While we're on the topic of pixels, in Qt 4 we in effect have two coordinate systems. The antialiased coordinate system where (0, 0) corresponds to the top left corner of the top left pixel, and the aliased coordinate system where (0, 0) corresponds to the center of the top left pixel. That means if you fill a rectangle QRectF(0.1, 0.1, 10, 10) without QPainter::Antialiasing enabled it will actually round up and start filling at (1, 1).
The reason for the aliased coordinate system being that way goes all the way back to X11 only having integer coordinate line drawing and polygon filling. The raster paint engine was made to match the behavior of the X11 paint engine and thus the half pixel offset became the de facto standard for the fill rules in our painting systems. _Except_ in cases like image / pixmap painting, where both the X11 and raster paint engine simply rounded the QPointF coordinate. Thus drawing a rect and an image of the same size at the same sub-pixel coordinates would lead to a one pixel delta between the two (a bug that occasionally gets reported but typically closed as "Won't fix"). Once we became fully aware of this inconsistency (well into the 4.x series) we were too afraid to fix it, to avoid breaking existing applications that relied on the current behavior. Well, this has left us with a coordinate system that is neither internally consistent, nor consistent with the antialiased coordinate system (which closely matches how the OpenGL coordinate system works, although in that case the y axis is flipped). The result of these inconsistencies can easily be seen in the filltest.qps arthur test run with the lance tool against 4.8: http://chaos.troll.no/~sroedal/fillrules.png As the arthur test shows, in some cases the way we fill and the way we sample brush textures don't even match. Although it's a bit late, I think Qt 5 is a good time to fix this so that we finally get a consistent coordinate system without gotchas at every corner when using sub-pixel coordinates. I have a work in progress patch in Code Review that gets rid of the half pixel aliased coordinate offset by default: https://codereview.qt-project.org/#change,36757 Here is how the arthur test looks with the new behavior: http://chaos.troll.no/~sroedal/fillrules-qt5.png For applications that do depend on the old behavior, I introduce a new render hint, QPainter::LegacyFillRules, which restores the 4.x behavior. The patch makes use of this render hint in some of Qt's style painting code (mostly to make the Windows "95" and Cleanlooks styles look good, as they draw aliased polygons in some places). It's unfortunate to potentially cause some extra trouble for a subset of existing applications that wish to port to Qt 5, but weighed against the utter embarrassment of the current fill rules I think we need this change. -- Samuel _______________________________________________ Development mailing list Development@qt-project.org http://lists.qt-project.org/mailman/listinfo/development
