QPainter: line drawing along a circle

[Jonny174]

Hi All!
I have a class:
widget.h

class Widget : public QWidget
{
  Q_OBJECT
 
  QPoint mouseOldPos;
  QPoint mousePos;
  QImage *image;
 
  public:
    Widget( QWidget *parent = 0 );
    ~Widget();
 
  protected:
    void paintEvent( QPaintEvent *e );
    void draw( QPainter *painter );
    void mousePressEvent( QMouseEvent *e );
    void mouseReleaseEvent( QMouseEvent *e );
    void mouseMoveEvent( QMouseEvent *e );
 
  protected slots:
    void createImage();
};

class Widget : public QWidget
{
  Q_OBJECT

  QPoint mouseOldPos;
  QPoint mousePos;
  QImage *image;

  public:
    Widget( QWidget *parent = 0 );
    ~Widget();

  protected:
    void paintEvent( QPaintEvent *e );
    void draw( QPainter *painter );
    void mousePressEvent( QMouseEvent *e );
    void mouseReleaseEvent( QMouseEvent *e );
    void mouseMoveEvent( QMouseEvent *e );
        
  protected slots:
    void createImage();
};

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widget.cpp

Widget::Widget( QWidget *parent ):
  QWidget( parent ),
  image(0)
{
  setAttribute( Qt::WA_StaticContents );
  setCursor( Qt::CrossCursor );
  setMouseTracking( true );
  setFocusPolicy( Qt::WheelFocus );
  setFocus();
 
  setFixedSize( 800, 600 );
 
  QTimer::singleShot( 500, this, SLOT(createImage()) );
}
//=================================================================================================
Widget::~Widget()
{
  if (image)
  {
    delete image;
    image = 0;
  }
}
//=================================================================================================
void Widget::paintEvent( QPaintEvent *e )
{
  if (image)
  {
    QPainter imagePainter( image );
    imagePainter.setRenderHint( QPainter::Antialiasing, true );
    imagePainter.eraseRect( rect() );
    draw( &imagePainter );
    imagePainter.end();
 
    QPainter painter( this );
    QRect dirtyRect = e->rect();
    painter.drawImage( dirtyRect, (*image), dirtyRect );
  }
}
//=================================================================================================
void Widget::draw( QPainter *painter )
{
  painter->save();
    painter->translate( width() / 2, height() / 2 );
    painter->setPen( QColor( 0, 0, 0, 255 ) );
    painter->drawEllipse( QPoint( 0, 90 ), 90, 90 );
  painter->restore();
 
  painter->save();
    painter->setPen( QPen( Qt::green, 2, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin ) );
    painter->drawLine( QPoint( width() / 2, height() / 2 + 90 ), mousePos );
  painter->restore();
}
//=================================================================================================
void Widget::mousePressEvent( QMouseEvent *e )
{
  if (e->buttons() & Qt::LeftButton)
  {
    mouseOldPos = e->pos();
    update();
  }
}
//=================================================================================================
void Widget::mouseReleaseEvent( QMouseEvent *e )
{
  update();
  e->accept();
}
//=================================================================================================
void Widget::mouseMoveEvent( QMouseEvent *e )
{
  mousePos = e->pos();
  update();
}
//=================================================================================================
void Widget::createImage()
{
  if (!image)
  {
    image = new QImage( size(), QImage::Format_ARGB32_Premultiplied );
    image->fill( Qt::white );
 
    update();
  }
}

Widget::Widget( QWidget *parent ):
  QWidget( parent ),
  image(0)
{
  setAttribute( Qt::WA_StaticContents );
  setCursor( Qt::CrossCursor );
  setMouseTracking( true );
  setFocusPolicy( Qt::WheelFocus );
  setFocus();

  setFixedSize( 800, 600 );

  QTimer::singleShot( 500, this, SLOT(createImage()) );
}
//=================================================================================================
Widget::~Widget()
{
  if (image)
  {
    delete image;
    image = 0;
  }
}
//=================================================================================================
void Widget::paintEvent( QPaintEvent *e )
{
  if (image)
  {
    QPainter imagePainter( image );
    imagePainter.setRenderHint( QPainter::Antialiasing, true );
    imagePainter.eraseRect( rect() );
    draw( &imagePainter );
    imagePainter.end();

