The GNOME canvas is an engine for displaying structured graphics
and simplifying the development of complex graphic-based
applications.
Canvas Items
The GNOME Canvas basic building blocks are the GNOME Canvas
Items (GnomeCanvasItem): lines, rectangles, text, ellipses,
polylines, images and embedded widgets. You can use any of
those directly in your application.
The CanvasItem system is designed to be extensible.
Applications can define their own GnomeCanvasItem objects for
special purpose tasks. For example, the GNOME Gnumeric
spreadsheet defines a number of special Canvas Items that are
specialized for the task of spreadsheets.
Specialized canvas items allow the developer to write
custom items that can adapt to their needs for speed,
scalability and gives the user the power to extend the
canvas.
Items on the canvas can be reconfigured by using the Gtk
argument system. Users can reconfigure the parameters of
the canvas items and the changes on the parameters will be
reflected immediately on the screen.
Flicker free display
The GNOME Canvas uses off-screen buffers to render the images
before transferring them to the screen. Transfers can take
place at the command of the programmer (by explicitly
requesting a repaint update) or done automatically by the
engine (during the idle look handler).
Event dispatching
Each
GnomeCanvasItem can receive mouse events, keyboard events,
mouse-enter and mouse leave events. In addition a canvas
item can grab the mouse (for example to implement reliable
dragging of objects).
Canvas types
The Canvas can be run in two different modes: X11 mode and Art
mode. The mode is chosen at the creation time of the widget
by either calling gnome_canvas_new() or gnome_canvas_new_aa(),
the former creates an X11 canvas, while the latter creates an
Art-based canvas.
The X11 mode uses the X server to draw the items and it takes
advantage of the X server acceleration features for drawing on
the screen. The only drawback is that the output quality and
the imaging model are restricted to the X11 quality and
imaging model.
The Art mode of the canvas has an advanced imaging model based
on LibArt and it allows any GnomeCanvasItem (with the
exception of the embedded widget item) to be rotated, scaled
and translated (this is done by means of applying an affine
transformation on the object).
Details
struct GnomeCanvas
struct GnomeCanvas;
Most of the fields in this structure are for private use only.
However, canvas item implementations may make use of some of them.
GNOME_CANVAS_EPSILON
#define GNOME_CANVAS_EPSILON 1e-10
This macro defines a ‘small’ floating-point value for
the internal computations that the canvas performs. It can be
used by item implementations as a test to see whether a number is
“almost zero”.
GNOME_CANVAS_COLOR()
#define GNOME_CANVAS_COLOR(r, g, b)
This macro is used to build a 32-bit integer with an RGB color
specification. The specified values must be integers in the range
[0, 255].
r :
Red component of the color.
g :
Green component of the color.
b :
Blue component of the color.
GNOME_CANVAS_COLOR_A()
#define GNOME_CANVAS_COLOR_A(r, g, b, a)
This macro is used to build a 32-bit integer with an RGBA color
specification. This is the same as an RGB color specification,
but with an added alpha or opacity value. The specified values
must be integers in the range [0, 255].
r :
Red component of the color.
g :
Green component of the color.
b :
Blue component of the color.
a :
Opacity component of the color.
struct GnomeCanvasBuf
typedef struct {
/* 24-bit RGB buffer for rendering */
guchar *buf;
/* Rowstride for the buffer */
int buf_rowstride;
/* Rectangle describing the rendering area */
ArtIRect rect;
/* Background color, given as 0xrrggbb */
guint32 bg_color;
/* Invariant: at least one of the following flags is true. */
/* Set when the render rectangle area is the solid color bg_color */
unsigned int is_bg : 1;
/* Set when the render rectangle area is represented by the buf */
unsigned int is_buf : 1;
} GnomeCanvasBuf;
This structure is passed to the render method
of canvas items when they need to paint themselves on an
antialiased canvas. The buf field
points to a 24-bit RGB buffer for rendering. The
buf_rowstride field specifies the
number of bytes in each row in the buffer, which should be used to
calculate byte offsets inside it. The buffer's pixel offsets in
canvas pixel coordinates are given by the
rect rectangle. The
is_bg and
is_buf fields are flags that items can
use to implement rendering optimizations, and they are used in
conjunction with the bg_color field.
The is_buf flag specifies whether the
contents of the buffer are an accurate representation of the state
of the canvas. If this flag is true, then the RGB data in the
buf is valid, that is, it contains
meaningful data.
The is_bg flag specifies whether the
buffer has all its pixels set to the same color. This allows
canvas items to optimize for this case by doing alpha compositing
for a smaller set of values than if the buffer had pixels of
different colors.
At least one of these flags is on at any one time. The meaning of
their combinations is as follows:
Table 1. Values for is_bg and
is_buf
is_buf
is_bg
Meaning
FALSE
TRUE
The buffer does not contain meaningful data. However,
it should be considered as if it were filled with the
solid color specified in the
bg_color field. Item
implementations may want to call
gnome_canvas_buf_ensure_buf() to
fill the buffer automatically.
TRUE
FALSE
The buffer contains meaningful data and not all of its
pixels may be the same color. Item implementations can
use the buffer data as-is for alpha compositing.
TRUE
TRUE
The buffer contains meaningful data, and all the pixels
are of the same color. Item implementations can use the
buffer data as-is for alpha compositing, or be smarter
and do less operations since they can just composite
over a single color.
