Stellarium uses CPM to automatically download several dependencies during build process, if they are missing from the target system. Follow that page for more details, but here's summary.
If you want to use an external library, use
find_package().
If you want to provide a fallback of downloading it when they are missing, use
CPMFindPackage(). If find_package() is impossible to use because the
library doesn't provide <Foo>Config.cmake and there is no Find<Foo>.cmake,
use e.g. find_library() followed by CPMAddPackage(): CPMFindPackage()
itself is essentially find_package() followed by CPMAddPackage().
Then it depends on the library. If it can be used without any changes and
provides a good CMakeLists.txt, the simplest way is to just use that file.
This is the default operation mode of CPM. However, if CMakeLists.txt is
missing or poorly-written, or if some other changes (patches etc) are
necessary, use DOWNLOAD_ONLY YES option, and use the files from
${foo_SOURCE_DIR} as you please. After making use of the files provide an
alias for the library to match the name exported by find_package(Foo), so
that the rest of cmake config doesn't need to care whether the dependency was
found locally, or downloaded automatically.
If you already have the dependent library packaged, it should be picked up normally.
It may be a good idea to provide -DCPM_USE_LOCAL_PACKAGES=yes to cmake to
ask CPM to show an error if the package is missing, instead of trying to download
anything.
PNG images can frequently be more efficiently compressed by using a tool designed for the purpose, such as pngcrush, oxipng, or ect.
When adding new images to the Stellarium project, contributors should make sure that added PNGs (and exported images from SVG sources) are compressed as much as is reasonably achievable.
Benchmarks suggest that ect is the
most efficient recompression tool for full color images, particularly larger ones,
so that is what we recommend.
ECT is an open source tool that compiles and runs on Linux, Windows, and macOS. To compress a PNG image in place, run the following command:
ect -9 image.png
The -9 specifies the maximum effort in searching for optimal compression
parameters, and is not appropriate for use in CIs or build scripts due to its
low speed. -3 is the default compression speed, which is much faster and
compresses almost as well.
Note: when recompressing an entire directory of existing images, it is advisable
to add the option --strict. This prevents ect from stripping metadata out of
images. In rare cases, this metadata could affect how the image is displayed.
If you did not create an image and cannot verify changes to it, avoid making
changes to image metadata.