问题
More specifically, I'd like to accurately convert a CMYK value (probably from the ISO Coated v2 space) to an RGB value (probably from the sRGB space) on the Ruby platform (probably using ICC profiles).
ImageMagick seemed like a good place to start, but I've also heard that LittleCMS might have been ported/wrapped to work with Ruby.
Once again, I'm looking to convert single colors, NOT image files. Any ideas?
回答1:
In ImageMagick, you can do the following:
convert xc:"cmyk(0,255,255,0)" -colorspace sRGB -format "%[pixel:u.p{0,0}]\n" info:
red
convert xc:"cmyk(0,255,255,0)" -profile /Users/fred/images/profiles/USWebCoatedSWOP.icc -profile /Users/fred/images/profiles/sRGB.icc -format "%[pixel:u.p{0,0}]\n" info:
srgb(93%,11%,14%)
回答2:
Is there anything you can tweak in format to ensure more significant digits in the srgb(X%,X%,X%)
Likely due to different IM versions. IM 7.0.7.8 shows srgb(93.0648%,11.1254%,14.1741%). IM 6.9.9.20 shows integers. I tried adding -precision 4 to IM 6 command line, but still get integers. To get more precision, one has to parse the txt: output format.
For example without parsing:
convert xc:"cmyk(0,255,255,0)" -profile /Users/fred/images/profiles/USWebCoatedSWOP.icc -profile /Users/fred/images/profiles/sRGB.icc txt:
# ImageMagick pixel enumeration: 1,1,65535,srgb
0,0: (60990,7291,9289) #EE3E1C7B2449 srgb(93%,11%,14%)
So you need to parse the 16-bit values (for IM Q16) in parenthesis, namely, (60990,7291,9289)
vals=`convert xc:"cmyk(0,255,255,0)" \
-profile /Users/fred/images/profiles/USWebCoatedSWOP.icc \
-profile /Users/fred/images/profiles/sRGB.icc txt: |\
tail -n +2 | sed -n 's/^.*[(]\(.*\)[)][ ]*\#.*$/\1/p'`
red=`echo $vals | cut -d, -f1`
green=`echo $vals | cut -d, -f2`
blue=`echo $vals | cut -d, -f3`
red=`convert -precision 4 xc: -format "%[fx:100*$red/quantumrange]" info:`
green=`convert -precision 4 xc: -format "%[fx:100*$green/quantumrange]" info:`
blue=`convert -precision 4 xc: -format "%[fx:100*$blue/quantumrange]" info:`
color="srgb($red%,$green%,$blue%)"
echo "$color"
srgb(93.06%,11.13%,14.17%)
Adjust -precision, for the number of significant digits you want.
NOTE: In IM 7, -precision does work.
magick xc:"cmyk(0,255,255,0)" -profile /Users/fred/images/profiles/USWebCoatedSWOP.icc -profile /Users/fred/images/profiles/sRGB.icc -format "%[pixel:u.p{0,0}]\n" info:
srgb(93.0648%,11.1254%,14.1741%)
magick -precision 4 xc:"cmyk(0,255,255,0)" -profile /Users/fred/images/profiles/USWebCoatedSWOP.icc -profile /Users/fred/images/profiles/sRGB.icc -format "%[pixel:u.p{0,0}]\n" info:
srgb(93.06%,11.13%,14.17%)
magick -precision 2 xc:"cmyk(0,255,255,0)" -profile /Users/fred/images/profiles/USWebCoatedSWOP.icc -profile /Users/fred/images/profiles/sRGB.icc -format "%[pixel:u.p{0,0}]\n" info:
srgb(93%,11%,14%)
来源：https://stackoverflow.com/questions/47022566/converting-colors-not-images-with-imagemagick