QPainter::drawLine(QPoint, QPoint) incorrect drawing

[kamre]

Why drawing lines with integer coords is not correct:

#include <QApplication>
#include <QLabel>
#include <QCheckBox>
#include <QPainter>
#include <QMouseEvent>
#include <QLayout>
 
class DrawLine: public QWidget {
Q_OBJECT
public:
    DrawLine() : size(17), scale(40), radius(5), active(-1), hover(-1),
                 image(size, size, QImage::Format_RGB32), round(false),
                 antialiasing(false) {
        int sz = size*scale;
        setMinimumSize(sz, sz);
        int half = sz/2;
        pts[0] = QPoint(half/2, half);
        pts[1] = QPoint(half*3/2, half);
        renderLine();
        setMouseTracking(true);
    }
    void emitCoords() {
        QString str;
        if (round) {
            QPoint p0 = roundCoords(0);
            QPoint p1 = roundCoords(1);
            str.sprintf("P0: (%d, %d) P1: (%d, %d)", p0.x(), p0.y(), p1.x(), p1.y());
        } else {
            QPointF p0 = coords(0);
            QPointF p1 = coords(1);
            str.sprintf("P0: (%.2f, %.2f) P1: (%.2f, %.2f)", p0.x(), p0.y(), p1.x(), p1.y());
        }
        emit coordsUpdated(str);
    }
public slots:
    void setRound(bool round) {
        this->round = round;
        renderLine();
    }
    void setAntialiasing(bool antialiasing) {
        this->antialiasing = antialiasing;
        renderLine();
    }
signals:
    void coordsUpdated(QString coords);
protected:
    virtual void paintEvent(QPaintEvent*) {
        QPainter p(this);
        for (int i = 0; i < size; ++i) {
            for (int j = 0; j < size; ++j) {
                QRgb cur = image.pixel(i, j);
                p.fillRect(i*scale, j*scale, scale, scale, QColor(cur));
            }
        }
        p.setPen(Qt::lightGray);
        int sz = size*scale;
        for (int i = 0; i < size; ++i) {
            int x = i*scale;
            p.drawLine(x, 0, x, sz);
            p.drawLine(0, x, sz, x);
        }
        p.setRenderHint(QPainter::Antialiasing);
        p.setPen(Qt::blue);
        p.drawLine(pts[0], pts[1]);
        p.setPen(Qt::NoPen);
        for (int i = 0; i < 2; ++i) {
            p.setBrush(i == hover ? Qt::yellow : Qt::blue);
            p.drawEllipse(pts[i], radius, radius);
        }
    }
    virtual void mousePressEvent(QMouseEvent *event) {
        if (active == -1 && event->button() == Qt::LeftButton) {
            int current = index(event->pos());
            if (current != -1)
                active = current;
        }
    }
    virtual void mouseReleaseEvent(QMouseEvent *event) {
        if (active != -1 && event->button() == Qt::LeftButton) {
            active = -1;
        }
    }
    virtual void mouseMoveEvent(QMouseEvent *event) {
        if (active != -1) {
            pts[active] = event->pos();
            renderLine();
        }
        int current = index(event->pos());
        if (current != hover) {
            hover = current;
            update();
        }
    }
private:
    QPointF coords(int idx) {
        return QPointF(pts[idx])/scale;
    }
    QPoint roundCoords(int idx) {
        return (coords(idx) - QPointF(0.5, 0.5)).toPoint();
    }
    int index(QPoint p) {
        for (int i = 0; i < 2; ++i) {
            if (QLineF(p, pts[i]).length() <= radius)
                return i;
        }
        return -1;
    }
    void renderLine() {
        image.fill(qRgb(255, 255, 255));
        QPainter p(&image);
        p.setRenderHint(QPainter::Antialiasing, antialiasing);
        p.setPen(Qt::red);
        if (round) {
            p.drawLine(roundCoords(0), roundCoords(1));
        } else {
            p.drawLine(coords(0), coords(1));
        }
        emitCoords();
        update();
    }
    int size;
    int scale;
    int radius;
    int active;
    int hover;
    QImage image;
    QPoint pts[2];
    bool round;
    bool antialiasing;
};
 
