Smooth text scrolling

[francesco.frankie]

Hello!
I'm writing a program that should render the text embedded in a QTextDocument on a QGraphicsView, the text must be scaled and smoothly scrolled.
To do this job I've inherited the class QGraphicsItem and implemented the funciont paint, after the item is added to a QGraphicsScene that calls the function advance() every 30 milliseconds.

here is the code of the paint function:

void ScrollTextGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget){
 
    if(currentDocument){
 
   painter->setRenderHint(QPainter::SmoothPixmapTransform);
 
   painter->translate(0,-scrollValue);
 
    painter->scale(renderScale.width(),renderScale.height());
 
    QAbstractTextDocumentLayout::PaintContext pcontext;
    pcontext.palette.setColor(QPalette::Text, scrollScene->getDefaultTextColor());
 
    pcontext.clip = QRectF(0,scrollValue/renderScale.height(),
    renderSize.width()/renderScale.width(),
    renderSize.height()/renderScale.height());
 
    currentDocument->documentLayout()->draw(painter,pcontext);
 
    }
}

void ScrollTextGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget){

    if(currentDocument){

   painter->setRenderHint(QPainter::SmoothPixmapTransform);
 
   painter->translate(0,-scrollValue);

    painter->scale(renderScale.width(),renderScale.height());

    QAbstractTextDocumentLayout::PaintContext pcontext;
    pcontext.palette.setColor(QPalette::Text, scrollScene->getDefaultTextColor());

    pcontext.clip = QRectF(0,scrollValue/renderScale.height(),
    renderSize.width()/renderScale.width(),
    renderSize.height()/renderScale.height());

    currentDocument->documentLayout()->draw(painter,pcontext);

    }
}

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The matter is if the result is almost reasonable for light-weight documents and for low scale factor, the performances turn down even for a small portion of text (few lines) scaled and drawn in a window 800x600.
Did i forget some optimization? Are there ways for improve the draw algorithm (for example using a QGLWidget)?
Have you some hints or suggestions?


thanks for the attention
Regards

[wysota]

Is there a reason why you didn't use QGraphicsTextItem?

[francesco.frankie]

Thank you for your reply.
Yes, I've tried it, but the performance problem persist.
Do you have other advices?

[wysota]

What is the performance problem? Please provide an example.

[francesco.frankie]

OK, I've made some test. Although the CPU level is low, the scroll animation isn't smooth, indeed depends on the resolution of the window where the scene is placed. I think the matter is resize and scale the scene...
For the animation I connect a timeout event of a timer with the function "advance" of the scene. The painter of the graphics item is moved by a number of pixel depends on the time elapsed sine last drawing.

[tbscope]

That's still not an example.

Here are a few:
http://doc.qt.nokia.com/4.7/examples-animation.html

I made a little test with a graphicsview and a qgraphicsproxywidget containing a label and using qpropertyanimation. The animation is smooth.

[francesco.frankie]

thank you guys for the attention and the support, and sorry if i wasn't able to provide a clear question.
My program should scroll in a full screen window a very large QTextDocument, about 5 lines in the same moment. I've tried several ways to perform the animation and render the text, but I wasn't able to obtain a real smooth and flicker-free animation.
In your opinion, which is the best way to do that? I followed the Animation tutorial and read articles about QT Graphics View Framework, but I didn't find the answer.

[wysota]

Why don't you just write a short (up to 30 lines of code) example demonstrating the problem?

[francesco.frankie]

Why don't you just write a short (up to 30 lines of code) example demonstrating the problem?

Well, here is a quick example about what i told.
I made a main window with a QTextEdit and a PushButton, let's try to copy and paste some text and click "start"
In this case I used a QGraphicsTextItem (as suggested by wysota) a timer and the "translate" method to arrange the scrolling.
The result (at least on my computer) flickers and isn't smooth.
Ok, this is a real simple example, but starting from this code, how can i improve the results?
Or...should I chose a totally different way?

