QCanvas Class Reference
[canvas module]

Detailed Description

The QCanvas class manages its 2D graphic area and all the canvas items the area contains. The canvas has no visual appearance of its own. Instead, it is displayed on screen using a QCanvasView. Multiple QCanvasView widgets may be associated with a canvas to provide multiple views of the same canvas.

The canvas is optimized for large numbers of items, particularly where only a small percentage of the items change at any one time. If the entire display changes very regularly, you should consider using your own custom QScrollView subclass.

Qt provides a rich set of canvas item classes, e.g. QCanvasEllipse, QCanvasLine, QCanvasPolygon, QCanvasPolygonalItem, QCanvasRectangle, QCanvasSpline, QCanvasSprite and QCanvasText. You can subclass to create your own canvas items; QCanvasPolygonalItem is the most common base class used for this purpose.

Items appear on the canvas after their show() function has been called (or setVisible(TRUE)), and after update() has been called. The canvas only shows items that are visible, and then only if update() is called. (By default the canvas is white and so are canvas items, so if nothing appears try changing colors.)

If you created the canvas without passing a width and height to the constructor you must also call resize().

Although a canvas may appear to be similar to a widget with child widgets, there are several notable differences:

• Canvas items are usually much faster to manipulate and redraw than child widgets, with the speed advantage becoming especially great when there are many canvas items and non-rectangular items. In most situations canvas items are also a lot more memory efficient than child widgets.

• It's easy to detect overlapping items (collision detection).

• The canvas can be larger than a widget. A million-by-million canvas is perfectly possible. At such a size a widget might be very inefficient, and some window systems might not support it at all, whereas QCanvas scales well. Even with a billion pixels and a million items, finding a particular canvas item, detecting collisions, etc., is still fast (though the memory consumption may be prohibitive at such extremes).

• Two or more QCanvasView objects can view the same canvas.

• An arbitrary transformation matrix can be set on each QCanvasView which makes it easy to zoom, rotate or shear the viewed canvas.

• Widgets provide a lot more functionality, such as input (QKeyEvent, QMouseEvent etc.) and layout management (QGridLayout etc.).

A canvas consists of a background, a number of canvas items organized by x, y and z coordinates, and a foreground. A canvas item's z coordinate can be treated as a layer number -- canvas items with a higher z coordinate appear in front of canvas items with a lower z coordinate.

The background is white by default, but can be set to a different color using setBackgroundColor(), or to a repeated pixmap using setBackgroundPixmap() or to a mosaic of smaller pixmaps using setTiles(). Individual tiles can be set with setTile(). There are corresponding get functions, e.g. backgroundColor() and backgroundPixmap().

Note that QCanvas does not inherit from QWidget, even though it has some functions which provide the same functionality as those in QWidget. One of these is setBackgroundPixmap(); some others are resize(), size(), width() and height(). QCanvasView is the widget used to display a canvas on the screen.

Canvas items are added to a canvas by constructing them and passing the canvas to the canvas item's constructor. An item can be moved to a different canvas using QCanvasItem::setCanvas().

Canvas items are movable (and in the case of QCanvasSprites, animated) objects that inherit QCanvasItem. Each canvas item has a position on the canvas (x, y coordinates) and a height (z coordinate), all of which are held as floating-point numbers. Moving canvas items also have x and y velocities. It's possible for a canvas item to be outside the canvas (for example QCanvasItem::x() is greater than width()). When a canvas item is off the canvas, onCanvas() returns FALSE and the canvas disregards the item. (Canvas items off the canvas do not slow down any of the common operations on the canvas.)

Canvas items can be moved with QCanvasItem::move(). The advance() function moves all QCanvasItem::animated() canvas items and setAdvancePeriod() makes QCanvas move them automatically on a periodic basis. In the context of the QCanvas classes, to `animate' a canvas item is to set it in motion, i.e. using QCanvasItem::setVelocity(). Animation of a canvas item itself, i.e. items which change over time, is enabled by calling QCanvasSprite::setFrameAnimation(), or more generally by subclassing and reimplementing QCanvasItem::advance(). To detect collisions use one of the QCanvasItem::collisions() functions.

The changed parts of the canvas are redrawn (if they are visible in a canvas view) whenever update() is called. You can either call update() manually after having changed the contents of the canvas, or force periodic updates using setUpdatePeriod(). If you have moving objects on the canvas, you must call advance() every time the objects should move one step further. Periodic calls to advance() can be forced using setAdvancePeriod(). The advance() function will call QCanvasItem::advance() on every item that is animated and trigger an update of the affected areas afterwards. (A canvas item that is `animated' is simply a canvas item that is in motion.)

QCanvas organizes its canvas items into chunks; these are areas on the canvas that are used to speed up most operations. Many operations start by eliminating most chunks (i.e. those which haven't changed) and then process only the canvas items that are in the few interesting (i.e. changed) chunks. A valid chunk, validChunk(), is one which is on the canvas.

