[cairo-bugs] [Bug 104042] text failing to display on SVG and X11 surface even though it works correctly on PDF and PS surfaces

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Thu Dec 28 13:33:06 UTC 2017


--- Comment #5 from Uli Schlachter <psychon at znc.in> ---
Actually... what are the semantics of drawing an alpha-only surface to a
color-only target with operator OVER? I'm not quite sure and this seems to be
the root of the problem. https://www.cairographics.org/operators/ doesn't
answer this either.

I noticed that with cairo-xcb the example program works, while with cairo-xlib
still nothing is displayed. After staring at protocol traces for far too long,
I noticed that the significant difference seems to be in the drawing to the
intermediate surface:

- creates an uninitialized(???) intermediate pixmap
- draws the glyphs to it via operator OVER
- with a black solid source

- creates an intermediate pixmap and clears it
- draws the glyphs to it via operator ADD
- with a white solid source

Both backends then OVER this intermediate to the target picture which uses a
RGB24 format. I'm not actually sure why this produces no result with
cairo-xlib, but the missing clearing of the intermediate pixmap strikes me as
quite odd.

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