Legacy ImageMagick Discussions Archive

I try to combine two CMYK based images into one image. The first image serves as background, the 2nd image as overlay. However, from the second image I only want to use pixels that have a certain pre-defined colour.

image1.jpg JPEG 7121x1434 7121x1434+0+0 8-bit CMYK 918KB 0.000u 0:00.000
image2.eps EPT 6008x298 6008x298+0+0 16-bit ColorSeparation CMYK 7.162MB 0.000u 0:00.000

from image2 I only want to use pixels with CMYK colour 60 30 20 80 for the overlay.

the images should be center aligned (-gravity center)

How can this be accomplished using imagemagick?

I think you want to composite the second image over the first, but only after making other colours transparent, like this:
The icc-color numbers are in the range 0.0 to 1.0. I'm not sure how you would specify 60 30 20 80 which I assume are out of 255.

I'm also unsure about the icc-colours & CMYK relation.

The second image has only two colours, CMYK 60 30 20 80 and white. Maybe this information helps solving the problem.

What I used sofar was:
But the colours CMYK of output.jpg seem to be slightly different from the image2.jpg, probably because it is the result of a pixel multiplication process of both images.

What version of IM, on what platform?

I suggest you don't use the "composite" command. Instead, use "convert" in v6 or "magick" in v7, with a "-composite" option.

Why are you multiplying pixels?

I'm using

Version: ImageMagick 6.9.7-4 Q16 x86_64 20170114 http://www.imagemagick.org

on

Ubuntu 18.10 LTS

I multiplied the pixels because optically it came close to what I was looking for. Image2 has only two colours white and CMYK 60 30 20 80 (which comes close to black)

If image2 has only white and some other colour, and you don't want the white copied, then... I write to tiff instead of jpg because tiff is by default lossless.

This command does the trick for me, everything in image 2 that is white (cmyk,0,0,0,0) is made transparant before using image2 as an overlay on top of image1:
Thank you for all your support!

PS:

Finding 1) I also found out that imagemagick reports the CMYK colours on a scale from 0 to 255. The CMYK colour I was using (60 30 20 80) translated to image magicks definition of CMYK as cmyk,153,76,51,204

To determine the colour of a certain pixel I used the following command:

convert image2.tiff -format '%[pixel:p{1,149}]' info:-;echo

with the following result: cmyk(153,76,51,204), the same pixel is reported in Photoshop as CMYK 60,30,20,80.

Finding 2) However converting the command like this (making everything transparant that has not the pre-defined cmyk colour) didn't work for some reason.