Hi Mark, On Feb 14, 2012, at 12:03 AM, ext Mark wrote: > While using Qt's elidedText function > http://developer.qt.nokia.com/doc/qt-4.8/qfontmetricsf.html#elidedText i > noticed the function was dog slow. For just 1000 entries (each 250 chars) > eliding them to 200px took about 190ms on windows and 100ms on linux. Using > that for drawing is noticeable slow. I haven't actually tried that, but i > certainly would notice that and in the longer run that is my intention. > > So, i started playing with it in an attempt to get it blazing fast. And i got > there. On windows i have a speedup of 39x (same job only takes 5ms) and on > Linux i got a speedup of 10x (same job takes 10ms, side effect is that linux > while 10x faster is now slower then windows.. odd), Those speedups are nice > but it tasted like more so i went on to profilling. I already knew the > QFontMetricsF::width was the function dragging everything down and a > kcachegrind seems to indicate the same. > > Note for the speedups, i got them mostly by cashing the width of the used > character thus avoiding the width function as much as possible. > > This is the path that width() takes till the point of calling the freetype > functions: > QFontEngineFT::stringToCMap > QFontEngineFT::recalcAdvances > QFontEngineFT::loadGlyph > > loadGlyph is where i think something odd is going on. > http://qt.gitorious.org/qt/qt/blobs/4.8/src/gui/text/qfontengine_ft.cpp#line812 > > Note the last parameter: "fetchMetricsOnly" ... The path it took had > fetchMetricsOnly set to true : > http://qt.gitorious.org/qt/qt/blobs/4.8/src/gui/text/qfontengine_ft.cpp#line1611. > That leads me to think that this function is only going to fetch the metrics > of the glyph, not render it! The opposite is the case. It's rendering it and > that is the part that takes long. > > So, am i right here? Is this function doing to much? Remember, we come from > the "width()" function. All we actually need is the width of the glyph, > that's it. > > Actually, if you look in that function for the fetchMetricsOnly occurrence > you'll see that it's only used in one place and there it doesn't seem to do a > lot with metrics. It kinda looks like the original implementer of this has > some idea, but never finished implementing it. Or am i wrong here?
Your guess is mostly correct. This fetchMetricsOnly flag was added a while ago (Nov 5, 2008) to suppress a warning message but it's actually useful to avoid doing rendering in that case. I think we should change (set->outline_drawing && fetchMetricsOnly) to (set->outline_drawing || fetchMetricsOnly) if not regression is caused. How is that working for you? - Jiang _______________________________________________ Development mailing list Development@qt-project.org http://lists.qt-project.org/mailman/listinfo/development
