Why does QPainter::drawPoint draw a horizontal line segment? - c++

Question

I'm trying to draw a 3-pixel large point with QPainter. But the following code instead draws a horizontal line with width of 3 pixels.
#include <QPainter>
#include <QImage>
int main()
{
const int w=1000, h=1000;
QImage img(w, h, QImage::Format_RGBX8888);
{
QPainter p(&img);
p.fillRect(0,0,w,h,Qt::black);
p.scale(w,h);
p.setPen(QPen(Qt::red, 3./w, Qt::SolidLine, Qt::RoundCap));
p.drawPoint(QPointF(0.1,0.1));
}
img.save("test.png");
}
Here's the top left corner of the resulting image:
I am expecting to get a point which is red circle, or at least a square — but instead I get this line segment. If I comment out p.scale(w,h) and draw the point with width 3 (instead of 3./w) at position (100,100), then I get a small mostly symmetric 3-pixel in height and width point.
What's going on? Why do I get a line segment instead of point as expected? And how to fix it, without resorting to drawing an ellipse or to avoiding QPainter::scale?
I'm using Qt 5.10.0 on Linux x86 with g++ 5.5.0. The same happens on Qt 5.5.1.

Answer 1

It appears that QPaintEngineEx::drawPoints renders points as line segments of length 1/63.. See the following code from qtbase/src/gui/painting/qpaintengineex.cpp in the Qt sources:
void QPaintEngineEx::drawPoints(const QPointF *points, int pointCount)
{
QPen pen = state()->pen;
if (pen.capStyle() == Qt::FlatCap)
pen.setCapStyle(Qt::SquareCap);
if (pen.brush().isOpaque()) {
while (pointCount > 0) {
int count = qMin(pointCount, 16);
qreal pts[64];
int oset = -1;
for (int i=0; i<count; ++i) {
pts[++oset] = points[i].x();
pts[++oset] = points[i].y();
pts[++oset] = points[i].x() + 1/63.;
pts[++oset] = points[i].y();
}
QVectorPath path(pts, count * 2, qpaintengineex_line_types_16, QVectorPath::LinesHint);
stroke(path, pen);
pointCount -= 16;
points += 16;
}
} else {
for (int i=0; i<pointCount; ++i) {
qreal pts[] = { points[i].x(), points[i].y(), points[i].x() + qreal(1/63.), points[i].y() };
QVectorPath path(pts, 2, 0);
stroke(path, pen);
}
}
}
Notice the pts[++oset] = points[i].x() + 1/63.; line in the opaque brush branch. This is the second vertex of the path — shifted with respect to the desired position of the point.
This explains why the line extends to the right of the position requested and why it depends on the scale. So, it seems the code in the OP isn't wrong for an ideal QPainter implementation, but just has come across a Qt bug (be it in the implementation of the method or in its documentation).
So the conclusion: one has to work around this problem by either using different scale, or drawing ellipses, or drawing line segments with much smaller lengths than what QPainter::drawPoints does.
I've reported this as QTBUG-70409.

Answer 2

Though I have not been able to pin point why exactly the problem occurs, I have got relatively close to the solution. The problem lies with scaling. I did a lots of trial and error with different scaling and width of point with the below code.
const int w=500, h=500;
const int scale = 100;
float xPos = 250;
float yPos = 250;
float widthF = 5;
QImage img(w, h, QImage::Format_RGBX8888);
{
QPainter p(&img);
p.setRenderHints(QPainter::Antialiasing);
p.fillRect(0,0,w,h,Qt::black);
p.scale(scale, scale);
p.setPen(QPen(Qt::red, widthF/(scale), Qt::SolidLine, Qt::RoundCap));
p.drawPoint(QPointF(xPos/scale, yPos/scale));
}
img.save("test.png");
The above code produces the image
My observations are
1) Due to high scaling, the point (which is just 3 pixel wide) is not able to scale proportionally at lower width (width of point), if you set width as something like 30 the round shape is visible.
2) If you want to keep the width of point low then you have to decrease the scaling.
Sadly I can not explain why at high scaling it is not expanding proportionally.