The customer reports and I've verified that Ghostscript 9.02 and head (b7d52f725a7c4ef939469f757687a86252fade9c) produce a PNG file with a correct transparent background at 150 dpi but not at 300 dpi. The command lines I'm using for testing: bin/gs -r150 -sDEVICE=pngalpha -o test150.png line_demo.pdf and bin/gs -r300 -sDEVICE=pngalpha -o test300.png line_demo.pdf
This has actually nothing to do with transparency but is a clist issue (the file contains no transparency and the pdf14 device is not installed). The pngalpha devices alpha channel appears is not being handled properly during clist writing. Assigning to Ray to look at.
*** Bug 691372 has been marked as a duplicate of this bug. ***
Fixed with commit 5ce12e8 Handle transparent background with custom fillpage in pngalpha