Qt: Overlapping semitransparent QgraphicsItem

Question

I've been working with QGraphicsView for a while and I am facing a requisite that I am not sure if it can be fulfilled by using this framework.

Putting it as simple as possible, I have 2 overlapping RectItem with a semitransparent QBrush (the same one for both). Is it possible to prevent the overlapping area from becoming more opaque? I just want the whole area to have the same color (this will occur only if both rects are fully opaque, but sometimes that is not the case)

I know it might seem a weird requisite, but the old graphics engine my colleagues used allowed it.

Any ideas?

Answer 1

Qt provides various blend (composition) modes for the QPainter . Deriving your RectItem class from QGraphicsItem or QGraphicsObject, allows you to customise the painting and using the composition modes , create various effects, as demonstrated in the Qt Example .

If you want two semi-transparent items overlapping without changing the colour (assuming their colour is the same), either the QPainter::CompositionMode_Difference mode , or CompositionMode_Exclusion will do this. Here's example code of such an object: -

Header

#ifndef RECTITEM_H
#define RECTITEM_H

#include <QGraphicsItem>
#include <QColor>

class RectItem : public QGraphicsItem
{
public:
    RectItem(int width, int height, QColor colour);
    ~RectItem();

    QRectF boundingRect() const;

private:
    QRectF m_boundingRect;
    QColor m_colour;

    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget = 0);
};

#endif // RECTITEM_H

Implementation

#include "rectitem.h"
#include <QPainter>

RectItem::RectItem(int width, int height, QColor colour)
    : QGraphicsItem(), m_boundingRect(-width/2, -height/2, width, height), m_colour(colour)
{    
    setFlag(QGraphicsItem::ItemIsSelectable);
    setFlag(QGraphicsItem::ItemIsMovable);
}

RectItem::~RectItem()
{
}

QRectF RectItem::boundingRect() const
{
    return m_boundingRect;
}

void RectItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
{
    painter->setCompositionMode(QPainter::CompositionMode_Difference);
    painter->setBrush(m_colour);
    painter->drawRect(m_boundingRect);
}

You can now create two RectItem objects of the same semi-transparent colour and add them to the scene

// assuming the scene and view are setup and m_pScene is a pointer to the scene

RectItem* pItem = new RectItem(50, 50, QColor(255, 0, 0, 128));
pItem->setPos(10, 10);
m_pScene->addItem(pItem);

pItem = new RectItem(50, 50, QColor(255, 0, 0, 128));
pItem->setPos(80, 80);
m_pScene->addItem(pItem);