Whenever an item paints to an RGB buffer in which the
is_bg field was true, the item is then
responsible for turning off this flag if it knows that the result
will not be pixels all of the same color. If a large item, like a
solid rectangle, knows that it will be filling the buffer with a
solid color, then it take any one of the following actions:
Fill the actual pixels in the buffer with the solid color
and turn off the is_bg flag. It
should then turn on the is_buf
flag.
Fill the actual pixels in the buffer with the solid color,
set the bg_color field to that
same color, and turn on both the
is_bg and
is_buf flags. This is correct,
but is wasteful, since it could have done just the following
instead.
Just set the bg_color to the
solid color, turn on the is_bg
flag, and turn off the is_buf
flag. This means that the buffer does not contain the
actual meaningful data, and the next item to be repainted
should look at the solid color instead. This is the most
efficient version.
Most item implementations may only need to perform the actions for
the first case described above. The other two are simply
optimizations they can perform.
gnome_canvas_new ()
GtkWidget* gnome_canvas_new (void);
Creates a new empty canvas in non-antialiased mode. If you wish to use the
&GnomeCanvasImage item inside this canvas, then you must push the gdk_imlib
visual and colormap before calling this function, and they can be popped
afterwards.
Returns :
A newly-created canvas.
gnome_canvas_new_aa ()
GtkWidget* gnome_canvas_new_aa (void);
Creates a new empty canvas in antialiased mode. You should push the GdkRGB
visual and colormap before calling this functions, and they can be popped
afterwards.
Sets the scrolling region of a canvas to the specified rectangle. The canvas
will then be able to scroll only within this region. The view of the canvas
is adjusted as appropriate to display as much of the new region as possible.
Sets the zooming factor of a canvas by specifying the number of pixels that
correspond to one canvas unit.
canvas :
A canvas.
n :
The number of pixels that correspond to one canvas unit.
gnome_canvas_scroll_to ()
void gnome_canvas_scroll_to (GnomeCanvas *canvas,
int cx,
int cy);
Makes a canvas scroll to the specified offsets, given in canvas pixel units.
The canvas will adjust the view so that it is not outside the scrolling
region. This function is typically not used, as it is better to hook
scrollbars to the canvas layout's scrolling adjusments.
canvas :
A canvas.
cx :
Horizontal scrolling offset in canvas pixel units.
cy :
Vertical scrolling offset in canvas pixel units.
gnome_canvas_get_scroll_offsets ()
void gnome_canvas_get_scroll_offsets (GnomeCanvas *canvas,
int *cx,
int *cy);
Queries the scrolling offsets of a canvas. The values are returned in canvas
pixel units.
Forces an immediate update and redraw of a canvas. If the canvas does not
have any pending update or redraw requests, then no action is taken. This is
typically only used by applications that need explicit control of when the
display is updated, like games. It is not needed by normal applications.
Informs a canvas that the specified area, given as a microtile array, needs
to be repainted. To be used only by item implementations.
canvas :
A canvas.
uta :
Microtile array that specifies the area to be redrawn.
gnome_canvas_request_redraw ()
void gnome_canvas_request_redraw (GnomeCanvas *canvas,
int x1,
int y1,
int x2,
int y2);
Convenience function that informs a canvas that the specified rectangle needs
to be repainted. This function converts the rectangle to a microtile array
and feeds it to gnome_canvas_request_redraw_uta(). The rectangle includes
x1 and y1, but not x2 and y2. To be used only by item implementations.
canvas :
A canvas.
x1 :
Leftmost coordinate of the rectangle to be redrawn.
y1 :
Upper coordinate of the rectangle to be redrawn.
x2 :
Rightmost coordinate of the rectangle to be redrawn, plus 1.
y2 :
Lower coordinate of the rectangle to be redrawn, plus 1.
Converts window-relative coordinates into world coordinates. You can use
this when you need to convert mouse coordinates into world coordinates, for
example.
Converts world coordinates into window-relative coordinates.
canvas :
A canvas.
worldx :
World X coordinate.
worldy :
World Y coordinate.
winx :
X window-relative coordinate.
winy :
Y window-relative coordinate.
gnome_canvas_get_color ()
int gnome_canvas_get_color (GnomeCanvas *canvas,
char *spec,
GdkColor *color);
Allocates a color based on the specified X color specification. As a
convenience to item implementations, it returns TRUE if the color was
allocated, or FALSE if the specification was NULL. A NULL color
specification is considered as "transparent" by the canvas.
canvas :
A canvas.
spec :
X color specification, or NULL for "transparent".
color :
Returns the allocated color.
Returns :
TRUE if spec is non-NULL and the color is allocated. If spec
is NULL, then returns FALSE.
Sets the stipple origin of the specified GC as is appropriate for the canvas,
so that it will be aligned with other stipple patterns used by canvas items.
This is typically only needed by item implementations.
Controls dithered rendering for antialiased canvases. The value of dither
should be; GDK_RGB_DITHER_NONE, GDK_RGB_DITHER_NORMAL, or
GDK_RGB_DITHER_MAX. The default canvas setting is GDK_RGB_DITHER_NORMAL.
canvas :
A canvas.
dither :
Type of dither used to render an antialiased canvas.