The chunk size is a key factor to QCanvas's speed: if there are too many chunks, the speed benefit of grouping canvas items into chunks is reduced. If the chunks are too large, it takes too long to process each one. The QCanvas constructor tries to pick a suitable size, but you can call retune() to change it at any time. The chunkSize() function returns the current chunk size. The canvas items always make sure they're in the right chunks; all you need to make sure of is that the canvas uses the right chunk size. A good rule of thumb is that the size should be a bit smaller than the average canvas item size. If you have moving objects, the chunk size should be a bit smaller than the average size of the moving items.

The foreground is normally nothing, but if you reimplement drawForeground(), you can draw things in front of all the canvas items.

Areas can be set as changed with setChanged() and set unchanged with setUnchanged(). The entire canvas can be set as changed with setAllChanged(). A list of all the items on the canvas is returned by allItems().

An area can be copied (painted) to a QPainter with drawArea().

If the canvas is resized it emits the resized() signal.

The examples/canvas application and the 2D graphics page of the examples/demo application demonstrate many of QCanvas's facilities.

See also QCanvasView, QCanvasItem, Abstract Widget Classes, Graphics Classes, and Image Processing Classes.

Member Function Documentation

QCanvas::QCanvas ( QObject * parent = 0, const char * name = 0 )

Warning: You must call resize() at some time after creation to be able to use the canvas.

QCanvas::QCanvas ( int w, int h )

QCanvas::QCanvas ( QPixmap p, int h, int v, int tilewidth, int tileheight )

The pixmap p is a list of tiles, arranged left to right, (and in the case of pixmaps that have multiple rows of tiles, top to bottom), with tile 0 in the top-left corner, tile 1 next to the right, and so on, e.g.

The QCanvas is initially sized to show exactly the given number of tiles horizontally and vertically. If it is resized to be larger, the entire matrix of tiles will be repeated as often as necessary to cover the area. If it is smaller, tiles to the right and bottom will not be visible.

See also setTiles().

QCanvas::~QCanvas () [virtual]

void QCanvas::advance () [virtual slot]

The advance takes place in two phases. In phase 0, the QCanvasItem::advance() function of each QCanvasItem::animated() canvas item is called with paramater 0. Then all these canvas items are called again, with parameter 1. In phase 0, the canvas items should not change position, merely examine other items on the canvas for which special processing is required, such as collisions between items. In phase 1, all canvas items should change positions, ignoring any other items on the canvas. This two-phase approach allows for considerations of "fairness", although no QCanvasItem subclasses supplied with Qt do anything interesting in phase 0.

The canvas can be configured to call this function periodically with setAdvancePeriod().

See also update().

QCanvasItemList QCanvas::allItems ()

QColor QCanvas::backgroundColor () const

This function is not a reimplementation of QWidget::backgroundColor() (QCanvas is not a subclass of QWidget), but all QCanvasViews that are viewing the canvas will set their backgrounds to this color.

See also setBackgroundColor() and backgroundPixmap().

QPixmap QCanvas::backgroundPixmap () const

See also setBackgroundPixmap() and backgroundColor().

int QCanvas::chunkSize () const

Returns the chunk size of the canvas.

See also retune().

QCanvasItemList QCanvas::collisions ( const QPoint & p ) const

QCanvasItemList QCanvas::collisions ( const QRect & r ) const

Returns a list of items which collide with the rectangle r. The list is ordered by z coordinates, from highest z coordinate (front-most item) to lowest z coordinate (rear-most item).

QCanvasItemList QCanvas::collisions ( const QPointArray & chunklist, const QCanvasItem * item, bool exact ) const

Returns a list of canvas items which intersect with the chunks listed in chunklist, excluding item. If exact is TRUE, only those which actually collide with item are returned; otherwise canvas items are included just for being in the chunks.

This is a utility function mainly used to implement the simpler QCanvasItem::collisions() function.

void QCanvas::drawArea ( const QRect & clip, QPainter * painter, bool dbuf = FALSE )

e.g. to print the canvas to a printer:

    QPrinter pr;
    if ( pr.setup() ) {
        QPainter p(&pr);
        canvas.drawArea( canvas.rect(), &p );
    }
    

Example: canvas/canvas.cpp.

void QCanvas::drawBackground ( QPainter & painter, const QRect & clip ) [virtual protected]

If the graphics for an area change, you must explicitly call setChanged(const QRect&) for the result to be visible when update() is next called.

See also setBackgroundColor(), setBackgroundPixmap(), and setTiles().

void QCanvas::drawForeground ( QPainter & painter, const QRect & clip ) [virtual protected]

If the graphics for an area change, you must explicitly call setChanged(const QRect&) for the result to be visible when update() is next called.

The default is to draw nothing.

int QCanvas::height () const

Returns the height of the canvas, in pixels.

