Table of Contents
The DrawingArea
widget is a blank window that gives
you the freedom to create any graphic you desire. Along with that freedom
comes the responsibility to handle expose events on the widget. When a
widget is first shown, or when it is covered and then uncovered again it
needs to redraw itself. Most widgets have code to do this, but the
DrawingArea does not, allowing you to write your own expose event signal
handler to determine how the contents of the widget will be drawn. This is
most often done by overriding the virtual
on_expose_event()
member function.
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Note |
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Before gtkmm version 2.10, drawing was mostly done with Graphics Contexts
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You can draw very sophisticated shapes using Cairo, but the methods to do so are quite basic. Cairo provides methods for drawing straight lines, curved lines, and arcs (including circles). These basic shapes can be combined to create more complex shapes and paths which can be filled with solid colors, gradients, patterns, and other things. In addition, Cairo can perform complex transformations, do compositing of images, and render antialiased text.
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Cairo and Pango |
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Although Cairo can render text, it's not meant to be a replacement for Pango. Pango is a better choice if you need to perform more advanced text rendering such as wrapping or ellipsizing text. Drawing text with Cairo should only be done if the text is part of a graphic. |
In this section of the tutorial, we'll cover the basic Cairo drawing model, describe each of the basic drawing elements in some detail (with examples), and then present a simple application that uses Cairo to draw a custom clock widget.
The basic concept of drawing in Cairo involves defining 'invisible' paths and then stroking or filling them to make them visible.
To do any drawing in gtkmm with Cairo, you must first create a
Cairo::Context
object. This class holds all of the graphics state parameters that
describe how drawing is to be done. This includes information such as
line width, color, the surface to draw to, and many other things. This
allows the actual drawing functions to take fewer arguments to simplify
the interface. In gtkmm, a Cairo::Context
is
created by calling the
Gdk::Window::create_cairo_context()
function.
Since Cairo context are reference-counted objects, this function
returns a Cairo::RefPtr<Cairo::Context>
object.
The following example shows how to set up a Cairo context with a foreground color of red and a width of 2. Any drawing functions that use this context will use these settings.
Gtk::DrawingArea myArea; Cairo::RefPtr<Cairo::Context> myContext = myArea.get_window()->create_cairo_context(); myContext->set_source_rgb(1.0, 0.0, 0.0); myContext->set_line_width(2.0);
Each Cairo::Context
is associated with a
particular Gdk::Window
, so the first line of the
above example creates a Gtk::DrawingArea
widget
and the second line uses its associated
Gdk::Window
to create a
Cairo::Context
object. The final two lines
change the graphics state of the context.
There are a number of graphics state variables that can be set for a
Cairo context. The most common context attributes are color (using
set_source_rgb()
or
set_source_rgba()
for translucent colors), line
width (using set_line_width()
), line dash pattern
(using set_dash()
), line cap style (using
set_line_cap()
), and line join style (using
set_line_join()
), and font styles (using
set_font_size()
,
set_font_face()
and others).
There are many other settings as well, such as transformation matrices,
fill rules, whether to perform antialiasing, and others. For further
information, see the Cairomm API documentation.
The current state of a Cairo::Context
can be
saved to an internal stack of saved states and later be restored to the
state it was in when you saved it. To do this, use the
save()
method and the restore()
method. This can be
useful if you need to temporarily change the line width and color (or
any other graphics setting) in order to draw something and then return
to the previous settings. In this situation, you could call
Cairo::Context::save()
, change the graphics
settings, draw the lines, and then call
Cairo::Context::restore()
to restore the original
graphics state. Multiple calls to save()
and
restore()
can be nested; each call to
restore()
restores the state from the
matching paired save()
.
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Tip |
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It is good practice to put all modifications to the graphics state
between void doSomething(Cairo::RefPtr<Cairo::Context> context, int x) { context->save(); // change graphics state // peform drawing operations context->restore(); } |