Draw over an image, without replacing the pixels, save drawn image as standalone

[Charvi]

But if I make the overlay-ed pixmap transparent then user can't see what he draws ?
If I make it opaque then user can't see the base image ?

How to make the user see the overlay where he draws and background where he hasn't drawn ?

Thanks,
Charvi

[^NyAw^]

Hi,

The widget must not be transparent but the pixels on it must have the Alpha value to full transparency. When you draw on int you have to change the Alpha value and the RGB values to desired color.

[wysota]

But if I make the overlay-ed pixmap transparent then user can't see what he draws ?
If I make it opaque then user can't see the base image ?

How to make the user see the overlay where he draws and background where he hasn't drawn ?

Geez....

#include <QtWidgets>
 
class Widget : public QWidget {
public:
  Widget(QWidget *parent = 0) : QWidget(parent) {}
  void setImage(const QString &path) {
    m_base = QPixmap(path);
    m_mask = QImage(m_base.size(), QImage::Format_ARGB32);
    if(!m_mask.isNull()) m_mask.fill(Qt::transparent);
    updateGeometry();
  }
  QSize sizeHint() const { return m_base.size(); }
protected:
  void mousePressEvent(QMouseEvent *e) {
    if(!m_base.rect().contains(e->pos())) return;
    m_mask.setPixel(e->pos(), qRgba(255, 0, 0, 255));
    update();
  }
  void mouseMoveEvent(QMouseEvent *e) {
    if(!m_base.rect().contains(e->pos())) return;
    m_mask.setPixel(e->pos(), qRgba(255, 0, 0, 255));
    update();
  }
  void paintEvent(QPaintEvent *pe) {
    QPainter p(this);
    p.drawPixmap(0, 0, m_base);
    p.drawImage(0, 0, m_mask);
  }
private:
  QPixmap m_base;
  QImage m_mask;
};
 
int main(int argc, char **argv) {
  QApplication app(argc, argv);
  Widget w;
  w.setImage("/usr/share/icons/oxygen/256x256/apps/kword.png");
  w.show();
  return app.exec();
}

#include <QtWidgets>

class Widget : public QWidget {
public:
  Widget(QWidget *parent = 0) : QWidget(parent) {}
  void setImage(const QString &path) {
    m_base = QPixmap(path);
    m_mask = QImage(m_base.size(), QImage::Format_ARGB32);
    if(!m_mask.isNull()) m_mask.fill(Qt::transparent);
    updateGeometry();
  }
  QSize sizeHint() const { return m_base.size(); }
protected:
  void mousePressEvent(QMouseEvent *e) {
    if(!m_base.rect().contains(e->pos())) return;
    m_mask.setPixel(e->pos(), qRgba(255, 0, 0, 255));
    update();
  }
  void mouseMoveEvent(QMouseEvent *e) {
    if(!m_base.rect().contains(e->pos())) return;
    m_mask.setPixel(e->pos(), qRgba(255, 0, 0, 255));
    update();
  }
  void paintEvent(QPaintEvent *pe) {
    QPainter p(this);
    p.drawPixmap(0, 0, m_base);
    p.drawImage(0, 0, m_mask);
  }
private:
  QPixmap m_base;
  QImage m_mask;
};

int main(int argc, char **argv) {
  QApplication app(argc, argv);
  Widget w;
  w.setImage("/usr/share/icons/oxygen/256x256/apps/kword.png");
  w.show();
  return app.exec();
}

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[Charvi]

Also, how would i construct QPen to erase the overlay ?

[wysota]

Why would you need a pen for that?

[d_stranz]

Also, how would i construct QPen to erase the overlay ?

You take the code wysota already wrote for you, and you replace the alpha value with 0 instead of 255 in the setPixel() calls.

You seem to be confused about the difference between the virtual "pen", "eraser" and other tools you might use in a drawing application and a QPen used by QPainter. Totally unrelated concepts.

