c++ - QT is fuzzing up my arc -

i'm trying draw beautiful arc, qt seems making pix-elated , jaggy. want smooth, , thought anti-aliasing that.

edit: using qt 4.7.4

here screen shot:

screen shot of mainwindow jagged arc

you can notice parts of red line (arc) relatively smooth, ugly looking goes around.

here self-contained paintevent. test, drop template mainwindow gui project:

void mainwindow::paintevent(qpaintevent *e) {     qpainter painter(this);     qbrush fillbrush = qbrush(qt::white, qt::solidpattern);     qpen markpen(qt::black);     qpoint centrept(this->width() / 2, this->height() / 2);     int arcwidth = 8;                             // width of arc     int radius = (this->width() / 2) - arcwidth; // radius maximum minus width of arc line     int maxspeed = 80;                          // 80 mph max speed     int maxangle = -270 * 16;                   // 80 mph -270, *16 drawarc     int speedlimit = 40;                        // speed limit mph     double ratio = speedlimit / static_cast<float>(maxspeed); // 80 mph max speed     int anglespan_ = maxangle * ratio;     int startangle_ = -135 * 16;                // starting angle -135 0 mph, *16 drawarc      qrect bounds(centrept.x()-radius, centrept.y()-radius, (2*radius), (2*radius));       painter.setrenderhint(qpainter::antialiasing, true); // suppose smooth out?     setbackgroundrole(qpalette::base);     setautofillbackground(false);     painter.setbrush(fillbrush);      // draw circle     fillbrush = qbrush(qt::black, qt::solidpattern);     painter.drawellipse(centrept, radius, radius);          //  draw arc         markpen.setcapstyle(qt::flatcap);         markpen.setjoinstyle(qt::roundjoin);         markpen.setstyle(qt::solidline);         markpen.setcolor(qt::red);         markpen.setwidth(arcwidth);         painter.setpen(markpen);         fillbrush = qbrush(qt::red, qt::solidpattern);         painter.setbrush(fillbrush);          painter.drawarc(bounds, startangle_, anglespan_);          painter.end();  }

by "relatively smooth" referring parts arc perfect line or staircase, result of anti-aliasing not visible:

enter image description here

at other angles it's more difficult result fine on low dpi screen, what's why these jagged smoothing:

enter image description here

there not can do, besides applying sharpening effect. @ end same result no antialiasing:

enter image description here

however can still improve black line snapping nearest grid. need use floating-point versions of qpoint , qrect:

qpointf centrept(this->width() / 2 + 0.5, this->height() / 2 + 0.5); ... qrectf bounds(centrept.x()-radius + 0.5, centrept.y()-radius + 0.5, (2*radius), (2*radius));

original:

enter image description here