Converting single RGB color value to CMYK with ICC profiles

[lehik]

I'm sure this has been asked before, but I couldn't find a solution from earlier threads, so here goes: I'm developing a PHP application that deals with color values incoming in RGB and the app should convert them to CMYK. The PHP runs ImageMagick convert command with exec() -function. I've used ImageMagick for some basic image crop/resize -stuff, but I feel that my understanding is a bit lacking here...

For example, let's say that I have RGB color #dfd3c1 which equals 223,211,193. Based on what I found from other threads, I managed to build something like this:

convert xc:"rgb(223,211,193)" -profile sRGB.icc -profile ISOnewspaper26v4.icc -format "%[fx:int(100*c)], %[fx:int(100*m)], %[fx:int(100*y)], %[fx:int(100*k)]\r\n":

The output in percentages should be something like 3,6,16,14, but I'm getting 0.894179%, 5.71908%, 13.8964%, 2.82902%

The questions:
1) What's wrong with the command? Why the values aren't what they _should_ be?
2) In PHP, how can I just "echo" out the value to store the output from the exec to an variable? I managed to make the convert command to write the output to a file, but it would be much easier to have it output as string which I could store to a variable.

[snibgo]

You need "info:" at the end. That's probably just a copy-paste typo.

I get slightly different values to you.

Code: Select all

convert xc:"rgb(223,211,193)" -profile sRGB.icc -profile ISOnewspaper26v4.icc -format "%[fx:(100*r)], %[fx:(100*g)], %[fx:(100*b)], %[fx:(100*k)]\r\n" info:

0.968948, 6.12955, 13.9956, 2.80156

Perhaps this is a different version of the ICC.

A plain CMYK conversion, without prfiles, is:

Code: Select all

convert xc:"rgb(223,211,193)" -colorspace CMYK -format "%[fx:(100*r)], %[fx:(100*g)], %[fx:(100*b)], %[fx:(100*k)]\r\n" info:

0, 5.38186, 13.4524, 12.549

This is more or less the same, but with a greater K value.

The output in percentages should be something like 3,6,16,14

Where did those numbers come from?

I don't know PHP, sorry. I think you can send stdout to an array, one (string) element per line of text.

[lehik]

Thanks a lot for such a quick reply!

The missing "info" on my example was indeed a copy-paste error. Also, I managed to get the output to PHP as desired.

The plain conversion works as expected, but when I try to use profiles, I get just 0,0,0,0 so there is something fishy going on. Maybe the path to the profile is not correct, I'll have to double check that one.

In my previous message, the numbers 3,6,16,14 were supposed to be CMYK percent values, sorry for causing confusion by leaving the % sign out. I got the values from a graphic designer who said that those should be the CMYK values for RGB 223,211,193 when using the profiles.