[Charvi]

To get smooth drawing I used the following code taking reference from the Scribble example.

QPainter painter(&m_mask);
//    if (eraseFlag == true)
//        painter.setPen(QPen(QBrush(QColor(0, 0, 0, 0)), myPenWidth, Qt::SolidLine, Qt::RoundCap,
//                        Qt::RoundJoin));
//    else
        painter.setPen(QPen(myPenColor, myPenWidth, Qt::SolidLine, Qt::RoundCap,
                        Qt::RoundJoin));
    painter.drawLine(lastPoint, endPoint);
 
    int rad = (myPenWidth / 2) + 2;
    update(QRect(lastPoint, endPoint).normalized()
                                     .adjusted(-rad, -rad, +rad, +rad));
    lastPoint = endPoint;

QPainter painter(&m_mask);
//    if (eraseFlag == true)
//        painter.setPen(QPen(QBrush(QColor(0, 0, 0, 0)), myPenWidth, Qt::SolidLine, Qt::RoundCap,
//                        Qt::RoundJoin));
//    else
        painter.setPen(QPen(myPenColor, myPenWidth, Qt::SolidLine, Qt::RoundCap,
                        Qt::RoundJoin));
    painter.drawLine(lastPoint, endPoint);

    int rad = (myPenWidth / 2) + 2;
    update(QRect(lastPoint, endPoint).normalized()
                                     .adjusted(-rad, -rad, +rad, +rad));
    lastPoint = endPoint;

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where

myPenWidth = 3;
    myPenColor = Qt::red;

myPenWidth = 3;
    myPenColor = Qt::red;

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Trying to implement erasing in this.

[Charvi]

One more feature, sorry to extend in the same question but it's because the context has already been set.

In order to implement scaling on the wheel event I did the following:

void
LT_DrawingCanvas::wheelEvent(QWheelEvent *event)
{
    QWidget::wheelEvent(event);
    scaleFactor += ((float)event->delta()/1200.0);
    update();
}

void
LT_DrawingCanvas::wheelEvent(QWheelEvent *event)
{
    QWidget::wheelEvent(event);
    scaleFactor += ((float)event->delta()/1200.0);
    update();
}

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and in the paintEvent:

void
LT_DrawingCanvas::paintEvent(QPaintEvent *pe)
{
    QPainter p(this);
    p.scale(scaleFactor, scaleFactor);
    p.drawPixmap(0, 0, m_base);
    p.drawImage(0, 0, m_mask);
}

void
LT_DrawingCanvas::paintEvent(QPaintEvent *pe)
{
    QPainter p(this);
    p.scale(scaleFactor, scaleFactor);
    p.drawPixmap(0, 0, m_base);
    p.drawImage(0, 0, m_mask);
}

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My widget hierarchy is as below:
Mainwindow
- Grid Layout
- Scrollarea
- MyCustomDrawing Canvas Widget (created just like the code wysota suggested above)

I am not getting the scrollbars on scaling the image. I realize I am drawing the scaled image again, instead of resizing the widget. I tried using resize on the widget but it didn't work. What I am doing wrong ?

[wysota]

Set the widgetResizable property of the scroll area to true.

[Charvi]

Okay so for erasing I did the following, may be this will be useful to someone.

QPainter painter(&m_mask);
painter.setCompositionMode(QPainter::CompositionMode_Source);
painter.setPen(QPen(QBrush(Qt::transparent), myPenWidth, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));

QPainter painter(&m_mask);
painter.setCompositionMode(QPainter::CompositionMode_Source);
painter.setPen(QPen(QBrush(Qt::transparent), myPenWidth, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));

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To understand the Composition Modes : http://doc.qt.io/qt-5/qpainter.html#...itionMode-enum


Thanks.

P.S.: Still working on the scrollarea stuff. Once done will update here.

[Kryzon]

I think it might be faster to use composition mode CLEAR. It erases whatever pixels are there.