/**************************************************************************** ** ** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies). ** Contact: http://www.qt-project.org/ ** ** This file is part of the documentation of the Qt Toolkit. ** ** $QT_BEGIN_LICENSE:FDL$ ** GNU Free Documentation License ** Alternatively, this file may be used under the terms of the GNU Free ** Documentation License version 1.3 as published by the Free Software ** Foundation and appearing in the file included in the packaging of ** this file. ** ** Other Usage ** Alternatively, this file may be used in accordance with the terms ** and conditions contained in a signed written agreement between you ** and Nokia. ** ** ** ** ** ** $QT_END_LICENSE$ ** ****************************************************************************/ /*! \page coordsys.html \title Coordinate System \ingroup qt-graphics \ingroup best-practices \brief Information about the coordinate system used by the paint system. The coordinate system is controlled by the QPainter class. Together with the QPaintDevice and QPaintEngine classes, QPainter form the basis of Qt's painting system, Arthur. QPainter is used to perform drawing operations, QPaintDevice is an abstraction of a two-dimensional space that can be painted on using a QPainter, and QPaintEngine provides the interface that the painter uses to draw onto different types of devices. The QPaintDevice class is the base class of objects that can be painted: Its drawing capabilities are inherited by the QWidget, QPixmap, QPicture, QImage, and QPrinter classes. The default coordinate system of a paint device has its origin at the top-left corner. The \e x values increase to the right and the \e y values increase downwards. The default unit is one pixel on pixel-based devices and one point (1/72 of an inch) on printers. The mapping of the logical QPainter coordinates to the physical QPaintDevice coordinates are handled by QPainter's transformation matrix, viewport and "window". The logical and physical coordinate systems coincide by default. QPainter also supports coordinate transformations (e.g. rotation and scaling). \tableofcontents \section1 Rendering \section2 Logical Representation The size (width and height) of a graphics primitive always correspond to its mathematical model, ignoring the width of the pen it is rendered with: \table \row \o \inlineimage coordinatesystem-rect.png \o \inlineimage coordinatesystem-line.png \row \o QRect(1, 2, 6, 4) \o QLine(2, 7, 6, 1) \endtable \section2 Aliased Painting When drawing, the pixel rendering is controlled by the QPainter::Antialiasing render hint. The \l {QPainter::RenderHint}{RenderHint} enum is used to specify flags to QPainter that may or may not be respected by any given engine. The QPainter::Antialiasing value indicates that the engine should antialias edges of primitives if possible, i.e. smoothing the edges by using different color intensities. But by default the painter is \e aliased and other rules apply: When rendering with a one pixel wide pen the pixels will be rendered to the \e {right and below the mathematically defined points}. For example: \table \row \o \inlineimage coordinatesystem-rect-raster.png \o \inlineimage coordinatesystem-line-raster.png \row \o \snippet doc/src/snippets/code/doc_src_coordsys.cpp 0 \o \snippet doc/src/snippets/code/doc_src_coordsys.cpp 1 \endtable When rendering with a pen with an even number of pixels, the pixels will be rendered symetrically around the mathematical defined points, while rendering with a pen with an odd number of pixels, the spare pixel will be rendered to the right and below the mathematical point as in the one pixel case. See the QRectF diagrams below for concrete examples. \table \header \o {3,1} QRectF \row \o \inlineimage qrect-diagram-zero.png \o \inlineimage qrectf-diagram-one.png \row \o Logical representation \o One pixel wide pen \row \o \inlineimage qrectf-diagram-two.png \o \inlineimage qrectf-diagram-three.png \row \o Two pixel wide pen \o Three pixel wide pen \endtable Note that for historical reasons the return value of the QRect::right() and QRect::bottom() functions deviate from the true bottom-right corner of the rectangle. QRect's \l {QRect::right()}{right()} function returns \l {QRect::left()}{left()} + \l {QRect::width()}{width()} - 1 and the \l {QRect::bottom()}{bottom()} function returns \l {QRect::top()}{top()} + \l {QRect::height()}{height()} - 1. The bottom-right green point in the diagrams shows the return coordinates of these functions. We recommend that you simply use QRectF instead: The QRectF class defines a rectangle in the plane using floating point coordinates for accuracy (QRect uses integer coordinates), and the QRectF::right() and QRectF::bottom() functions \e do return the true bottom-right corner. Alternatively, using QRect, apply \l {QRect::x()}{x()} + \l {QRect::width()}{width()} and \l {QRect::y()}{y()} + \l {QRect::height()}{height()} to find the bottom-right corner, and avoid the \l {QRect::right()}{right()} and \l {QRect::bottom()}{bottom()} functions. \section2 Anti-aliased Painting If you set QPainter's \l {QPainter::Antialiasing}{anti-aliasing} render hint, the pixels will be rendered symetrically on both sides of the mathematically defined points: \table \row \o \inlineimage coordinatesystem-rect-antialias.png \o \inlineimage coordinatesystem-line-antialias.png \row \o \snippet doc/src/snippets/code/doc_src_coordsys.cpp 