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QWidget widget;
    QLabel label;
    QCheckBox round("Round coords");
    QCheckBox antialiasing("Antialiasing");
    DrawLine draw;
    QObject::connect(&draw, SIGNAL(coordsUpdated(QString)), &label, SLOT(setText(QString)));
    QObject::connect(&round, SIGNAL(toggled(bool)), &draw, SLOT(setRound(bool)));
    QObject::connect(&antialiasing, SIGNAL(toggled(bool)), &draw, SLOT(setAntialiasing(bool)));
    QGridLayout layout;
    layout.addWidget(&round, 0, 0);
    layout.addWidget(&antialiasing, 0, 1);
    layout.addWidget(&label, 0, 2);
    layout.addWidget(&draw, 1, 0, 1, 3);
    layout.setColumnStretch(2, 1);
    widget.setLayout(&layout);
    widget.show();
    draw.emitCoords();
    return app.exec();
}
 
#include "main.moc"

#include <QApplication>
#include <QLabel>
#include <QCheckBox>
#include <QPainter>
#include <QMouseEvent>
#include <QLayout>

class DrawLine: public QWidget {
Q_OBJECT
public:
    DrawLine() : size(17), scale(40), radius(5), active(-1), hover(-1),
                 image(size, size, QImage::Format_RGB32), round(false),
                 antialiasing(false) {
        int sz = size*scale;
        setMinimumSize(sz, sz);
        int half = sz/2;
        pts[0] = QPoint(half/2, half);
        pts[1] = QPoint(half*3/2, half);
        renderLine();
        setMouseTracking(true);
    }
    void emitCoords() {
        QString str;
        if (round) {
            QPoint p0 = roundCoords(0);
            QPoint p1 = roundCoords(1);
            str.sprintf("P0: (%d, %d) P1: (%d, %d)", p0.x(), p0.y(), p1.x(), p1.y());
        } else {
            QPointF p0 = coords(0);
            QPointF p1 = coords(1);
            str.sprintf("P0: (%.2f, %.2f) P1: (%.2f, %.2f)", p0.x(), p0.y(), p1.x(), p1.y());
        }
        emit coordsUpdated(str);
    }
public slots:
    void setRound(bool round) {
        this->round = round;
        renderLine();
    }
    void setAntialiasing(bool antialiasing) {
        this->antialiasing = antialiasing;
        renderLine();
    }
signals:
    void coordsUpdated(QString coords);
protected:
    virtual void paintEvent(QPaintEvent*) {
        QPainter p(this);
        for (int i = 0; i < size; ++i) {
            for (int j = 0; j < size; ++j) {
                QRgb cur = image.pixel(i, j);
                p.fillRect(i*scale, j*scale, scale, scale, QColor(cur));
            }
        }
        p.setPen(Qt::lightGray);
        int sz = size*scale;
        for (int i = 0; i < size; ++i) {
            int x = i*scale;
            p.drawLine(x, 0, x, sz);
            p.drawLine(0, x, sz, x);
        }
        p.setRenderHint(QPainter::Antialiasing);
        p.setPen(Qt::blue);
        p.drawLine(pts[0], pts[1]);
        p.setPen(Qt::NoPen);
        for (int i = 0; i < 2; ++i) {
            p.setBrush(i == hover ? Qt::yellow : Qt::blue);
            p.drawEllipse(pts[i], radius, radius);
        }
    }
    virtual void mousePressEvent(QMouseEvent *event) {
        if (active == -1 && event->button() == Qt::LeftButton) {
            int current = index(event->pos());
            if (current != -1)
                active = current;
        }
    }
    virtual void mouseReleaseEvent(QMouseEvent *event) {
        if (active != -1 && event->button() == Qt::LeftButton) {
            active = -1;
        }
    }
    virtual void mouseMoveEvent(QMouseEvent *event) {
        if (active != -1) {
            pts[active] = event->pos();
            renderLine();
        }
        int current = index(event->pos());
        if (current != hover) {
            hover = current;
            update();
        }
    }
private:
    QPointF coords(int idx) {
        return QPointF(pts[idx])/scale;
    }
    QPoint roundCoords(int idx) {
        return (coords(idx) - QPointF(0.5, 0.5)).toPoint();
    }
    int index(QPoint p) {
        for (int i = 0; i < 2; ++i) {
            if (QLineF(p, pts[i]).length() <= radius)
                return i;
        }
        return -1;
    }
    void renderLine() {
        image.fill(qRgb(255, 255, 255));
        QPainter p(&image);
        p.setRenderHint(QPainter::Antialiasing, antialiasing);
        p.setPen(Qt::red);
        if (round) {
            p.drawLine(roundCoords(0), roundCoords(1));
        } else {
            p.drawLine(coords(0), coords(1));
        }
        emitCoords();
        update();
    }
    int size;
    int scale;
    int radius;
    int active;
    int hover;
    QImage image;
    QPoint pts[2];
    bool round;
    bool antialiasing;
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QWidget widget;
    QLabel label;
    QCheckBox round("Round coords");
    QCheckBox antialiasing("Antialiasing");
    DrawLine draw;
    QObject::connect(&draw, SIGNAL(coordsUpdated(QString)), &label, SLOT(setText(QString)));
    QObject::connect(&round, SIGNAL(toggled(bool)), &draw, SLOT(setRound(bool)));
    QObject::connect(&antialiasing, SIGNAL(toggled(bool)), &draw, SLOT(setAntialiasing(bool)));
    QGridLayout layout;
    layout.addWidget(&round, 0, 0);
    layout.addWidget(&antialiasing, 0, 1);
    layout.addWidget(&label, 0, 2);
    layout.addWidget(&draw, 1, 0, 1, 3);
    layout.setColumnStretch(2, 1);
    widget.setLayout(&layout);
    widget.show();
    draw.emitCoords();
    return app.exec();
}

