QGraphicsScene/QGraphicsView performance problems

**bnilsson** · 5th January 2008, 11:39

Quote:
I suspect I would never get it working for a 150Mb data file if I use QGraphicsItems with QGraphicsScene/QGraphicsView.
I'm not so sure. One graphics item takes about 50 bytes of memory on average. How many items would you have in a 150MB file? 1M? That's less than 100MB of RAM used for the items.

I have a hard time believing that. A file of 600,000 shapes costs me about 500Mb of additional RAM, which indicates almost 1kb per item. I must be doing something seriously wrong....

I wouldn't use polygons here. You have a pretty simple shape, you can create your own item class which will be much faster.

Sorry, I am too new to Qt to implement such a thing. Could you direct me to some example?
Also, polygons are mostly only about 10-20% of the total population, the rest are rectangles. Still worthwile to make a new class?

**wysota** · 5th January 2008, 12:45

Originally Posted by bnilsson

I have a hard time believing that. A file of 600,000 shapes costs me about 500Mb of additional RAM, which indicates almost 1kb per item. I must be doing something seriously wrong....

Because you are using QGraphicsPolygonItem. You can probably use something simpler instead.

Originally Posted by bnilsson

Sorry, I am too new to Qt to implement such a thing. Could you direct me to some example?

See examples bundled with Qt.

Also, polygons are mostly only about 10-20% of the total population, the rest are rectangles. Still worthwile to make a new class?

Yes, because they are the most heavy of your items so they probably occupy about 50% of the memory used by items. A simple esitmate of the memory can be calculated as follows:
total_per_item = sizeof(QGraphicsItem) + sizeof(data_stored_within_item) where the latter is:

in case of a rect item: sizeof(QRectF) + sizeof(QPen) + sizeof(QBrush) = sizeof(QPointF)+sizeof(QSizeF) + sizeof(QPen)+sizeof(QBrush) = 2*sizeof(double) + 2*sizeof(double) + ...

in case of a polygon item: sizeof(QPolygonF) + sizeof(QPen) + sizeof(QBrush) = sizeof(QVector<QPointF>) + ... = n * sizeof(QPointF) + ... = n*2*double + ... = at least twice as much as for QRectF with n>=4.

So a rect item data occupies probably about 70-80 bytes and a polygon item probably around 100. Add all the additional stuff kept by all the classes to that and multiply by the number of items.

BTW. Also consider using fewer more complex items instead of more simpler ones. Maybe you can "connect" your trapezoids into more complex shapes?

**bnilsson** · 5th January 2008, 14:32

Thanks for your engagement, but I don't seen go get anywhere.
I managed to implement a subclass of QGraphicsItem, but it does not draw, I have no idea why. Could you help me?

Qt Code:

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#ifndef SHAPEITEM_H
#define SHAPEITEM_H
 
#include <QtGui> 
#include <QGraphicsItem>
#include <QObject>
 
class ShapeItem : public QGraphicsItem
{
 
 public:
				ShapeItem(void);
				ShapeItem(const QRect *rect);
				ShapeItem(const QPolygon *poly);
 
	QRectF		boundingRect() const;
	QPainterPath shape(void) const;
 
	virtual void		paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
						QWidget *widget);
private:
 
	int x0,y0,x1,y1,x2,y2,x3,y3;
	int xmin,ymin,xmax,ymax;
 };
 
#endif

#ifndef SHAPEITEM_H
#define SHAPEITEM_H

#include <QtGui> 
#include <QGraphicsItem>
#include <QObject>

class ShapeItem : public QGraphicsItem
{

 public:
				ShapeItem(void);
				ShapeItem(const QRect *rect);
				ShapeItem(const QPolygon *poly);

	QRectF		boundingRect() const;
	QPainterPath shape(void) const;

	virtual void		paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
						QWidget *widget);
private:
	
	int x0,y0,x1,y1,x2,y2,x3,y3;
	int xmin,ymin,xmax,ymax;
 };

#endif

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Qt Code:

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#include "shapeitem.h"
 
ShapeItem::ShapeItem(const QRect *rect)
	: QGraphicsItem()
{
	x0 = rect->x();
	y0 = rect->y();
 
	x1 = rect->x() + rect->width();
	y1 = y0;
 
	x2 = x1;
	y2 = y0 + rect->height();
 
	x3 = x0;
	y3 = y2;
 
	xmin = x0;
	xmax = x1;
	ymin = y0;
	ymax = y2;
 
}
 
ShapeItem::ShapeItem(const QPolygon *poly)
	: QGraphicsItem()
{
	poly->point(0,&x0,&y0);
	poly->point(1,&x1,&y1);
	poly->point(2,&x2,&y2);
	poly->point(3,&x3,&y3);
 
	xmin = xmax = x0;
	ymin = ymax = y0;
 
	if(x1<xmin) xmin = x1;
	if(x2<xmin) xmin = x2;
	if(x3<xmin) xmin = x3;
 
	if(y1<ymin) ymin = y1;
	if(y2<ymin) ymin = y2;
	if(y3<ymin) ymin = y3;
 
	if(x1>xmax) xmax = x1;
	if(x2>xmax) xmax = x2;
	if(x3>xmax) xmax = x3;
 
	if(y1>ymax) ymax = y1;
	if(y2>ymax) ymax = y2;
	if(y3>ymax) ymax = y3;
 