2 \o \snippet doc/src/snippets/code/doc_src_coordsys.cpp 3 \endtable \section1 Transformations By default, the QPainter operates on the associated device's own coordinate system, but it also has complete support for affine coordinate transformations. You can scale the coordinate system by a given offset using the QPainter::scale() function, you can rotate it clockwise using the QPainter::rotate() function and you can translate it (i.e. adding a given offset to the points) using the QPainter::translate() function. \table \row \o \inlineimage qpainter-clock.png \o \inlineimage qpainter-rotation.png \o \inlineimage qpainter-scale.png \o \inlineimage qpainter-translation.png \row \o nop \o \l {QPainter::rotate()}{rotate()} \o \l {QPainter::scale()}{scale()} \o \l {QPainter::translate()}{translate()} \endtable You can also twist the coordinate system around the origin using the QPainter::shear() function. See the \l {painting/affine}{Affine Transformations} example for a visualization of a sheared coordinate system. All the transformation operations operate on QPainter's transformation matrix that you can retrieve using the QPainter::worldTransform() function. A matrix transforms a point in the plane to another point. If you need the same transformations over and over, you can also use QTransform objects and the QPainter::worldTransform() and QPainter::setWorldTransform() functions. You can at any time save the QPainter's transformation matrix by calling the QPainter::save() function which saves the matrix on an internal stack. The QPainter::restore() function pops it back. One frequent need for the transformation matrix is when reusing the same drawing code on a variety of paint devices. Without transformations, the results are tightly bound to the resolution of the paint device. Printers have high resolution, e.g. 600 dots per inch, whereas screens often have between 72 and 100 dots per inch. \table 100% \header \o {2,1} Analog Clock Example \row \o \inlineimage coordinatesystem-analogclock.png \o The Analog Clock example shows how to draw the contents of a custom widget using QPainter's transformation matrix. Qt's example directory provides a complete walk-through of the example. Here, we will only review the example's \l {QWidget::paintEvent()}{paintEvent()} function to see how we can use the transformation matrix (i.e. QPainter's matrix functions) to draw the clock's face. We recommend compiling and running this example before you read any further. In particular, try resizing the window to different sizes. \row \o {2,1} \snippet examples/widgets/analogclock/analogclock.cpp 9 First, we set up the painter. We translate the coordinate system so that point (0, 0) is in the widget's center, instead of being at the top-left corner. We also scale the system by \c side / 100, where \c side is either the widget's width or the height, whichever is shortest. We want the clock to be square, even if the device isn't. This will give us a 200 x 200 square area, with the origin (0, 0) in the center, that we can draw on. What we draw will show up in the largest possible square that will fit in the widget. See also the \l {Window-Viewport Conversion} section. \snippet examples/widgets/analogclock/analogclock.cpp 18 We draw the clock's hour hand by rotating the coordinate system and calling QPainter::drawConvexPolygon(). Thank's to the rotation, it's drawn pointed in the right direction. The polygon is specified as an array of alternating \e x, \e y values, stored in the \c hourHand static variable (defined at the beginning of the function), which corresponds to the four points (2, 0), (0, 2), (-2, 0), and (0, -25). The calls to QPainter::save() and QPainter::restore() surrounding the code guarantees that the code that follows won't be disturbed by the transformations we've used. \snippet examples/widgets/analogclock/analogclock.cpp 24 We do the same for the clock's minute hand, which is defined by the four points (1, 0), (0, 1), (-1, 0), and (0, -40). These coordinates specify a hand that is thinner and longer than the minute hand. \snippet examples/widgets/analogclock/analogclock.cpp 27 Finally, we draw the clock face, which consists of twelve short lines at 30-degree intervals. At the end of that, the painter is rotated in a way which isn't very useful, but we're done with painting so that doesn't matter. \endtable For a demonstation of Qt's ability to perform affine transformations on painting operations, see the \l {painting/affine}{Affine Transformations} example which allows the user to experiment with the transformation operations. See also the \l {painting/transformations}{Transformations} example which shows how transformations influence the way that QPainter renders graphics primitives. In particular, it shows how the order of transformations affects the result. For more information about the transformation matrix, see the QTransform documentation. \section1 Window-Viewport Conversion When drawing with QPainter, we specify points using logical coordinates which then are converted into the physical coordinates of the paint device. The mapping of the logical coordinates to the physical coordinates are handled by QPainter's world transformation \l {QPainter::worldTransform()}{worldTransform()} (described in the \l Transformations section), and QPainter's \l {QPainter::viewport()}{viewport()} and \l {QPainter::window()}{window()}. The viewport represents the physical