#include "main.moc"

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QPainter-DrawLine.jpg

[kamre]

Nobody uses QPainter::drawLine(QPoint, QPoint) or I am doing something wrong with this function?

[high_flyer]

What you are doing wrong is how you ask your question.
http://www.catb.org/esr/faqs/smart-questions.html#code

Specifically, it looks like the problem is in your position math - check it.

[kamre]

What you are doing wrong is how you ask your question.

Provided rather simple compilable example that reproduces the problem. What is wrong about it?

Specifically, it looks like the problem is in your position math - check it.

Ok, lets remove all user interaction and use hard coded values for the function:

#include <QApplication>
#include <QPainter>
#include <QWidget>
 
class DrawLine: public QWidget {
public:
    DrawLine() : size(15), scale(40), image(size, size, QImage::Format_RGB32) {
        int sz = size*scale;
        setMinimumSize(sz, sz);
        renderLine();
    }
protected:
    virtual void paintEvent(QPaintEvent*) {
        QPainter p(this);
        for (int i = 0; i < size; ++i) {
            for (int j = 0; j < size; ++j) {
                QRgb cur = image.pixel(i, j);
                p.fillRect(i*scale, j*scale, scale, scale, QColor(cur));
            }
        }
        p.setPen(Qt::lightGray);
        int sz = size*scale;
        for (int i = 0; i < size; ++i) {
            int x = i*scale;
            p.drawLine(x, 0, x, sz);
            p.drawLine(0, x, sz, x);
        }
    }
private:
    void renderLine() {
        image.fill(qRgb(255, 255, 255));
        QPainter p(&image);
        p.setPen(Qt::red);
        p.drawLine(QPoint(1, 4), QPoint(11, 2));
    }
    int size;
    int scale;
    QImage image;
};
 
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    DrawLine draw;
    draw.show();
    return app.exec();
}

#include <QApplication>
#include <QPainter>
#include <QWidget>

class DrawLine: public QWidget {
public:
    DrawLine() : size(15), scale(40), image(size, size, QImage::Format_RGB32) {
        int sz = size*scale;
        setMinimumSize(sz, sz);
        renderLine();
    }
protected:
    virtual void paintEvent(QPaintEvent*) {
        QPainter p(this);
        for (int i = 0; i < size; ++i) {
            for (int j = 0; j < size; ++j) {
                QRgb cur = image.pixel(i, j);
                p.fillRect(i*scale, j*scale, scale, scale, QColor(cur));
            }
        }
        p.setPen(Qt::lightGray);
        int sz = size*scale;
        for (int i = 0; i < size; ++i) {
            int x = i*scale;
            p.drawLine(x, 0, x, sz);
            p.drawLine(0, x, sz, x);
        }
    }
private:
    void renderLine() {
        image.fill(qRgb(255, 255, 255));
        QPainter p(&image);
        p.setPen(Qt::red);
        p.drawLine(QPoint(1, 4), QPoint(11, 2));
    }
    int size;
    int scale;
    QImage image;
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    DrawLine draw;
    draw.show();
    return app.exec();
}

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Look at how y-coord is not respected.

[high_flyer]

Provided rather simple compilable example that reproduces the problem. What is wrong about it?

1. That you expect people to compile code for you.
Many people, me included, do this along side their work - I have not time to compile your code and debug for you.
I can help you narrow down the problem and offer potential problems - but you have to make it easy for me to focus on the problem - if you expect help.
2. You have provided no hint to help any potential helper to the area of the problem, rather expect people to read all the code check all of it and tell you where the problem is.
Nothing is wrong with doing that really, just don't act so surprised when you get no answers, that is all.

