QWMatrix

NAME

SYNOPSIS

Public Members

Static Public Members

RELATED FUNCTION DOCUMENTATION

DESCRIPTION

The standard coordinate system of a paint device has the origin located at the top-left position. X values increase to the right; Y values increase downward.

This coordinate system is the default for the QPainter, which renders graphics in a paint device. A user-defined coordinate system can be specified by setting a QWMatrix for the painter.

Example:

        MyWidget::paintEvent( QPaintEvent * )


        {


            QPainter p;                      // our painter


            QWMatrix m;                      // our transformation matrix


            m.rotate( 22.5 );                // rotated coordinate system


            p.begin( this );                 // start painting


            p.setWorldMatrix( m );           // use rotated coordinate system


            p.drawText( 30,20, "detator" );  // draw rotated text at 30,20


            p.end();                         // painting done


        }

A matrix specifies how to translate, scale, shear or rotate the graphics; the actual transformation is performed by the drawing routines in QPainter and by QPixmap::xForm().

The QWMatrix class contains a 3x3 matrix of the form:

A matrix transforms a point in the plane to another point:

        x' = m11*x + m21*y + dx


        y' = m22*y + m12*x + dy

The point (x, y) is the original point, and (x', y') is the transformed point. (x', y') can be transformed back to (x, y) by performing the same operation on the inverted matrix.

The elements dx and dy specify horizontal and vertical translation. The elements m11 and m22 specify horizontal and vertical scaling. The elements m12 and m21 specify horizontal and vertical shearing.

The identity matrix has m11 and m22 set to 1; all others are set to 0. This matrix maps a point to itself.

Translation is the simplest transformation. Setting dx and dy will move the coordinate system dx units along the X axis and dy units along the Y axis.

Scaling can be done by setting m11 and m22. For example, setting m11 to 2 and m22 to 1.5 will double the height and increase the width by 50%.

Shearing is controlled by m12 and m21. Setting these elements to values different from zero will twist the coordinate system.

Rotation is achieved by carefully setting both the shearing factors and the scaling factors. The QWMatrix also has a function that sets rotation directly.

QWMatrix lets you combine transformations like this:

        QWMatrix m;           // identity matrix


        m.translate(10, -20); // first translate (10,-20)


        m.rotate(25);         // then rotate 25 degrees


        m.scale(1.2, 0.7);    // finally scale it

Here's the same example using basic matrix operations:

        double a    = pi/180 * 25;         // convert 25 to radians


        double sina = sin(a);


        double cosa = cos(a);


        QWMatrix m1(1, 0, 0, 1, 10, -20);  // translation matrix


        QWMatrix m2( cosa, sina,           // rotation matrix


                    -sina, cosa, 0, 0 );


        QWMatrix m3(1.2, 0, 0, 0.7, 0, 0); // scaling matrix


        QWMatrix m;


        m = m3 * m2 * m1;                  // combine all transformations

QPainter has functions to translate, scale, shear and rotate the coordinate system without using a QWMatrix. Although these functions are very convenient, it can be more efficient to build a QWMatrix and call QPainter::setWorldMatrix() if you want to perform more than a single transform operation.

See also QPainter::setWorldMatrix(), QPixmap::xForm(), Graphics Classes, and Image Processing Classes.  

Member Type Documentation

QWMatrix::TransformationMode

QWMatrix offers two transformation modes. Calculations can either be done in terms of points (Points mode, the default), or in terms of area (Area mode).

In Points mode the transformation is applied to the points that mark out the shape's bounding line. In Areas mode the transformation is applied in such a way that the area of the contained region is correctly transformed under the matrix.

QWMatrix::Points - transformations are applied to the shape's points.

QWMatrix::Areas - transformations are applied (e.g. to the width and height) so that the area is transformed.

Example:

Suppose we have a rectangle, QRect( 10, 20, 30, 40 ) and a transformation matrix QWMatrix( 2, 0, 0, 2, 0, 0 ) to double the rectangle's size.

In Points mode, the matrix will transform the top-left (10,20) and the bottom-right (39,59) points producing a rectangle with its top-left point at (20,40) and its bottom-right point at (78,118), i.e. with a width of 59 and a height of 79.

In Areas mode, the matrix will transform the top-left point in the same way as in Points mode to (20/40), and double the width and height, so the bottom-right will become (69,99), i.e. a width of 60 and a height of 80.

Because integer arithmetic is used (for speed), rounding differences mean that the modes will produce slightly different results given the same shape and the same transformation, especially when scaling up. This also means that some operations are not commutative.

