When converting DocBook, pandoc can't find image - cmake

Question

I'm writing documentation for a program and would like to include a screenshot. The CMakeLists.txt file, which is in the doc subdirectory, looks like this:
find_program(PANDOC pandoc)
if (PANDOC)
add_custom_command(OUTPUT perfecttin.pdf MAIN_DEPENDENCY perfecttin.xml COMMAND pandoc -f docbook -o perfecttin.pdf -V papersize=a4 --data-dir=${PROJECT_SOURCE_DIR}/doc ${PROJECT_SOURCE_DIR}/doc/perfecttin.xml)
add_custom_target(perfecttin-doc ALL DEPENDS perfecttin.pdf)
endif ()
The section of the DocBook file looks like this:
<section id="window">
<title>Main Window</title>
<mediaobject>
<imageobject>
<imagedata fileref="window1.png" format="PNG"/>
</imageobject>
</mediaobject>
</section>
window1.png is in the same directory as CMakeLists.txt and perfecttin.xml. I've tried it with and without --data-dir=${PROJECT_SOURCE_DIR}/doc, and I get the same error:
[pandoc warning] Could not find image `window1.png', skipping...
How do I tell pandoc where to look for the image?

Answer 1

The option to do this is --resource-path, but that is not available on the version of pandoc on Ubuntu Bionic. So to build the documentation on a newer version, but not break the entire build on an older version, I added code to check whether pandoc supports the option. Here is the working CMakeLists.txt:
find_program(PANDOC pandoc)
if (PANDOC)
execute_process(COMMAND ${PANDOC} "--resource-path=/" INPUT_FILE /dev/null ERROR_FILE /dev/null RESULT_VARIABLE pandoc_exit)
endif ()
if (PANDOC AND NOT pandoc_exit)
add_custom_command(OUTPUT perfecttin.pdf MAIN_DEPENDENCY perfecttin.xml COMMAND pandoc -f docbook -o perfecttin.pdf -V papersize=a4 --resource-path=${PROJECT_SOURCE_DIR}/doc ${PROJECT_SOURCE_DIR}/doc/perfecttin.xml)
add_custom_target(perfecttin-doc ALL DEPENDS perfecttin.pdf)
endif ()
if (PANDOC AND NOT pandoc_exit) sounds backward, but the exit code for success is 0, which CMake takes as false, while the code for error is 2, which it takes as true.