Again, looks like a math problem - my guess its the rounding.
What happens if you remove the rounding?

[kamre]

Nothing is wrong with doing that really, just don't act so surprised when you get no answers, that is all.

Ok, I see, thank you for your attention to my problem!

Again, looks like a math problem - my guess its the rounding.
What happens if you remove the rounding?

Look at the small piece of code posted above:

QPainter p(&image);
p.setPen(Qt::red);
p.drawLine(QPoint(1, 4), QPoint(11, 2));

QPainter p(&image);
p.setPen(Qt::red);
p.drawLine(QPoint(1, 4), QPoint(11, 2));

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No rounding at all, exact integer numbers are passed.

Paint event also doesn't use any floating point calculations:

virtual void paintEvent(QPaintEvent*) {
        QPainter p(this);
        for (int i = 0; i < size; ++i) {
            for (int j = 0; j < size; ++j) {
                QRgb cur = image.pixel(i, j);
                p.fillRect(i*scale, j*scale, scale, scale, QColor(cur));
            }
        }
        p.setPen(Qt::lightGray);
        int sz = size*scale;
        for (int i = 0; i < size; ++i) {
            int x = i*scale;
            p.drawLine(x, 0, x, sz);
            p.drawLine(0, x, sz, x);
        }
    }

virtual void paintEvent(QPaintEvent*) {
        QPainter p(this);
        for (int i = 0; i < size; ++i) {
            for (int j = 0; j < size; ++j) {
                QRgb cur = image.pixel(i, j);
                p.fillRect(i*scale, j*scale, scale, scale, QColor(cur));
            }
        }
        p.setPen(Qt::lightGray);
        int sz = size*scale;
        for (int i = 0; i < size; ++i) {
            int x = i*scale;
            p.drawLine(x, 0, x, sz);
            p.drawLine(0, x, sz, x);
        }
    }

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Note that size and scale are integer values.

Now lets look at the picture:
Qt-5.1-DrawLine.png

The question is: why points (1, 4) and (11, 2) are not painted at all even if they were passed to the call of QPainter::drawLine?

[wysota]

Have you verified using the line drawing algorithm that they should be painted?

[kamre]

Have you verified using the line drawing algorithm that they should be painted?

Do you mean that drawing of line segment from pixel (x0, y0) to pixel (x1, y1) doesn't require filling these two pixels?

[wysota]

Do you mean that drawing of line segment from pixel (x0, y0) to pixel (x1, y1) doesn't require filling these two pixels?

I don't know, I haven't checked that using the line drawing algorithm. I'm asking whether you have done that since you claim something is wrong.

[kamre]

I don't know, I haven't checked that using the line drawing algorithm. I'm asking whether you have done that since you claim something is wrong.

I suppose you are joking, but lets look at the algorithms.

[wysota]

No, I'm not joking. You say something is wrong with the line so I'm asking whether this conclusion comes from your internal belief that the start and end points should be forced to be part of the line or whether you actually checked Qt's algorithm and found a flaw in it.

[kamre]

I'm asking whether this conclusion comes from your internal belief that the start and end points should be forced to be part of the line

This is the expected behavior of drawing line segment with pixel-based coords, is not it? It works so in other 2d drawing libraries like Cairo/Java2D.

But in Qt I have the following inconsistent behavior:

void renderLine() {
        image.fill(qRgb(255, 255, 255));
        QPainter p(&image);
        // these line segments with red color fill start/end points
        p.setPen(Qt::red);
        p.drawLine(QPoint(7, 2), QPoint(7, 5));
        p.drawLine(QPoint(9, 7), QPoint(12, 7));
        p.drawLine(QPoint(7, 9), QPoint(7, 12));
        p.drawLine(QPoint(2, 7), QPoint(5, 7));
        p.drawLine(QPoint(5, 5), QPoint(3, 3));
        p.drawLine(QPoint(9, 9), QPoint(11, 11));
        // these line segments with blue color doesn't fill start/end points
        // looks like y-coord is not respected
        p.setPen(Qt::blue);
        p.drawLine(QPoint(9, 5), QPoint(11, 3));
        p.drawLine(QPoint(5, 9), QPoint(3, 11));
    }

void renderLine() {
        image.fill(qRgb(255, 255, 255));
        QPainter p(&image);
        // these line segments with red color fill start/end points
        p.setPen(Qt::red);
        p.drawLine(QPoint(7, 2), QPoint(7, 5));
        p.drawLine(QPoint(9, 7), QPoint(12, 7));
        p.drawLine(QPoint(7, 9), QPoint(7, 12));
        p.drawLine(QPoint(2, 7), QPoint(5, 7));
        p.drawLine(QPoint(5, 5), QPoint(3, 3));
        p.drawLine(QPoint(9, 9), QPoint(11, 11));
        // these line segments with blue color doesn't fill start/end points
        // looks like y-coord is not respected
        p.setPen(Qt::blue);
        p.drawLine(QPoint(9, 5), QPoint(11, 3));
        p.drawLine(QPoint(5, 9), QPoint(3, 11));
    }

