Created attachment 109815 [details]
test case

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=770297 has a pdf that loses the minus signs after running through pdftocairo.

I've reduced the pdf to a minimal test case that draws a single minus sign (the minus sign is drawn as an image mask).

Reproduce with:

pdftocairo -pdf test.pdf out.pdf

or any other vector backend. The minus sign is lost.

eg

pdftocairo -ps test.pdf out.ps

and observe that the image is not included in the output:

%%EndPageSetup
q 0 883 0 1 rectclip q
Q Q
showpage


I found _cairo_pattern_get_extents was rounding the height down to 0. It seems to be assuming that all backends are pixel based.