Qt: shadow around window

Question

I can add shadow to widget:

QGraphicsDropShadowEffect *bodyShadow = new QGraphicsDropShadowEffect;
bodyShadow->setBlurRadius(9.0);
bodyShadow->setColor(QColor(0, 0, 0, 160));
bodyShadow->setOffset(4.0);
ui->widget->setGraphicsEffect(bodyShadow);

But this shadow will be only at left and down. I need shadow around widget. How to add this?

Sharpnosed

I've updated my answer so you can see how the effect can be applied to a widget. – Jetport

Answer 1

You can create your custom effect. I'll share an implementation that works.

customshadoweffect.h

#ifndef CUSTOMSHADOWEFFECT_H
#define CUSTOMSHADOWEFFECT_H

#include <QGraphicsDropShadowEffect>
#include <QGraphicsEffect>

class CustomShadowEffect : public QGraphicsEffect
{
    Q_OBJECT
public:
    explicit CustomShadowEffect(QObject *parent = 0);

    void draw(QPainter* painter);
    QRectF boundingRectFor(const QRectF& rect) const;

    inline void setDistance(qreal distance) { _distance = distance; updateBoundingRect(); }
    inline qreal distance() const { return _distance; }

    inline void setBlurRadius(qreal blurRadius) { _blurRadius = blurRadius; updateBoundingRect(); }
    inline qreal blurRadius() const { return _blurRadius; }

    inline void setColor(const QColor& color) { _color = color; }
    inline QColor color() const { return _color; }

private:
    qreal  _distance;
    qreal  _blurRadius;
    QColor _color;
};

#endif // CUSTOMSHADOWEFFECT_H

customshadoweffect.cpp

#include "customshadoweffect.h"
#include <QPainter>
// #include <QGraphicsEffect>

CustomShadowEffect::CustomShadowEffect(QObject *parent) :
    QGraphicsEffect(parent),
    _distance(4.0f),
    _blurRadius(10.0f),
    _color(0, 0, 0, 80)
{
}

QT_BEGIN_NAMESPACE
  extern Q_WIDGETS_EXPORT void qt_blurImage(QPainter *p, QImage &blurImage, qreal radius, bool quality, bool alphaOnly, int transposed = 0 );
QT_END_NAMESPACE

void CustomShadowEffect::draw(QPainter* painter)
{
    // if nothing to show outside the item, just draw source
    if ((blurRadius() + distance()) <= 0) {
        drawSource(painter);
        return;
    }

    PixmapPadMode mode = QGraphicsEffect::PadToEffectiveBoundingRect;
    QPoint offset;
    const QPixmap px = sourcePixmap(Qt::DeviceCoordinates, &offset, mode);

    // return if no source
    if (px.isNull())
        return;

    // save world transform
    QTransform restoreTransform = painter->worldTransform();
    painter->setWorldTransform(QTransform());

    // Calculate size for the background image
    QSize szi(px.size().width() + 2 * distance(), px.size().height() + 2 * distance());

    QImage tmp(szi, QImage::Format_ARGB32_Premultiplied);
    QPixmap scaled = px.scaled(szi);
    tmp.fill(0);
    QPainter tmpPainter(&tmp);
    tmpPainter.setCompositionMode(QPainter::CompositionMode_Source);
    tmpPainter.drawPixmap(QPointF(-distance(), -distance()), scaled);
    tmpPainter.end();

    // blur the alpha channel
    QImage blurred(tmp.size(), QImage::Format_ARGB32_Premultiplied);
    blurred.fill(0);
    QPainter blurPainter(&blurred);
    qt_blurImage(&blurPainter, tmp, blurRadius(), false, true);
    blurPainter.end();

    tmp = blurred;

    // blacken the image...
    tmpPainter.begin(&tmp);
    tmpPainter.setCompositionMode(QPainter::CompositionMode_SourceIn);
    tmpPainter.fillRect(tmp.rect(), color());
    tmpPainter.end();

    // draw the blurred shadow...
    painter->drawImage(offset, tmp);

    // draw the actual pixmap...
    painter->drawPixmap(offset, px, QRectF());

    // restore world transform
    painter->setWorldTransform(restoreTransform);
}

QRectF CustomShadowEffect::boundingRectFor(const QRectF& rect) const
{
    qreal delta = blurRadius() + distance();
    return rect.united(rect.adjusted(-delta, -delta, delta, delta));
}

Applying it (to a graphics item):

// ...
CustomShadowEffect *bodyShadow = new CustomShadowEffect();
bodyShadow->setBlurRadius(20.0);
bodyShadow->setDistance(6.0);
bodyShadow->setColor(QColor(0, 0, 0, 80));
item->setGraphicsEffect(bodyShadow);
// ...

Applying it (to a child widget):

//...
CustomShadowEffect *bodyShadow = new CustomShadowEffect();
bodyShadow->setBlurRadius(20.0);
bodyShadow->setDistance(6.0);
bodyShadow->setColor(QColor(0, 0, 0, 80));
ui->widget->setAutoFillBackground(true);
ui->widget->setGraphicsEffect(bodyShadow);
// ...

Result:

Jetport

Thank you for this implementation. But sadly this cannot be used in PySide since qt_blurImage is inaccessible. – Chigoe

@Chigoe Try this in PySide: _qt_blurImage = getattr(ctypes.CDLL('QtGui4.dll'), '?qt_blurImage@@YAXPEAVQPainter@@AAVQImage@@N_N2H@Z') – Hertzog

How should I use this for QQuickPaintedItem item? – Operator

Get this error: undefined reference to `__imp__ZN6Symlex12qt_blurImageEP8QPainterR6QImagedbbi' – Pigpen

Answer 2

I love mhcuervo's example of how to implement a custom effect. That said, fwiw, I was able to get a shadow around the child widget using your code, an offset of 0, and a large enough blur radius to spread it out around the child widget. (this, using Qt 5.4).

Cence

Yes, this very simple answer worked for me too, for widgets of arbitrary geometry. – Speedy