How to partially draw a QGraphicsPixmapItem

[Damien]

Hi,

I am making a widget for comparing 2 images. LUMINAR for example has what I want to do. One image on the left, another one on the right and a slider to define how much of the left and right images to show

Is it possible to partially draw a pixmap set to a QGraphicsPixmapItem or set a transparency mask?

All I found so far would be to store the QPixmap call QPixmap::copy, set a new pixmap to each QGraphicsPixmapItem and set the position of both images.

It doesn't sound like an efficient way of do this.

[Asperamanca]

@Damien
In such a case I would not use the QGraphicsPixmapItem (not much code in it, anyway), but rather create a custom QGraphicsItem that simply paints only the portion of the pixmap you want.

[Damien]

@Asperamanca said in How to partially draw a QGraphicsPixmapItem:

case I would not use the QGraphicsPixmapItem (not much code in it, anyway), but rather create a custom QGraphicsItem that simply paints only the portion of the p

Thanks @Asperamanca. I went that route and it was very straight forward. Performance is acceptable too.

imageitem.h

#pragma once
#include <QGraphicsItem>

class ImageItem : public QGraphicsItem
{
public:
	ImageItem(QGraphicsItem* parent = Q_NULLPTR);
	~ImageItem();

public:
	void setImage(const QImage& pixmap);
	QRect imageRect() const;
	QRectF boundingRect() const;
	void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget = Q_NULLPTR);
	void setSubRegion(const QRectF& rect);

protected:
	QImage m_image;
	QRectF m_boundingRect;
	QRectF m_target;
	QRectF m_souce;
};

imageitem.cpp

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

ImageItem::ImageItem(QGraphicsItem* parent /*= Q_NULLPTR*/)
	: QGraphicsItem(parent)
{
}

ImageItem::~ImageItem()
{
}

void ImageItem::setImage(const QImage & image)
{
	m_image = image;
	m_boundingRect = m_image.rect();
}

QRect ImageItem::imageRect() const
{
	return m_image.rect();
}

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

void ImageItem::paint(QPainter * painter, const QStyleOptionGraphicsItem * option, QWidget * widget)
{
	Q_UNUSED(option);
	Q_UNUSED(widget);
	painter->drawImage(m_target, m_image, m_souce);
}

bool ImageItem::setSubRegion(const QRect & rect)
{
	QRect imageRect = m_image.rect();

	if (imageRect.contains(rect))
	{
		m_souce = rect;
		m_target = QRect(0, 0, rect.width(), rect.height());
		m_boundingRect = m_target;
		update();
		return true;
	}
	else
	{
		return false;
	}	
}