National Oceanic and
Atmospheric Administration
United States Department of Commerce

CMYK format for postscript files

CMYK format for postscript files

Question:

I understand that ferret/Fprintoutputs postscript color files in RGB format. However, our new printeris set to print CMYK color, and some journals require CMYK color formatfor figures that are submitted. How can I get CMYK format?

Example:

Postscript files from Fprint lose color resolution or simply look bad when printed on a CMYK printer. For example, the old default rainbow palette we are used to seeing (on the left) might look like the right-hand panel below, with the blues and greens most affected.

cmyk_example1

 

Explanation:

RGB color is used by computer monitors, where color is created by adding red, green, and blue light. CMYK is used for printing on paper, a combination of cyan, magenta, yellow, and black. The RGB color space is capable of producing many more colors than the CMYK color space.

Solution:

If you are only concerned with the output of your local printer, then as a starting point, you should try to set up your printer to use RGB translation. This tells the printer to do the best that it can with an RGB specification.

You may still need to make postscript plots with palettes that work when translated to CMYK. We have made a number of palettes which, when printed on a CMYK printer, give a reasonable range of colors. If you use these palettes they will look odd on the screen but will work when printed on a printer that uses CMYK. These palettes will be included with the palettes distributed with Ferret, starting with version 5.41, or you can download them in this tar file. Put them in a directory that is pointed to by the environment variable FER_PALETTE.

cmyk_palettes.tar.Z contains the palettes listed below.

These palettes, created by Gabe Vecchi, should give you what you need to highlight the characteristics of your data. Note you can easily combine two or more of them to make your own new palette files.

 

exciting_cmyk.spk Good for showing variances/standard deviations, etc. Shows high values as bright red/orange/yellow colors, and low values as paste lblue/purples.
dynamic_cmyk.spk Like exciting_cmyk.spk, except drops to a lower level of pastel purple at the bottom
rainbow_cmyk.spk Like the Ferret default rainbow palette, but attempts to fix the blues/greens.
rainbow2_cmyk.spk Like rainbow_cmyk.spk. More purple less green.
rain_cmyk.spk Good for showing raininess. A palette with brown/yellows at negative,blues at positive to show raininess anomaly.
rain_hole_cmyk.spk Similar to rain_cmyk, but with a ~30% of the center missing as white,to show large raininess anomaly.
norm_cent_cmyk.spk Centered for currents, winds, anomalies and such. This one is light centered with reds on the positive
rev_cent_cmyk.spk Another centered palette, this time with blues on the positive.
blues_cmyk.spk
reds_cmyk.spk
greens_cmyk.spk		 Monochromatic palettes; these work best with few levels (8 or less).You can make new palettes by combining these, for example combine blues with inverted reds in a new .spk file.
sunny_cmyk.spk yellows/oranges
warm_cmyk.spk oranges/reds

 

If you have a specialized palette that you wish to convert to a "cmyk" palette,you will need to use a graphics program such as Adobe Illustrator. Here arethe steps:

  • Make a plot with the palette you want to convert. It should have a colorbar that shows all the colors you want.

  • Open the postscript file in Illustrator (or your graphics tool).

  • Select the colorbar, and remove everything else.

  • Translate to CMYK representation. The colors change to look as they will on paper.

  • Open up a palette of colors from the graphics tool. You will select the colors you want from this.

  • In your colorbar, select a color that needs to be changed. Find a new color from the graphics tool's palette. Replace the color in the colorbar with the new color.

  • Continue to substitute colors in the colorbar until it appears as you want it.

  • For each color in your new colorbar, get the RGB representation of it. Displaying the RGB equivalent should be an option in your graphics tool.

  • Put these RGB values in a new .spk file for Ferret to use, with the list of RGB colors. Note that Ferret uses a scale of 0 - 100 for the red, green, and blue values. If your tool shows value of 0-255 you need to rescale the numbers.

  • Now you can use this palette in Ferret, generate postscript plot files, and when they are printed on a CMYK printer, they will have the colors you chose.

Your input and suggestions are welcome, as well as contributions of any palettes you develop, or tools you find to make this process simpler!