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Qt-DrawLine-Inconsistent.png

whether you actually checked Qt's algorithm and found a flaw in it

Don't know about algorithm used in Qt and whether it has flaws, as user of the Qt I have just tried to draw something simple with QPainter::drawLine and it didn't work in expected and consistent way.

My question is: why QPainter::drawLine works so inconsistent and what can I do to get expected results from it?

[wysota]

Sorry, I don't understand what you mean. This is the result of your code on an image scaled up 10 times:

star.png

What is inconsistent here?

The code used:

#include <QtGui>
 
void renderLine(QImage &image ) {
        image.fill(qRgb(255, 255, 255));
        QPainter p(&image);
        // these line segments with red color fill start/end points
        p.setPen(Qt::red);
        p.drawLine(QPoint(7, 2), QPoint(7, 5));
        p.drawLine(QPoint(9, 7), QPoint(12, 7));
        p.drawLine(QPoint(7, 9), QPoint(7, 12));
        p.drawLine(QPoint(2, 7), QPoint(5, 7));
        p.drawLine(QPoint(5, 5), QPoint(3, 3));
        p.drawLine(QPoint(9, 9), QPoint(11, 11));
        // these line segments with blue color doesn't fill start/end points
        // looks like y-coord is not respected
        p.setPen(Qt::blue);
        p.drawLine(QPoint(9, 5), QPoint(11, 3));
        p.drawLine(QPoint(5, 9), QPoint(3, 11));
}
 
 
int main(int argc, char **argv) {
  QApplication app(argc, argv);
  QLabel l;
  QImage img(16, 16, QImage::Format_ARGB32_Premultiplied);
  renderLine(img);
  img = img.scaled(160, 160);
  l.setPixmap(QPixmap::fromImage(img));
  l.show();
  return app.exec();
}

#include <QtGui>

void renderLine(QImage &image ) {
        image.fill(qRgb(255, 255, 255));
        QPainter p(&image);
        // these line segments with red color fill start/end points
        p.setPen(Qt::red);
        p.drawLine(QPoint(7, 2), QPoint(7, 5));
        p.drawLine(QPoint(9, 7), QPoint(12, 7));
        p.drawLine(QPoint(7, 9), QPoint(7, 12));
        p.drawLine(QPoint(2, 7), QPoint(5, 7));
        p.drawLine(QPoint(5, 5), QPoint(3, 3));
        p.drawLine(QPoint(9, 9), QPoint(11, 11));
        // these line segments with blue color doesn't fill start/end points
        // looks like y-coord is not respected
        p.setPen(Qt::blue);
        p.drawLine(QPoint(9, 5), QPoint(11, 3));
        p.drawLine(QPoint(5, 9), QPoint(3, 11));
}


int main(int argc, char **argv) {
  QApplication app(argc, argv);
  QLabel l;
  QImage img(16, 16, QImage::Format_ARGB32_Premultiplied);
  renderLine(img);
  img = img.scaled(160, 160);
  l.setPixmap(QPixmap::fromImage(img));
  l.show();
  return app.exec();
}

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[high_flyer]

I don't know what is different in the code you supplied here to the actual application you are coding.
I just ran your example code from the first post (because I could not see anything wrong with it and got curious) and it works perfectly, see attachment.
What ever it is which is going wrong its not in that code.
ScreenHunter_56 Sep. 18 13.51.jpg

EDIT: ah, wysota beat me to it :-)

[kamre]

What is inconsistent here?

This is perfectly fine result! And I also would like to obtain the same one.

But running your code (changed include to "QtWidgets") with my Qt 5.1.1 MinGW-OpenGL on Windows 7 looks like this:
Qt-DrawLine-Consistent.png
Why do I have a different picture?

[wysota]

I have the same error you have when using Qt5.1 beta on Linux. I don't know whether this is caused by Qt5 or OpenGL. Using OpenGL would explain it but I can hardly believe QImage uses OpenGL to draw a line. You could file a bug report on this or compare line drawing algorithms in Qt4 and Qt5.