As discussed in this post and also this post,
pdftex has the ability of copying PNG files as-is if they are in the right format. This is very useful for me (I’m experimenting with 1024×786px background images for my beamer slides).
My files are in the right format, because compiling with
pdflatex, I see
</home/jojo/texmf/tex/latex/substrate-beamer/substrate0.png (PNG copy)>
But compiling the same file with
xelatex (which I’d like to use for the proper font), compilation takes a lot of extra time (and does so only if I include the pictures), and there are no indication in the
.log that the PNGs are copied. So I assume that they are extracted and recompressed, which explains the long wait.
xelatex have different requirements for PNG files to be copied? Or does it simply not support this feature?