all

Image: cmyk.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Mime type: image/jpeg
  Class: DirectClass
  Geometry: 512x512+0+0
  Units: Undefined
  Type: ColorSeparation
  Endianess: Undefined
  Colorspace: CMYK
  Depth: 8-bit
  Channel depth:
    cyan: 8-bit
    magenta: 8-bit
    yellow: 8-bit
    black: 8-bit
  Channel statistics:
    Pixels: 262144
    Cyan:
      min: 0 (0)
      max: 72 (0.282353)
      mean: 1.79731 (0.00704826)
      standard deviation: 3.72137 (0.0145936)
      kurtosis: 47.4156
      skewness: 5.43311
      entropy: 0.399022
    Magenta:
      min: 0 (0)
      max: 253 (0.992157)
      mean: 125.014 (0.49025)
      standard deviation: 47.6472 (0.186852)
      kurtosis: -0.652458
      skewness: -0.148792
      entropy: 0.944862
    Yellow:
      min: 0 (0)
      max: 251 (0.984314)
      mean: 102.496 (0.401944)
      standard deviation: 37.1756 (0.145787)
      kurtosis: -0.0591612
      skewness: -0.787875
      entropy: 0.92373
    Black:
      min: 0 (0)
      max: 200 (0.784314)
      mean: 74.5686 (0.292426)
      standard deviation: 48.9337 (0.191897)
      kurtosis: -0.757827
      skewness: 0.713265
      entropy: 0.948229
  Image statistics:
    Overall:
      min: 2 (0.00784314)
      max: 255 (1)
      mean: 179.031 (0.702083)
      standard deviation: 60.5663 (0.237515)
      kurtosis: -1.17538
      skewness: 0.210131
      entropy: 0.803961
  Total ink density: 232.157%
  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: cmyk(255,255,255,0)
  Border color: cmyk(223,223,223,0)
  Matte color: cmyk(189,189,189,0)
  Transparent color: cmyk(0,0,0,0)
  Interlace: None
  Intensity: Undefined
  Compose: Over
  Page geometry: 512x512+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: JPEG
  Quality: 92
  Orientation: Undefined
  Properties:
    date:create: 2017-07-20T23:28:13-07:00
    date:modify: 2017-07-19T07:51:40-07:00
    jpeg:colorspace: 4
    jpeg:sampling-factor: 1x1,1x1,1x1,1x1
    signature: 709c112052f908e0223ef07da691c67349a277791cf29133b6250f1167491a6d
  Artifacts:
    filename: cmyk.jpg
    verbose: true
  Tainted: False
  Filesize: 195767B
  Number pixels: 262144
  Pixels per second: 26.2144MB
  User time: 0.010u
  Elapsed time: 0:01.009
  Version: ImageMagick 6.9.9-0 Q16 x86_64 2017-07-16 http://www.imagemagick.org

Code: Select all

Image: cmyk_incorrectly_decoded.png
  Format: PNG (Portable Network Graphics)
  Mime type: image/png
  Class: DirectClass
  Geometry: 512x512+0+0
  Resolution: 37.8x37.8
  Print size: 13.545x13.545
  Units: PixelsPerCentimeter
  Type: TrueColorAlpha
  Endianess: Undefined
  Colorspace: sRGB
  Depth: 8-bit
  Channel depth:
    red: 8-bit
    green: 8-bit
    blue: 8-bit
    alpha: 1-bit
  Channel statistics:
    Pixels: 262144
    Red:
      min: 75 (0.294118)
      max: 255 (1)
      mean: 181.326 (0.711081)
      standard deviation: 40.4909 (0.158788)
      kurtosis: -0.822027
      skewness: -0.546814
      entropy: 0.949539
    Green:
      min: 29 (0.113725)
      max: 249 (0.976471)
      mean: 119.69 (0.469373)
      standard deviation: 43.0388 (0.16878)
      kurtosis: -0.672884
      skewness: 0.132667
      entropy: 0.951982
    Blue:
      min: 34 (0.133333)
      max: 221 (0.866667)
      mean: 102.647 (0.402539)
      standard deviation: 31.3499 (0.122941)
      kurtosis: -0.0195183
      skewness: 0.695498
      entropy: 0.915276
    Alpha:
      min: 255 (1)
      max: 255 (1)
      mean: 255 (1)
      standard deviation: 0 (0)
      kurtosis: -3
      skewness: 0
      entropy: nan
  Image statistics:
    Overall:
      min: 0 (0)
      max: 255 (1)
      mean: 154.084 (0.604252)
      standard deviation: 73.2801 (0.287373)
      kurtosis: -1.09992
      skewness: -0.000353022
      entropy: nan
  Rendering intent: Perceptual
  Gamma: 0.45455
  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: 512x512+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: Zip
  Orientation: Undefined
  Properties:
    date:create: 2017-07-20T23:28:13-07:00
    date:modify: 2017-07-19T07:54:36-07:00
    png:bKGD: chunk was found (see Background color, above)
    png:cHRM: chunk was found (see Chromaticity, above)
    png:gAMA: gamma=0.45454544 (See Gamma, above)
    png:IHDR.bit-depth-orig: 8
    png:IHDR.bit_depth: 8
    png:IHDR.color-type-orig: 6
    png:IHDR.color_type: 6 (RGBA)
    png:IHDR.interlace_method: 0 (Not interlaced)
    png:IHDR.width,height: 512, 512
    png:pHYs: x_res=3780, y_res=3780, units=1
    png:sRGB: intent=0 (Perceptual Intent)
    png:text: 2 tEXt/zTXt/iTXt chunks were found
    signature: 1753dbe34ebca0d24802b4e63474369496ef8699fdb46ddbbbe11a7344600575
  Artifacts:
    filename: cmyk_incorrectly_decoded.png
    verbose: true
  Tainted: False
  Filesize: 496175B
  Number pixels: 262144
  Pixels per second: 26.2144MB
  User time: 0.010u
  Elapsed time: 0:01.009
  Version: ImageMagick 6.9.9-0 Q16 x86_64 2017-07-16 http://www.imagemagick.org

I do not understand what you are trying to do and why? What was the ImageMagick command used to make the "bad" one? What did you do to create the "good" one?

[arbingordon]

I do not understand what you are trying to do and why?

Like it says in the title, I'm trying to simulate CMYK colorspace images being (incorrectly) decoded as RGB (well technically sRGB) by non-compliant software. (eg, ms paint, windows photo viewer, don't know of any non-compliant decoders that are cross platform yet.
I understand that this is an unconventional request.

What was the ImageMagick command used to make the "bad" one?

convert Lenna.png -colorspace CMYK cmyk.jpg, then opened in ms paint, and saved as PNG.
(the latter part is what I am trying to simulate, outdated non-compliant software)

What did you do to create the "good" one?

convert Lenna.png -colorspace CMYK cmyk.jpg

[snibgo]

If it was misinterpreting CMYK as sRGB, you might get something like this:

Code: Select all

convert lena.png -colorspace CMYK -separate -delete 3 -set colorspace sRGB -combine -quality 40 bad_lena.jpg

[fmw42]

PNG does not support CMYK. What your software does to "treat" it as sRGB is unknown. Perhaps it just drops the K channel and has CMY put into the RGB channels or perhaps negates them first.

But what I don't understand is why you want to recreate such bad behavior? Why not just convert properly?