Example: canvas/canvas.cpp.

bool QCanvas::onCanvas ( int x, int y ) const

Returns TRUE if the pixel position (x, y) is on the canvas; otherwise returns FALSE.

See also validChunk().

bool QCanvas::onCanvas ( const QPoint & p ) const

Returns TRUE if the pixel position p is on the canvas; otherwise returns FALSE.

See also validChunk().

QRect QCanvas::rect () const

Returns a rectangle the size of the canvas.

void QCanvas::resize ( int w, int h ) [virtual]

Examples: canvas/canvas.cpp and chart/chartform.cpp.

void QCanvas::resized () [signal]

This signal is emitted whenever the canvas is resized. Each QCanvasView connects to this signal to keep the scrollview's size correct.

void QCanvas::retune ( int chunksze, int mxclusters = 100 ) [virtual]

The canvas is divided into chunks which are rectangular areas chunksze wide by chunksze high. Use a chunk size which is about the average size of the canvas items. If you choose a chunk size which is too small it will increase the amount of calculation required when drawing since each change will affect many chunks. If you choose a chunk size which is too large the amount of drawing required will increase because for each change, a lot of drawing will be required since there will be many (unchanged) canvas items which are in the same chunk as the changed canvas items.

Internally, a canvas uses a low-resolution "chunk matrix" to keep track of all the items in the canvas. A 64x64 chunk matrix is the default for a 1024x1024 pixel canvas, where each chunk collects canvas items in a 16x16 pixel square. This default is also affected by setTiles(). You can tune this default using this function. For example if you have a very large canvas and want to trade off speed for memory then you might set the chunk size to 32 or 64.

The mxclusters argument is the number of rectangular groups of chunks that will be separately drawn. If the canvas has a large number of small, dispersed items, this should be about that number. Our testing suggests that a large number of clusters is almost always best.

void QCanvas::setAdvancePeriod ( int ms ) [virtual]

If ms is less than 0 advancing will be stopped.

Example: canvas/main.cpp.

void QCanvas::setAllChanged () [virtual]

void QCanvas::setBackgroundColor ( const QColor & c ) [virtual]

See also backgroundColor(), setBackgroundPixmap(), and setTiles().

void QCanvas::setBackgroundPixmap ( const QPixmap & p ) [virtual]

See also backgroundPixmap(), setBackgroundColor(), and setTiles().

void QCanvas::setChanged ( const QRect & area ) [virtual]

void QCanvas::setDoubleBuffering ( bool y ) [virtual]

Turning off double-buffering causes the redrawn areas to flicker a little and also gives a (usually small) performance improvement.

Example: canvas/canvas.cpp.

void QCanvas::setTile ( int x, int y, int tilenum ) [virtual]

The images are taken from the pixmap set by setTiles() and are arranged left to right, (and in the case of pixmaps that have multiple rows of tiles, top to bottom), with tile 0 in the top-left corner, tile 1 next to the right, and so on, e.g.

See also tile() and setTiles().

void QCanvas::setTiles ( QPixmap p, int h, int v, int tilewidth, int tileheight ) [virtual]

The pixmap p is a list of tiles, arranged left to right, (and in the case of pixmaps that have multiple rows of tiles, top to bottom), with tile 0 in the top-left corner, tile 1 next to the right, and so on, e.g.

If the canvas is larger than the matrix of tiles, the entire matrix is repeated as necessary to cover the whole canvas. If it is smaller, tiles to the right and bottom are not visible.

The width and height of p must be a multiple of tilewidth and tileheight. If they are not the function will do nothing.

If you want to unset any tiling set, then just pass in a null pixmap and 0 for h, v, tilewidth, and tileheight.

void QCanvas::setUnchanged ( const QRect & area ) [virtual]

void QCanvas::setUpdatePeriod ( int ms ) [virtual]

If ms is less than 0 automatic updating will be stopped.

QSize QCanvas::size () const

Returns the size of the canvas, in pixels.

int QCanvas::tile ( int x, int y ) const

Returns the tile at position (x, y). Initially, all tiles are 0.

The parameters must be within range, i.e. 0 < x < tilesHorizontally() and 0 < y < tilesVertically().

See also setTile().

int QCanvas::tileHeight () const

Returns the height of each tile.

int QCanvas::tileWidth () const

Returns the width of each tile.

int QCanvas::tilesHorizontally () const

Returns the number of tiles horizontally.

int QCanvas::tilesVertically () const

Returns the number of tiles vertically.

void QCanvas::update () [virtual slot]

See also advance().

bool QCanvas::validChunk ( int x, int y ) const

Returns TRUE if the chunk position (x, y) is on the canvas; otherwise returns FALSE.

See also onCanvas().

bool QCanvas::validChunk ( const QPoint & p ) const

Returns TRUE if the chunk position p is on the canvas; otherwise returns FALSE.

See also onCanvas().

int QCanvas::width () const

Returns the width of the canvas, in pixels.

Example: canvas/canvas.cpp.

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