Preparation for offset printing - converting to CMYK

For commercial printing, you may need to provide an image in the CMYK color format, which describes the image in terms of Cyan, Magenta, Yellow and blacK ink, instead of the Red, Green and Blue pixels of computer screens and digital cameras. For inkjet printing at home you should not need this, even if similar colors are used.

Darktable doesn't offer a conversion to CMYK in itself. But free software provides a working solution by using the ImageMagick toolkit. A recipe for this:

1. Ask your printer for the color profile you should use for printing, and for the minimum highlight values and maximum ink coverage he recommends. In doubt, get a standard safe profile like ISO Coated v2 300% which is available for download at the ECI, and use a minimum of 5% for Cyan, Magenta and Yellow, a maximum of 85% for blacK, and don't exceed a maximum ink coverage of 300% in total.

2. You need to export the image from darktable. Choose "Adobe RGB compatible" as an output color profile, since that can transport some printable colors that are outside of the standard sRGB range. And use a lossless format like TIFF.

3. Convert the image to CMYK using convert from ImageMagick. Use the -profile option to specify the color profile that applies to your print job:

convert image-rgb.tif -profile ISOcoated_v2_300_eci.icc image-cmyk.tif

4. Use identify from ImageMagick with the -verbose option to check if your conversion is within the limits recommended by your printer. identify specifies the values twice, once in a range between 0 and 255 and second, in (), as a value between 0 and 1. Multiply this second value by 100 and you get a percent value as used in printing:

identify -verbose image-cmyk.tif

Image: image-cmyk.tif
  Channel statistics:
    Pixels: 10182900
      min: 0 (0)
      max: 224 (0.878431)
      mean: 71.7436 (0.281348)
      standard deviation: 41.0453 (0.160962)
      kurtosis: -1.15845
      skewness: 0.351658
      min: 0 (0)
      max: 255 (1)
      mean: 67.8804 (0.266198)
      standard deviation: 57.9146 (0.227116)
      kurtosis: -0.168102
      skewness: 0.765279
      min: 0 (0)
      max: 255 (1)
      mean: 109.522 (0.4295)
      standard deviation: 72.0618 (0.282595)
      kurtosis: -1.47142
      skewness: -0.175285
      min: 0 (0)
      max: 242 (0.94902)
      mean: 53.808 (0.211012)
      standard deviation: 50.9582 (0.199836)
      kurtosis: -0.845476
      skewness: 0.468255
  Image statistics:
      min: 0 (0)
      max: 255 (1)
      mean: 75.7386 (0.297014)
      standard deviation: 56.6314 (0.222084)
      kurtosis: -0.0961796
      skewness: 0.623551
  Total ink density: 300%

5. Correct the color levels as needed using convert. Use the -channel option to specify the color channel you wish to change and the options -evaluate Add and -evaluate Remove to modify. For Cyan, Magenta, Yellow, you need to apply all corrections similarly to all three channels to avoid color casts. In our example, we want to increase the minimum value for Cyan, Magenta and Yellow by 5%-points and decrease the maximum value for blacK by 10%-points. Be aware that the percent values are relating to the full color, not to the value already there: adding 10% to a value of 10% results in 20%, not 11%.

convert image-cmyk.tif -channel CMY -evaluate Add 5% -channel K -evaluate Subtract 10% image-cmyk-corrected.tif

6. Check your result visually: Loading the image into scribus is probably the most accurate preview available in free software.

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