Why drawing lines with integer coords is not correct:
Qt Code: 
  1. #include <QApplication>
  2. #include <QLabel>
  3. #include <QCheckBox>
  4. #include <QPainter>
  5. #include <QMouseEvent>
  6. #include <QLayout>
  7.  
  8. class DrawLine: public QWidget {
  9. Q_OBJECT
  10. public:
  11.     DrawLine() : size(17), scale(40), radius(5), active(-1), hover(-1),
  12.                  image(size, size, QImage::Format_RGB32), round(false),
  13.                  antialiasing(false) {
  14.         int sz = size*scale;
  15.         setMinimumSize(sz, sz);
  16.         int half = sz/2;
  17.         pts[0] = QPoint(half/2, half);
  18.         pts[1] = QPoint(half*3/2, half);
  19.         renderLine();
  20.         setMouseTracking(true);
  21.     }
  22.     void emitCoords() {
  23.         QString str;
  24.         if (round) {
  25.             QPoint p0 = roundCoords(0);
  26.             QPoint p1 = roundCoords(1);
  27.             str.sprintf("P0: (%d, %d) P1: (%d, %d)", p0.x(), p0.y(), p1.x(), p1.y());
  28.         } else {
  29.             QPointF p0 = coords(0);
  30.             QPointF p1 = coords(1);
  31.             str.sprintf("P0: (%.2f, %.2f) P1: (%.2f, %.2f)", p0.x(), p0.y(), p1.x(), p1.y());
  32.         }
  33.         emit coordsUpdated(str);
  34.     }
  35. public slots:
  36.     void setRound(bool round) {
  37.         this->round = round;
  38.         renderLine();
  39.     }
  40.     void setAntialiasing(bool antialiasing) {
  41.         this->antialiasing = antialiasing;
  42.         renderLine();
  43.     }
  44. signals:
  45.     void coordsUpdated(QString coords);
  46. protected:
  47.     virtual void paintEvent(QPaintEvent*) {
  48.         QPainter p(this);
  49.         for (int i = 0; i < size; ++i) {
  50.             for (int j = 0; j < size; ++j) {
  51.                 QRgb cur = image.pixel(i, j);
  52.                 p.fillRect(i*scale, j*scale, scale, scale, QColor(cur));
  53.             }
  54.         }
  55.         p.setPen(Qt::lightGray);
  56.         int sz = size*scale;
  57.         for (int i = 0; i < size; ++i) {
  58.             int x = i*scale;
  59.             p.drawLine(x, 0, x, sz);
  60.             p.drawLine(0, x, sz, x);
  61.         }
  62.         p.setRenderHint(QPainter::Antialiasing);
  63.         p.setPen(Qt::blue);
  64.         p.drawLine(pts[0], pts[1]);
  65.         p.setPen(Qt::NoPen);
  66.         for (int i = 0; i < 2; ++i) {
  67.             p.setBrush(i == hover ? Qt::yellow : Qt::blue);
  68.             p.drawEllipse(pts[i], radius, radius);
  69.         }
  70.     }
  71.     virtual void mousePressEvent(QMouseEvent *event) {
  72.         if (active == -1 && event->button() == Qt::LeftButton) {
  73.             int current = index(event->pos());
  74.             if (current != -1)
  75.                 active = current;
  76.         }
  77.     }
  78.     virtual void mouseReleaseEvent(QMouseEvent *event) {
  79.         if (active != -1 && event->button() == Qt::LeftButton) {
  80.             active = -1;
  81.         }
  82.     }
  83.     virtual void mouseMoveEvent(QMouseEvent *event) {
  84.         if (active != -1) {
  85.             pts[active] = event->pos();
  86.             renderLine();
  87.         }
  88.         int current = index(event->pos());
  89.         if (current != hover) {
  90.             hover = current;
  91.             update();
  92.         }
  93.     }
  94. private:
  95.     QPointF coords(int idx) {
  96.         return QPointF(pts[idx])/scale;
  97.     }
  98.     QPoint roundCoords(int idx) {
  99.         return (coords(idx) - QPointF(0.5, 0.5)).toPoint();
  100.     }
  101.     int index(QPoint p) {
  102.         for (int i = 0; i < 2; ++i) {
  103.             if (QLineF(p, pts[i]).length() <= radius)
  104.                 return i;
  105.         }
  106.         return -1;
  107.     }
  108.     void renderLine() {
  109.         image.fill(qRgb(255, 255, 255));
  110.         QPainter p(&image);
  111.         p.setRenderHint(QPainter::Antialiasing, antialiasing);
  112.         p.setPen(Qt::red);
  113.         if (round) {
  114.             p.drawLine(roundCoords(0), roundCoords(1));
  115.         } else {
  116.             p.drawLine(coords(0), coords(1));
  117.         }
  118.         emitCoords();
  119.         update();
  120.     }
  121.     int size;
  122.     int scale;
  123.     int radius;
  124.     int active;
  125.     int hover;
  126.     QImage image;
  127.     QPoint pts[2];
  128.     bool round;
  129.     bool antialiasing;
  130. };
  131.  
  132. int main(int argc, char *argv[])
  133. {
  134.     QApplication app(argc, argv);
  135.     QWidget widget;
  136.     QLabel label;
  137.     QCheckBox round("Round coords");
  138.     QCheckBox antialiasing("Antialiasing");
  139.     DrawLine draw;
  140.     QObject::connect(&draw, SIGNAL(coordsUpdated(QString)), &label, SLOT(setText(QString)));
  141.     QObject::connect(&round, SIGNAL(toggled(bool)), &draw, SLOT(setRound(bool)));
  142.     QObject::connect(&antialiasing, SIGNAL(toggled(bool)), &draw, SLOT(setAntialiasing(bool)));
  143.     QGridLayout layout;
  144.     layout.addWidget(&round, 0, 0);
  145.     layout.addWidget(&antialiasing, 0, 1);
  146.     layout.addWidget(&label, 0, 2);
  147.     layout.addWidget(&draw, 1, 0, 1, 3);
  148.     layout.setColumnStretch(2, 1);
  149.     widget.setLayout(&layout);
  150.     widget.show();
  151.     draw.emitCoords();
  152.     return app.exec();
  153. }
  154.  
  155. #include "main.moc"
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QPainter-DrawLine.jpg