QPainter::drawLine() draws some weird dashes instead of zero-length lines

[Brono]

Here is some code:

widget.cpp

#include "widget.h"

#define SIZE 400
#define LOGICAL_SIZE 30
#define LINE_WIDTH 0.5

Widget::Widget(QWidget *parent)
	: QWidget(parent),
	  img(SIZE, SIZE, QImage::Format_ARGB32)
{
	QPointF p1(1.23, 4.56);
	QPointF p2(1.23, 4.56);
	QPointF p3(6.54, 3.21);
	QPointF p4(6.54, 3.21);
	this->resize(SIZE, SIZE);
	img.fill(Qt::white);
	QPainter painter(&img);
	painter.setWindow(-LOGICAL_SIZE/2.0,
	                  -LOGICAL_SIZE/2.0,
	                  LOGICAL_SIZE,
	                  LOGICAL_SIZE);
	painter.setPen(QPen(Qt::black, LINE_WIDTH));
	painter.drawLine(p1, p2);
	painter.setPen(QPen(Qt::red, LINE_WIDTH));
	painter.drawLine(p3, p4);
	img.save("result.png");
}

//...

AFAIK this should generate blank image or two dots.
However it outputs two dashes:

What am i doing wrong?

[Asperamanca]

Try setting the cap style of the pen to Qt::FlatCap

[raven-worx]

@Brono
setting a line-width of 0.5 doesn't make much sense. See QPen::setWidth(). Even a pen with a width of 0 draws a 1-pixel line.

Either you want to draw or surround your drawing code with an if-condition and prevent drawing.

[Brono]

@raven-worx :
Help entry you linked to mentions "cosmetic pen", which is "independant to transformations".

Here setting width of the pen does make quite a sense, because i'm setting a "transformation" (QPainter::setWindow()).

P.S.: Image in the post is the real output image (the result.png one).

[raven-worx]

@Brono
but you are setting a pen width of a half pixel in a QWidget context where only full pixels can be drawn

[Brono]

@raven-worx
You can play around with the code (clickable link) and discover that setting pen width to .25 or .75 does indeed change the width of the line

[Brono]

@Asperamanca
Thanks, this helped.
Seems like square capping rounds up non-integer width to highest int.
(this does not apply to round caps)

Is it a bug?