//file: mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QtGui/QApplication>
#include <QMainWindow>
#include <QGraphicsTextItem>
#include <QPushButton>
#include <QTextEdit>
#include <QGraphicsView>
#include <QGraphicsTextItem>
#include <QTimeLine>
#include <QTimer>
#include <QTime>
#include <QRect>
#include <QPushButton>
#include <QVBoxLayout>
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0) : QMainWindow(parent){
        QVBoxLayout* layout = new QVBoxLayout();
 
        pushButton = new QPushButton("start");
        textEdit = new QTextEdit();
        layout->addWidget(textEdit);
        layout->addWidget(pushButton);
        QWidget* central = new QWidget();
        central->setLayout(layout);
        setCentralWidget(central);
        connect(pushButton, SIGNAL(clicked()),SLOT(start()));
    }
 
    ~MainWindow(){
 
    }
 
    QGraphicsView* view;
    QGraphicsTextItem * text ;
    QGraphicsScene* scene;
    QPushButton* pushButton;
    QTextEdit* textEdit;
    QTime* timeSinceLastUpdate;
 
 
public slots:
 
    void start(){
        scene = new QGraphicsScene();
        text = new QGraphicsTextItem();
        text->setDocument(textEdit->document());
        text->scale(6,6);
        text->setPos(-100,0);
        text->setTextWidth(140);
 
        scene->addItem(text);
        scene->setSceneRect(0,0,400,400);
        view = new QGraphicsView(scene);
        view->fitInView(view->sceneRect(),Qt::KeepAspectRatio);
        view->setRenderHint(QPainter::SmoothPixmapTransform);
        view->showFullScreen();
 
        QTimer* sctT = new QTimer();
        connect(sctT,SIGNAL(timeout()),SLOT(scroll()));
 
        sctT->start(30);
 
        timeSinceLastUpdate = new QTime();
        timeSinceLastUpdate->start();
    }
 
    void scroll(){
        int time = timeSinceLastUpdate->elapsed(); //make the scrolling indipendent from frame rate
        float scrollValue = (40.0 * float(time))/1000.0;
        text->translate(0,-scrollValue);
        timeSinceLastUpdate->restart();
 
    }
};
 
 
 
 
#endif // MAINWINDOW_H
 
file: main.cpp
 
#include <QtGui/QApplication>
 
#include "mainwindow.h"
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
 
    return a.exec();
}

//file: mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QtGui/QApplication>
#include <QMainWindow>
#include <QGraphicsTextItem>
#include <QPushButton>
#include <QTextEdit>
#include <QGraphicsView>
#include <QGraphicsTextItem>
#include <QTimeLine>
#include <QTimer>
#include <QTime>
#include <QRect>
#include <QPushButton>
#include <QVBoxLayout>

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0) : QMainWindow(parent){
        QVBoxLayout* layout = new QVBoxLayout();

        pushButton = new QPushButton("start");
        textEdit = new QTextEdit();
        layout->addWidget(textEdit);
        layout->addWidget(pushButton);
        QWidget* central = new QWidget();
        central->setLayout(layout);
        setCentralWidget(central);
        connect(pushButton, SIGNAL(clicked()),SLOT(start()));
    }

    ~MainWindow(){

    }

    QGraphicsView* view;
    QGraphicsTextItem * text ;
    QGraphicsScene* scene;
    QPushButton* pushButton;
    QTextEdit* textEdit;
    QTime* timeSinceLastUpdate;


public slots:

    void start(){
        scene = new QGraphicsScene();
        text = new QGraphicsTextItem();
        text->setDocument(textEdit->document());
        text->scale(6,6);
        text->setPos(-100,0);
        text->setTextWidth(140);

        scene->addItem(text);
        scene->setSceneRect(0,0,400,400);
        view = new QGraphicsView(scene);
        view->fitInView(view->sceneRect(),Qt::KeepAspectRatio);
        view->setRenderHint(QPainter::SmoothPixmapTransform);
        view->showFullScreen();

