drawRect function of Qpainter not giving rectangles of equal dimension - qt

Question

In Qt i have a Qpixmap of 5 px width and 4 px height ,I am filling it with rectangles of width .08 and height .08 but the rectangles formed are not of equal dimension as I have given`.
I want to draw alternate black and white rectangles of equal dimension
QLabel *a=new QLabel();
QPixmap b(5,4);
a->setFixedSize(b.size());
QPainter painter(&b);
heightOfCheckeredBox=.08;
widthOfCheckeredBox=.08;
bool switchBetweenBlackAndWhiteBoxRowDirection=true;
int switchBetweenBlackAndWhiteBoxColumnDirection=0;
for(qreal i=0;i<4;i+=heightOfCheckeredBox)
{
for(qreal j=0;j<5;j+=widthOfCheckeredBox)
{
if(switchBetweenBlackAndWhiteBoxRowDirection)
{
painter.setPen(Qt::white);
painter.setBrush(Qt::white);
switchBetweenBlackAndWhiteBoxRowDirection=false;
}
else
{
painter.setPen(Qt::gray);
painter.setBrush(Qt::gray);
switchBetweenBlackAndWhiteBoxRowDirection=true;
}
QRectF rectangle(j,i,widthOfCheckeredBox,heightOfCheckeredBox);
painter.drawRect(rectangle);
}
switchBetweenBlackAndWhiteBoxColumnDirection++;
if(int(switchBetweenBlackAndWhiteBoxColumnDirection)%2==0)
switchBetweenBlackAndWhiteBoxRowDirection=true;
else
switchBetweenBlackAndWhiteBoxRowDirection=false;
}
a->setPixmap(b);
a->show();

Answer 1

Since you're drawing without antialiasing, the tiny pixel size makes little sense - what did you expect?
The size of the pixmap is given in pixels. Your pixmap contains 5x4 = 20 pixels. You're trying to draw 20 rectangles on it. For this to work, each rectangle must be a square exactly one pixel wide and high. Alas, you're trying to draw squares 12.5x smaller than a pixel. That doesn't make much sense, and I have no idea where you got the value of 0.08 from. Just draw the squares one pixel wide.
Example:
// https://github.com/KubaO/stackoverflown/tree/master/questions/checkered-paint-42205180
#include <QtWidgets>
QPixmap checkers(const QSizeF & rectSize, const QSize & pixmapSize) {
QPixmap pixmap{pixmapSize};
QPainter painter{&pixmap};
painter.setPen(Qt::NoPen);
const QColor colors[] = {Qt::white, Qt::black};
QPointF pos{};
bool oddRow{}, color{};
while (pos.y() < pixmap.height()) {
painter.setBrush(colors[color ? 1 : 0]);
painter.drawRect({pos, rectSize});
color = !color;
pos.setX(pos.x() + rectSize.width());
if (pos.x() >= pixmap.width()) {
pos.setX({});
pos.setY(pos.y() + rectSize.height());
oddRow = !oddRow;
color = oddRow;
}
}
return pixmap;
}
int main(int argc, char ** argv) {
QApplication app{argc, argv};
QLabel label;
auto pix = checkers({1, 1}, {60, 30});
label.resize(200, 200);
label.setAlignment(Qt::AlignCenter);
label.setPixmap(pix);
label.show();
return app.exec();
}