Convert PDF to Tiff With CMYK and Transparency - pdf

Question

I’m attempting to convert from PDF to TIFF and maintain both the CMYK color space and the transparent areas in the PDF. There are many posts out there say “just use PNG” but for my application the CMYK color space is a must and PNG doesn’t support CMYK. I started working on this using Imagemagick (IM) and had limited success but had better results so far using Ghostscript directly (which is what IM uses under the hood anyway). The problem that continually occurs is that the transparent areas from the PDF are filled in with white.
Context:
Ubuntu 16.04.3 LTS (AWS), Ghostscript 9.23, ImageMagick 7.0.7-36
I created a test PDF that has a transparent background and overlapping blocks of solid and semi-transparent colors:
trans-test.pdf
I’ve verified that the PDF has the correct transparent areas. If I use PNG (and thus the RGB colorspace) I get the resulting image which has the matching transparent areas as the PDF but, alas, the wrong colorspace:
output.png
(remember: web browsers will show it with a white background but it’s really transparent)
For conversions I’ve tried variations of the following commands (simplified for posting):
Imagemagick:
convert -depth 8 -colorspace CMYK trans-test.pdf output.tif
Ghostscript:
gs -sDEVICE=tiff32nc -sOutputFile=output_gs.tif -r144 trans-test.pdf
In IM, the delegate for processing CMYK PDFs uses the pamcmyk32 device. Some suggest changing that to pngalpha but that forces an unwanted change to the RGB color space. Some suggest converting first to PNG then back to CMYK but that results in color data loss.
Using Ghostscript directly none of the device options for TIFF have any options for transparency. After digging around for a while I found an old post reply from a dev at Artifex (KenS) stating that “As for TIFF, there is no support in GS for making the unmarked areas transparent”.:
Conversion...does not maintain transparency
It was disheartening but it was from 2011 so I’m holding out hope that there is some workaround for this issue by now. I’m searching for some configuration change to Ghostscript that will enable me to set all unmarked areas to transparent, or get it to start with a page erased to transparent rather than white.
The TIFF file format supports both CMYK color space and transparency so there must be a way to get both in the same file. Any insight into how to get both inside a TIFF would be welcome at this point. Thanks for reading.

Answer 1

Some creative solutions would be needed to get around the delegate limitations -- as pointed out in the comments.
I would suggest extracting the transparency to an intermediate mask, and reapply it after enabling the CMYKA data channels.
# Create transparent mask (we don't care about colorspace, just grab the alpha channel values)
convert -depth 8 -colorspace sRGB trans-test.pdf -alpha Extract mask.png
# Apply mask _after_ enabling alpha channel
convert \( \
-depth 8 \
-colorspace CMYK \
trans-test.pdf \
-alpha Activate \
\) \
mask.png -compose CopyAlpha -composite output.tif
The -alpha Activate turn on the alpha channel, but there's no data so everything is transparent. Following mask.png -compose CopyAlpha -composite populates the alpha channel with the values extracted from the previous operation.

Answer 2

I compared my solution to that of emcconville. His approach should be a good one, but my profile solution seems to match colors better in saved tif files displayed in Mac OSX Sierra Preview, GraphicConverter and Photoshop. Commands below are unix syntax in IM 7.0.7.37 Q16 HDRI and LIBTIFF Version 4.0.9 Ghostscript 9.23. You can see your versions from magick -list format.
Input:
trans-test.pdf
My solution:
magick -depth 8 -colorspace sRGB trans-test.pdf \
-profile /Users/fred/images/profiles/sRGB.icc \
-profile /Users/fred/images/profiles/USWebCoatedSWOP.icc \
trans-test_profile.tif
trans-test_profile.tif
emcconville's solution:
magick -depth 8 -colorspace sRGB trans-test.pdf -alpha Extract mask.png
magick \( -depth 8 -colorspace CMYK trans-test.pdf \) \
mask.png -alpha off -compose CopyOpacity -composite \
trans-test_mask.tif
trans-test_mask.tif
Note that -compose copyOpacity or copyAlpha generally requires -alpha off. Therefore, I do not see any reason to add -alpha activate.
Here are my delegates:
Version: ImageMagick 7.0.7-37 Q16 x86_64 2018-05-30 https://www.imagemagick.org
Copyright: © 1999-2018 ImageMagick Studio LLC
License: https://www.imagemagick.org/script/license.php
Features: Cipher DPC HDRI Modules OpenMP
Delegates (built-in): bzlib cairo djvu fftw fontconfig freetype gslib gvc jbig jng jp2 jpeg lcms lqr ltdl lzma openexr png ps raw rsvg tiff webp x xml zlib