Under Points mode, matrix * ( region1 | region2 ) is not equal to matrix * region1 | matrix * region2. Under Area mode, matrix * (pointarray[i]) is not neccesarily equal to (matrix * pointarry)[i].

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MEMBER FUNCTION DOCUMENTATION

QWMatrix::QWMatrix ()

QWMatrix::QWMatrix ( double m11, double m12, double m21, double m22, double dx, double dy )

double QWMatrix::det () const

double QWMatrix::dx () const

double QWMatrix::dy () const

QWMatrix QWMatrix::invert ( bool * invertible = 0 ) const

If the matrix is singular (not invertible), the identity matrix is returned.

If invertible is not 0: the value of *invertible is set to TRUE if the matrix is invertible; otherwise *invertible is set to FALSE.

See also isInvertible().

Example: t14/cannon.cpp.  

bool QWMatrix::isIdentity () const

See also reset().  

bool QWMatrix::isInvertible () const

See also invert().  

double QWMatrix::m11 () const

double QWMatrix::m12 () const

double QWMatrix::m21 () const

double QWMatrix::m22 () const

void QWMatrix::map ( int x, int y, int * tx, int * ty ) const

        *tx = m11*x + m21*y + dx  (rounded to the nearest integer)


        *ty = m22*y + m12*x + dy  (rounded to the nearest integer)

Examples:  

void QWMatrix::map ( double x, double y, double * tx, double * ty ) const

Transforms ( x, y ) to ( *tx, *ty ) using the following formulae:

        *tx = m11*x + m21*y + dx


        *ty = m22*y + m12*x + dy

QPoint QWMatrix::map ( const QPoint & p ) const

Transforms p to using the formulae:

        retx = m11*px + m21*py + dx  (rounded to the nearest integer)


        rety = m22*py + m12*px + dy  (rounded to the nearest integer)

QRect QWMatrix::map ( const QRect & r ) const

Please use QWMatrix::mapRect() instead.

Note that this method does return the bounding rectangle of the r, when shearing or rotations are used.  

QPointArray QWMatrix::map ( const QPointArray & a ) const

Returns the point array a transformed by calling map for each point.  

QRegion QWMatrix::map ( const QRegion & r ) const

Transforms the region r.

Calling this method can be rather expensive, if rotations or shearing are used.  

QRect QWMatrix::mapRect ( const QRect & rect ) const

The bounding rectangle is returned if rotation or shearing has been specified.

If you need to know the exact region rect maps to use operator*().

See also operator*().

Example: xform/xform.cpp.  

QPointArray QWMatrix::mapToPolygon ( const QRect & rect ) const

Polygons and rectangles behave slightly differently when transformed (due to integer rounding), so matrix.map( QPointArray( rect ) ) is not always the same as matrix.mapToPolygon( rect ).  

QRegion QWMatrix::mapToRegion ( const QRect & rect ) const

A rectangle which has been rotated or sheared may result in a non-rectangular region being returned.

Calling this method can be expensive, if rotations or shearing are used. If you just need to know the bounding rectangle of the returned region, use mapRect() which is a lot faster than this function.

See also QWMatrix::mapRect().  

bool QWMatrix::operator!= ( const QWMatrix & m ) const

QWMatrix & QWMatrix::operator*= ( const QWMatrix & m )

bool QWMatrix::operator== ( const QWMatrix & m ) const

void QWMatrix::reset ()

All elements are set to zero, except m11 and m22 (scaling) which are set to 1.

See also isIdentity().  

QWMatrix & QWMatrix::rotate ( double a )

Returns a reference to the matrix.

See also translate(), scale(), and shear().

Examples:  

QWMatrix & QWMatrix::scale ( double sx, double sy )

Returns a reference to the matrix.

See also translate(), shear(), and rotate().

Examples:  

void QWMatrix::setMatrix ( double m11, double m12, double m21, double m22, double dx, double dy )

void QWMatrix::setTransformationMode ( QWMatrix::TransformationMode m ) [static]

See also QWMatrix::TransformationMode.  

QWMatrix & QWMatrix::shear ( double sh, double sv )

Returns a reference to the matrix.

See also translate(), scale(), and rotate().

Examples:  

TransformationMode QWMatrix::transformationMode () [static]

See also QWMatrix::TransformationMode.  

QWMatrix & QWMatrix::translate ( double dx, double dy )

Returns a reference to the matrix.

See also scale(), shear(), and rotate().

Examples:  

RELATED FUNCTION DOCUMENTATION

QDataStream & operator<< ( QDataStream & s, const QWMatrix & m )

See also Format of the QDataStream operators.  

QDataStream & operator>> ( QDataStream & s, QWMatrix & m )

See also Format of the QDataStream operators.

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