Can't paint points on my widget

[graniteDev]

@mrjj
Qt example I am trying to follow from: https://doc.qt.io/qt-5/qtwidgets-painting-basicdrawing-example.html

How do I pass parameters into the function? I suppose I could set global variables with the data, and then call that data from the override "paintEvent" but I'd prefer to just pass it in by reference if I can.

[mrjj]

@graniteDev
Ah. well it paints via
void RenderArea ::paintEvent(QPaintEvent *event) override;

You cant change paintEvent signature so you will have to copy the points to member variables
and then call update and in paintEvent use those member variables.
No reason for global vars. just let PointViewer store them and let paintEvent use those member vars.
something like
PointViewer::ShowPoint(int x, int y, int z) {
m_x=x; m_y=y; m_z=z;
update();
}

and paintEvent uses m_x,m_y etc.
you could let it point to original data in other class if its heavy but for some ints i dont think it matters.

[graniteDev]

@mrjj Yes, that's what I meant to say... I think of member variables as global because of their scope, but I didn't not mean global to the whole application.

I tried the code as you provided, but still nothing happened.

void PointViewer::paintEvent(QPaintEvent *)
{
    QPen pen(Qt::green, 20, Qt::SolidLine, Qt::RoundCap);
    QPainter painter(this);
    painter.begin(ui->liveFeedWidget);
    painter.setPen(pen);
    painter.drawPoint(100,200);
    painter.end();
}

[ofmrew]

@mrjj I usually create a new class, MyCanvas, and promote the Widget to it, mainly because I what to capture mouse and other events. Which is the Qt preferred way?

[mrjj]

@ofmrew
Well that is indeed a good way as it allows both design time handling and
custom drawing. I think PointViewer is sort of a MyCanvas type.

[mrjj]

@graniteDev
Did you use override ?
so we ar sure its correct ?
try put qDebug() << "im in paint";
to check it is indeed called.

Also what is
begin(ui->liveFeedWidget);
that looks wrong.
Are you trying to draw to multiple widgets from PointViewer ?
try with
painter.begin();

also, make sure widget is at least 100,200 in size.

[graniteDev]

@mrjj

begin(ui->liveFeedWidget);

is in the code you provided. That is the widget within my ui that I want to paint to. I don't want to paint directly to "this" as "this" has buttons, and labels. I want to paint to the QWidget inside it's form ui->liveFeedWidget

My header file

protected:
    void paintEvent(QPaintEvent *) override;

My source file

void PointViewer::paintEvent(QPaintEvent *)
{
    QPen pen(Qt::green, 20, Qt::SolidLine, Qt::RoundCap);
    QPainter painter(this);
    painter.begin(ui->liveFeedWidget);
    painter.setPen(pen);
    painter.drawPoint(100,200);
    painter.end();
}

[mrjj]

Hi
Sorry didnt spot it before.
So goal is to paint from
PointViewer to ui->liveFeedWidget
or IS the ui->liveFeedWidget a PointViewer instance?

If liveFeedWidget is completely other widget , then it need to have the paintEvent and not
PointViewer.

You cannot paint from other widget to other widgets.

[graniteDev]

@mrjj Ah, ok

So, liveFeedWidget is a widget I added to the PointViewer class's pointviewer.ui form file. Is it possible to access it from PointViewer and override it's paintEvent without my making another class and promoting it to that class? I don't like to make extra classes if I can avoid it, it makes the code harder to follow for those that come behind me. Self contained classes are best if it can be achieved.

[mrjj]

@graniteDev said in Can't paint points on my widget:

liveFeedWidget

But is it a custom control ?
What type is it ?

[graniteDev]

@mrjj it's just a QWidget

[mrjj]

@graniteDev
Ok, you could use QLabel and paint to a pixmap and assign pixmap
to the label.
Else only way is to add paintEvent to custom widget.
PaintEvent is virtual and only way to override is to subclass.

[graniteDev]

@mrjj

This still didn't work

void PointViewer::paintEvent(QPaintEvent *)
{
    QPen pen(Qt::green, 20, Qt::SolidLine, Qt::RoundCap);
    QPainter painter(this);
    painter.setPen(pen);
    painter.drawPoint(100,200);
}

I'm expecting, that at 20 pixels I should be able to see SOME dot on the screen, but I see nothing

[mrjj]

@graniteDev

There should be a point at 100,200

insert qDebug() to be sure its called.

If you really dont want a custom widget for drawing, you can do like

QPixmap pix(200, 200);
pix.fill(Qt::blue);
QPainter painter(&pix);
painter.drawpoint(50, 50);
ui->label->setPixmap(pix); 

[graniteDev]

@mrjj WE HAVE A DOT!!!!!

UGH, I have no idea why the previous code did not work. But this looks like it does. I'm going to see if this works for me. I'm hoping that as it's updated with new data it does not flicker.

[mrjj]

@graniteDev
\o/
Long live the DOT.

Qwidgets uses double buffer system internally so often there is no flicker.

Did you use pixmap or paintEvent ?

[graniteDev]

@mrjj Crud, now I have a different problem, how do I draw different points with different sizes at the same time? Drawing many points of the same size is easy, that's readily handled, but I need to draw different sized points to denote how far or near a marker is that the point represents.

[graniteDev]

@mrjj said in Can't paint points on my widget:

@graniteDev
\o/
Long live the DOT.

Qwidgets uses double buffer system internally so often there is no flicker.

Did you use pixmap or paintEvent ?

void PointViewer::paintMarkers(int x, int y, int z)
{
    QPen pen(Qt::green, z, Qt::SolidLine, Qt::RoundCap);
    QPixmap pix(800,600);
    pix.fill(Qt::black);
    QPainter painter(&pix);
    painter.setPen(pen);
    painter.drawPoint(x,y);
    ui->pointViewerLabel->setPixmap(pix);
}

and I got a large green dot about where I expected to

[mrjj]

@graniteDev
Just include pointSize with the points.
like
struct PointData {
int x;
int y;
int z;
int DotSize
}

and give that to drawing routine.
And in paintEvent / for image, simply set penSize from data member
Its easy to handle as a struct as else u get many loose variables.

Since you clear the pixmap pr run.
pix.fill(Qt::black);
you need to draw all points each time. or keep pixmap as member in class and
draw on top each time.

where does the points come from ?

[graniteDev]

@mrjj There is a tracking camera that provides live x-y-z data. I'll have to apply a transform to make it fit in my viewing space...but that feels like the easy part right now, as I already have ready access to that data.

I don't quite follow what your saying, won't that only draw one dot at a time with the specified size? Then clear, and paint another dot of a different size, etc?

I'm using the z data to determine size, but the issue as I'm looking at the pen, is I can't assign a size to the point, because the size is applied to the pen, and the pen is applied to the painter, not the individual points that "drawPoints" draws.