The canvas widget is a powerful and extensible
object-oriented display engine, useful in a wide variety
of Gnome applications. The widget itself is simply a
blank area; you can place
GnomeCanvasItems on it. A GnomeCanvasItem is a GtkObject representing some element
of the display, such as an image, a rectangle, an
ellipse, or some text. You can refer to this architecture
as structured graphics; the
canvas lets you deal with graphics in terms of items,
rather than an undifferentiated grid of pixels. Since a
GnomeCanvasItem is a
GtkObject, you can create
your own subclasses to supplement those that come with
Gnome. This gives you quite a bit of flexibility. Using
custom canvas items, the canvas can render almost
anything. At the same time, you can use stock canvas
items to save time and effort. The canvas puts you in
control.
GnomeCanvas has two modes,
representing a quality/speed tradeoff. In "GDK" mode,
canvas items render directly to a GdkPixmap buffer using the GDK
drawing primitives. The canvas copies the buffer to the
screen once all items are rendered. In antialiased or
"AA" mode, the canvas items write RGB pixels to a vector
of bytes; after all the items have drawn on the RGB
buffer, the canvas copies it to the screen. This RGB
buffer mode is called "antialiased" mode because all the
standard canvas items draw to the RGB buffer using the
high-quality antialiased routines in libart_lgpl. This results in smooth
lines and very high display quality, with some cost in
speed.