        QTimer* sctT = new QTimer();
        connect(sctT,SIGNAL(timeout()),SLOT(scroll()));

        sctT->start(30);

        timeSinceLastUpdate = new QTime();
        timeSinceLastUpdate->start();
    }

    void scroll(){
        int time = timeSinceLastUpdate->elapsed(); //make the scrolling indipendent from frame rate
        float scrollValue = (40.0 * float(time))/1000.0;
        text->translate(0,-scrollValue);
        timeSinceLastUpdate->restart();

    }
};




#endif // MAINWINDOW_H

file: main.cpp

#include <QtGui/QApplication>

#include "mainwindow.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

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Thank for the support!

[wysota]

The animation is fluent on my machine but you can try this:

#include <QtGui>
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0) : QMainWindow(parent){
        QVBoxLayout* layout = new QVBoxLayout();
 
        pushButton = new QPushButton("start");
        textEdit = new QTextEdit();
        layout->addWidget(textEdit);
        layout->addWidget(pushButton);
        QWidget* central = new QWidget();
        central->setLayout(layout);
        setCentralWidget(central);
        connect(pushButton, SIGNAL(clicked()),SLOT(start()));
    }
 
    QGraphicsView* view;
    QGraphicsTextItem * text ;
    QGraphicsScene* scene;
    QPushButton* pushButton;
    QTextEdit* textEdit;
 
public slots:
 
    void start(){
        scene = new QGraphicsScene();
        text = new QGraphicsTextItem();
        text->setDocument(textEdit->document());
        text->scale(6,6);
        text->setPos(-100,0);
        text->setTextWidth(140);
 
        scene->addItem(text);
        scene->setSceneRect(0,0,400,400);
        view = new QGraphicsView(scene);
        view->fitInView(view->sceneRect(),Qt::KeepAspectRatio);
        view->setRenderHint(QPainter::SmoothPixmapTransform);
        view->showFullScreen();
 
        // the important part:
        QPropertyAnimation *anim = new QPropertyAnimation(text, "pos", text);
	anim->setStartValue(text->pos());
	anim->setEndValue(QPointF(-100, -1000));
	anim->setDuration(5000);
	anim->start();
    }
};
 
#include "main.moc"
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
 
    return a.exec();
}

#include <QtGui>

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0) : QMainWindow(parent){
        QVBoxLayout* layout = new QVBoxLayout();

        pushButton = new QPushButton("start");
        textEdit = new QTextEdit();
        layout->addWidget(textEdit);
        layout->addWidget(pushButton);
        QWidget* central = new QWidget();
        central->setLayout(layout);
        setCentralWidget(central);
        connect(pushButton, SIGNAL(clicked()),SLOT(start()));
    }

    QGraphicsView* view;
    QGraphicsTextItem * text ;
    QGraphicsScene* scene;
    QPushButton* pushButton;
    QTextEdit* textEdit;

public slots:

    void start(){
        scene = new QGraphicsScene();
        text = new QGraphicsTextItem();
        text->setDocument(textEdit->document());
        text->scale(6,6);
        text->setPos(-100,0);
        text->setTextWidth(140);

        scene->addItem(text);
        scene->setSceneRect(0,0,400,400);
        view = new QGraphicsView(scene);
        view->fitInView(view->sceneRect(),Qt::KeepAspectRatio);
        view->setRenderHint(QPainter::SmoothPixmapTransform);
        view->showFullScreen();

        // the important part:
        QPropertyAnimation *anim = new QPropertyAnimation(text, "pos", text);
	anim->setStartValue(text->pos());
	anim->setEndValue(QPointF(-100, -1000));
	anim->setDuration(5000);
	anim->start();
    }
};

#include "main.moc"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

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