}
 
 
QRectF 
ShapeItem::boundingRect() const
{
         qreal penWidth = 1;
         return QRectF(xmin - penWidth / 2, ymin - penWidth / 2,
                       xmax + penWidth / 2, ymax + penWidth / 2);
}
 
QPainterPath
ShapeItem::shape() const
 {
     QPainterPath path;
     path.addRect(boundingRect());
     return path;
 }
 
 
void
ShapeItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
                QWidget *widget)
{
         painter->drawLine ( x0,y0,x1,y1 );
		 painter->drawLine ( x1,y1,x2,y2 );
		 painter->drawLine ( x2,y2,x3,y3 );
		 painter->drawLine ( x3,y3,x0,y0 );
}

#include "shapeitem.h"

ShapeItem::ShapeItem(const QRect *rect)
	: QGraphicsItem()
{
	x0 = rect->x();
	y0 = rect->y();
	
	x1 = rect->x() + rect->width();
	y1 = y0;
	
	x2 = x1;
	y2 = y0 + rect->height();
	
	x3 = x0;
	y3 = y2;
	
	xmin = x0;
	xmax = x1;
	ymin = y0;
	ymax = y2;

}

ShapeItem::ShapeItem(const QPolygon *poly)
	: QGraphicsItem()
{
	poly->point(0,&x0,&y0);
	poly->point(1,&x1,&y1);
	poly->point(2,&x2,&y2);
	poly->point(3,&x3,&y3);
	
	xmin = xmax = x0;
	ymin = ymax = y0;
	
	if(x1<xmin) xmin = x1;
	if(x2<xmin) xmin = x2;
	if(x3<xmin) xmin = x3;

	if(y1<ymin) ymin = y1;
	if(y2<ymin) ymin = y2;
	if(y3<ymin) ymin = y3;

	if(x1>xmax) xmax = x1;
	if(x2>xmax) xmax = x2;
	if(x3>xmax) xmax = x3;

	if(y1>ymax) ymax = y1;
	if(y2>ymax) ymax = y2;
	if(y3>ymax) ymax = y3;

}


QRectF 
ShapeItem::boundingRect() const
{
         qreal penWidth = 1;
         return QRectF(xmin - penWidth / 2, ymin - penWidth / 2,
                       xmax + penWidth / 2, ymax + penWidth / 2);
}

QPainterPath
ShapeItem::shape() const
 {
     QPainterPath path;
     path.addRect(boundingRect());
     return path;
 }


void
ShapeItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
                QWidget *widget)
{
         painter->drawLine ( x0,y0,x1,y1 );
		 painter->drawLine ( x1,y1,x2,y2 );
		 painter->drawLine ( x2,y2,x3,y3 );
		 painter->drawLine ( x3,y3,x0,y0 );
}

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Not at all optimized, as soon as it draws me anything in the scene, I will test different variants. At this stage, numItems=0 in myQGraphicsItems::drawIntems, so I'm stuck.

I am using it lile this:

Qt Code:

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ShapeItem item(&outRect);
	scene->addItem(&item);
	//scene->addRect(outRect,solidBlue);

ShapeItem item(&outRect);
	scene->addItem(&item);
	//scene->addRect(outRect,solidBlue);

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The commented-out line draws as it should, while the active lines does not.

**wysota** · 5th January 2008, 15:41

Create the item on heap.

Qt Code:

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ShapeItem *item = new ShapeItem(...);
scene->addItem(item);

ShapeItem *item = new ShapeItem(...);
scene->addItem(item);

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**bnilsson** · 5th January 2008, 15:56

Thanks, now it is working.
Unfortunately, now the app started gobbling up all memory even for a smaller file, so it got worse. I will have to figure out why...

**jacek** · 5th January 2008, 16:00

Originally Posted by bnilsson

Unfortunately, now the app started gobbling up all memory even for a smaller file, so it got worse. I will have to figure out why...

Use valgrind. The Massif tool might be especially helpful.

**bnilsson** · 5th January 2008, 16:36

Use valgrind. The Massif tool might be especially helpful.

For the moment I'm on MacOSX, so this is not applicable.

**bnilsson** · 5th January 2008, 16:39

The behaviour was a bit better using Qt4.4.0 than Qt4.3.3, so maybe it's not in my code entirely. But still in the 1Gb range for a 40Mb file.
The behaviour was very strange indeed (excessive cpu and memory load when zooming IN) on my amd46 Debian using Qt4.3.3, and this particular problem was fixed by the Qt4.4.0 snapshot.

Anyway, standard usage of QGraphicsView/QGraphiscScene was not dramatically improved using Qt4.4.0.

**wysota** · 5th January 2008, 18:30

Could you provide a minimal compilable example reproducing the problem so that we might try it ourselves?

**bnilsson** · 5th January 2008, 18:54

Sure, the project is not very big, I will try to prepare something.
I am new to this forum, what are the provisions for uploading?

**wysota** · 5th January 2008, 19:06

Use the attachments feature ("Manage attachments" button below the advanced editor).

**bnilsson** · 5th January 2008, 23:08

What would be your primary platform?

I will have to arrange some way for you to pick up the datafiles, they will be too large for an attachment. I think I have some space at my ISP's web hotel, I will do this tomorrow.
What will be your primary project platform?

Here is the project. It is currently tested only for Mac, I have not built this particular project for Linux yet.