coordinates specifying an arbitrary rectangle. The "window" describes the same rectangle in logical coordinates. By default the logical and physical coordinate systems coincide, and are equivalent to the paint device's rectangle. Using window-viewport conversion you can make the logical coordinate system fit your preferences. The mechanism can also be used to make the drawing code independent of the paint device. You can, for example, make the logical coordinates extend from (-50, -50) to (50, 50) with (0, 0) in the center by calling the QPainter::setWindow() function: \snippet doc/src/snippets/code/doc_src_coordsys.cpp 4 Now, the logical coordinates (-50,-50) correspond to the paint device's physical coordinates (0, 0). Independent of the paint device, your painting code will always operate on the specified logical coordinates. By setting the "window" or viewport rectangle, you perform a linear transformation of the coordinates. Note that each corner of the "window" maps to the corresponding corner of the viewport, and vice versa. For that reason it normally is a good idea to let the viewport and "window" maintain the same aspect ratio to prevent deformation: \snippet doc/src/snippets/code/doc_src_coordsys.cpp 5 If we make the logical coordinate system a square, we should also make the viewport a square using the QPainter::setViewport() function. In the example above we make it equivalent to the largest square that fit into the paint device's rectangle. By taking the paint device's size into consideration when setting the window or viewport, it is possible to keep the drawing code independent of the paint device. Note that the window-viewport conversion is only a linear transformation, i.e. it does not perform clipping. This means that if you paint outside the currently set "window", your painting is still transformed to the viewport using the same linear algebraic approach. \image coordinatesystem-transformations.png The viewport, "window" and transformation matrix determine how logical QPainter coordinates map to the paint device's physical coordinates. By default the world transformation matrix is the identity matrix, and the "window" and viewport settings are equivalent to the paint device's settings, i.e. the world, "window" and device coordinate systems are equivalent, but as we have seen, the systems can be manipulated using transformation operations and window-viewport conversion. The illustration above describes the process. \omit \section1 Related Classes Qt's paint system, Arthur, is primarily based on the QPainter, QPaintDevice, and QPaintEngine classes: \table \header \o Class \o Description \row \o QPainter \o The QPainter class performs low-level painting on widgets and other paint devices. QPainter can operate on any object that inherits the QPaintDevice class, using the same code. \row \o QPaintDevice \o The QPaintDevice class is the base class of objects that can be painted. Qt provides several devices: QWidget, QImage, QPixmap, QPrinter and QPicture, and other devices can also be defined by subclassing QPaintDevice. \row \o QPaintEngine \o The QPaintEngine class provides an abstract definition of how QPainter draws to a given device on a given platform. Qt 4 provides several premade implementations of QPaintEngine for the different painter backends we support; it provides one paint engine for each supported window system and painting frameworkt. You normally don't need to use this class directly. \endtable The 2D transformations of the coordinate system are specified using the QTransform class: \table \header \o Class \o Description \row \o QTransform \o A 3 x 3 transformation matrix. Use QTransform to rotate, shear, scale, or translate the coordinate system. \endtable In addition Qt provides several graphics primitive classes. Some of these classes exist in two versions: an \c{int}-based version and a \c{qreal}-based version. For these, the \c qreal version's name is suffixed with an \c F. \table \header \o Class \o Description \row \o \l{QPoint}(\l{QPointF}{F}) \o A single 2D point in the coordinate system. Most functions in Qt that deal with points can accept either a QPoint, a QPointF, two \c{int}s, or two \c{qreal}s. \row \o \l{QSize}(\l{QSizeF}{F}) \o A single 2D vector. Internally, QPoint and QSize are the same, but a point is not the same as a size, so both classes exist. Again, most functions accept either QSizeF, a QSize, two \c{int}s, or two \c{qreal}s. \row \o \l{QRect}(\l{QRectF}{F}) \o A 2D rectangle. Most functions accept either a QRectF, a QRect, four \c{int}s, or four \c {qreal}s. \row \o \l{QLine}(\l{QLineF}{F}) \o A 2D finite-length line, characterized by a start point and an end point. \row \o \l{QPolygon}(\l{QPolygonF}{F}) \o A 2D polygon. A polygon is a vector of \c{QPoint(F)}s. If the first and last points are the same, the polygon is closed. \row \o QPainterPath \o A vectorial specification of a 2D shape. Painter paths are the ultimate painting primitive, in the sense that any shape (rectange, ellipse, spline) or combination of shapes can be expressed as a path. A path specifies both an outline and an area. \row \o QRegion \o An area in a paint device, expressed as a list of \l{QRect}s. In general, we recommend using the vectorial QPainterPath class instead of QRegion for specifying areas, because QPainterPath handles painter transformations much better. \endtable \endomit \sa {Analog Clock Example}, {Transformations Example} */