Q3Canvas Class Reference
[Qt3Support module]

Detailed Description

The Q3Canvas class provides a 2D area that can contain Q3CanvasItem objects.

The Q3Canvas 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 Q3CanvasView. Multiple Q3CanvasView 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 frequently, you should consider using your own custom Q3ScrollView subclass.

Qt provides a rich set of canvas item classes, e.g. Q3CanvasEllipse, Q3CanvasLine, Q3CanvasPolygon, Q3CanvasPolygonalItem, Q3CanvasRectangle, Q3CanvasSpline, Q3CanvasSprite and Q3CanvasText. You can subclass to create your own canvas items; Q3CanvasPolygonalItem 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 Q3Canvas 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 Q3CanvasView objects can view the same canvas.
• An arbitrary transformation matrix can be set on each Q3CanvasView 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 Q3Canvas 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(). Q3CanvasView 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 Q3CanvasItem::setCanvas().

Canvas items are movable (and in the case of Q3CanvasSprites, animated) objects that inherit Q3CanvasItem. 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 Q3CanvasItem::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 Q3CanvasItem::move(). The advance() function moves all Q3CanvasItem::animated() canvas items and setAdvancePeriod() makes Q3Canvas move them automatically on a periodic basis. In the context of the Q3Canvas classes, to `animate' a canvas item is to set it in motion, i.e. using Q3CanvasItem::setVelocity(). Animation of a canvas item itself, i.e. items which change over time, is enabled by calling Q3CanvasSprite::setFrameAnimation(), or more generally by subclassing and reimplementing Q3CanvasItem::advance(). To detect collisions use one of the Q3CanvasItem::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 Q3CanvasItem::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.)

Q3Canvas 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 Q3Canvas'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 Q3Canvas 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 Q3Canvas's facilities.

See also Q3CanvasView, Q3CanvasItem, QtCanvas, and Porting to Graphics View.

Member Function Documentation

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