    QPainter painter( this );
    QRect dirtyRect = e->rect();
    painter.drawImage( dirtyRect, (*image), dirtyRect );
  }
}
//=================================================================================================
void Widget::draw( QPainter *painter )
{
  painter->save();
    painter->translate( width() / 2, height() / 2 );
    painter->setPen( QColor( 0, 0, 0, 255 ) );
    painter->drawEllipse( QPoint( 0, 90 ), 90, 90 );
  painter->restore();

  painter->save();
    painter->setPen( QPen( Qt::green, 2, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin ) );
    painter->drawLine( QPoint( width() / 2, height() / 2 + 90 ), mousePos );
  painter->restore();
}
//=================================================================================================
void Widget::mousePressEvent( QMouseEvent *e )
{
  if (e->buttons() & Qt::LeftButton)
  {
    mouseOldPos = e->pos();
    update();
  }
}
//=================================================================================================
void Widget::mouseReleaseEvent( QMouseEvent *e )
{
  update();
  e->accept();
}
//=================================================================================================
void Widget::mouseMoveEvent( QMouseEvent *e )
{
  mousePos = e->pos();
  update();
}
//=================================================================================================
void Widget::createImage()
{
  if (!image)
  {
    image = new QImage( size(), QImage::Format_ARGB32_Premultiplied );
    image->fill( Qt::white );

    update();
  }
}

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How can I draw part of line, selected in red?
Attachment 7504
http://saveimg.ru/show-image.php?id=...2f4d7b2c54d30a

[ChrisW67]

Try again. The attachment is broken.

Oh, and have a look at QPainter::drawArc() just in case that's what you were looking for.


Edit: Now you have posted a picture I have no idea what you want. What does "along a circle" have to do with the straight line segment in red? Seems like a simple case of calculate the end points, set a pen, draw a line.

[Jonny174]

SOLVE:

float mouse_radius = getDistanceBetweenPoints( width() / 2, height() / 2 + 90, mousePos.x(), mousePos.y() );
 
float pX = width() / 2 + 90 * (mousePos.x() - width() / 2) / mouse_radius;
float pY = (height() / 2 + 90) + 90 * (mousePos.y() - (height() / 2 + 90)) / mouse_radius;
 
float pX2 = width() / 2 + 80 * (mousePos.x() - width() / 2) / mouse_radius;
float pY2 = (height() / 2 + 90) + 80 * (mousePos.y() - (height() / 2 + 90)) / mouse_radius;
 
painter->drawLine( int( pX2 ), int( pY2 ), int( pX ), int( pY ) );
 
 
 
float getVectorLength( float dx, float dy )
{
  return qSqrt( dx * dx + dy * dy );
}
 
float getDistanceBetweenPoints( float x1, float y1, float x2, float y2 )
{
  return getVectorLength( x2 - x1, y2 - y1 );
}

float mouse_radius = getDistanceBetweenPoints( width() / 2, height() / 2 + 90, mousePos.x(), mousePos.y() );

float pX = width() / 2 + 90 * (mousePos.x() - width() / 2) / mouse_radius;
float pY = (height() / 2 + 90) + 90 * (mousePos.y() - (height() / 2 + 90)) / mouse_radius;

float pX2 = width() / 2 + 80 * (mousePos.x() - width() / 2) / mouse_radius;
float pY2 = (height() / 2 + 90) + 80 * (mousePos.y() - (height() / 2 + 90)) / mouse_radius;

painter->drawLine( int( pX2 ), int( pY2 ), int( pX ), int( pY ) );



float getVectorLength( float dx, float dy )
{
  return qSqrt( dx * dx + dy * dy );
}

float getDistanceBetweenPoints( float x1, float y1, float x2, float y2 )
{
  return getVectorLength( x2 - x1, y2 - y1 );
}

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