How to trace the missing pixels when using drawLine

Question

We know that for drawing on an image in qt, qpainter is used. Recently, I used drawLine() function to draw whatever an user is scribbling. This was done by passing the lastPoint and currentPoint from the mouseMoveEvent to a custom function which implements drawLine(). I have passed the arguments for that custom function as given below:

void myPaint::mouseMoveEvent(QMouseEvent *event) {  
qDebug() << event->pos();
   if ((event->buttons() & Qt::LeftButton) && scribbling) {
    pixelList.append(event->pos());
    drawLineTo(event->pos());
    lastPoint = event->pos();
   }
}

Now with the help of qDebug() I noticed that some pixels are missed while drawing but the drawing is precise. I looked up the source of qt-painting where I saw that drawLine() was calling drawLines() which was making use of qpainterPath to have a shape drawn on the image.

My question is that, is there anyway to track these "missed" pixels or any approach to find all the pixels which have been drawn?

Thanks!

void myPaint::drawLineTo(const QPoint &endPoint) {    
QPainter painter(image); //image is initialized in the constructor of myPaint
painter.setRenderHint(QPainter::Antialiasing);
painter.setPen(QPen(Qt::blue, myPenWidth, Qt::SolidLine, Qt::RoundCap,Qt::RoundJoin));
painter.drawLine(lastPoint, endPoint);
modified = true;
lastPoint = endPoint; //at the mousePressEvent, the event->pos() will be stored as
                      // lastPoint 
update();
}

Answer 1

For a start, don't draw in a mouseEvent(). Actually handling a mouseevent should be done as quick as possible. Also, it is not a good idea to look at the Qt source, it can be confusing. Rather assume that what Qt gives you work, and first try to answer "What I am doing wrong?". As I said drawing in a mouse event is definitely wrong.

Your description is really subjective, maybe an image of your output is better. Are you trying to emulate a pen (like in windows paint)? In this case do the mouse button has to be down ? is that the purpose of your variable scribbling?

There is more. following the documentation, QMouseEvent::buttons() always return a combination of all buttons for mouse move event. Which make sense : the mouse movements are independent of the buttons. It means

if ((event->buttons() & Qt::LeftButton)

will always be true.

Let's assume you want to draw the path of your mouse when the left button is pressed. Then you use something like :

void myPaint::mousePressEvent(QMouseEvent *event){
     scribbling = true;
     pixelList.clear();
}

void myPaint::mouseReleaseEvent(QMouseEvent *event){
     scribbling = false;
}

void myPaint::mouseMoveEvent(QMouseEvent *event) {  
  if ( scribbling) {
    pixelList.append(event->pos());
  }
}

void myPaint::paintEvent(){
  QPainter painter(this)
  //some painting here
  if ( scribbling) {
     painter.setRenderHint(QPainter::Antialiasing);
     painter.setPen(QPen(Qt::blue, myPenWidth, Qt::SolidLine, Qt::RoundCap,Qt::RoundJoin));
    // here draw your path
    // for example if your path can be made of lines, 
    // or you just put the points if they are close from each other
  }

  //other painting here
}

If after all of this you don't have a good rendering, try using float precision (slower), ie QMouseEvent::posF() instead of QMouseEvent::pos().

EDIT :
"I want to know whether there is any way to calculate all the sub-pixels between any two pixels that we send as arguments to drawLine"
Yes there is. I don't know why you need to do such thing but is really simple. A line can be characterized with the equation

   y = ax + b

Both of the endpoints of the line p0 = (x0, y0) and p1 = (x1, y1) satisfy this equation so you can easily find a and b. Now all you need to do is increment from x0 to x1 by the amount of pixels you want (say 1), and to compute the corresponding y value, each time saving point(x,y).

So will go over all of the points saved in pixelList and repeat this process for any two consecutive points.