## GnomeCanvas

GnomeCanvas — Main canvas widget

## Synopsis

    #include <libgnomecanvas/libgnomecanvas.h>

                        GnomeCanvas;
    #define             GNOME_CANVAS_EPSILON
    #define             GNOME_CANVAS_COLOR                  (r, g, b)
    #define             GNOME_CANVAS_COLOR_A                (r, g, b, a)
                        GnomeCanvasBuf;
    GtkWidget*          gnome_canvas_new                    (void);
    GtkWidget*          gnome_canvas_new_aa                 (void);
    GnomeCanvasGroup*   gnome_canvas_root                   (GnomeCanvas *canvas);
    void                gnome_canvas_set_scroll_region      (GnomeCanvas *canvas,
                                                             double x1,
                                                             double y1,
                                                             double x2,
                                                             double y2);
    void                gnome_canvas_get_scroll_region      (GnomeCanvas *canvas,
                                                             double *x1,
                                                             double *y1,
                                                             double *x2,
                                                             double *y2);
    void                gnome_canvas_set_center_scroll_region
                                                            (GnomeCanvas *canvas,
                                                             gboolean center_scroll_region);
    gboolean            gnome_canvas_get_center_scroll_region
                                                            (GnomeCanvas *canvas);
    void                gnome_canvas_set_pixels_per_unit    (GnomeCanvas *canvas,
                                                             double n);
    void                gnome_canvas_scroll_to              (GnomeCanvas *canvas,
                                                             int cx,
                                                             int cy);
    void                gnome_canvas_get_scroll_offsets     (GnomeCanvas *canvas,
                                                             int *cx,
                                                             int *cy);
    void                gnome_canvas_update_now             (GnomeCanvas *canvas);
    GnomeCanvasItem*    gnome_canvas_get_item_at            (GnomeCanvas *canvas,
                                                             double x,
                                                             double y);
    void                gnome_canvas_request_redraw_uta     (GnomeCanvas *canvas,
                                                             ArtUta *uta);
    void                gnome_canvas_request_redraw         (GnomeCanvas *canvas,
                                                             int x1,
                                                             int y1,
                                                             int x2,
                                                             int y2);
    void                gnome_canvas_w2c_affine             (GnomeCanvas *canvas,
                                                             double affine[6]);
    void                gnome_canvas_w2c                    (GnomeCanvas *canvas,
                                                             double wx,
                                                             double wy,
                                                             int *cx,
                                                             int *cy);
    void                gnome_canvas_w2c_d                  (GnomeCanvas *canvas,
                                                             double wx,
                                                             double wy,
                                                             double *cx,
                                                             double *cy);
    void                gnome_canvas_c2w                    (GnomeCanvas *canvas,
                                                             int cx,
                                                             int cy,
                                                             double *wx,
                                                             double *wy);
    void                gnome_canvas_window_to_world        (GnomeCanvas *canvas,
                                                             double winx,
                                                             double winy,
                                                             double *worldx,
                                                             double *worldy);
    void                gnome_canvas_world_to_window        (GnomeCanvas *canvas,
                                                             double worldx,
                                                             double worldy,
                                                             double *winx,
                                                             double *winy);
    int                 gnome_canvas_get_color              (GnomeCanvas *canvas,
                                                             const char *spec,
                                                             GdkColor *color);
    gulong              gnome_canvas_get_color_pixel        (GnomeCanvas *canvas,
                                                             guint rgba);
    void                gnome_canvas_set_stipple_origin     (GnomeCanvas *canvas,
                                                             GdkGC *gc);
    void                gnome_canvas_set_dither             (GnomeCanvas *canvas,
                                                             GdkRgbDither dither);
    GdkRgbDither        gnome_canvas_get_dither             (GnomeCanvas *canvas);

## Object Hierarchy

      GObject
       +----GInitiallyUnowned
             +----GtkObject
                   +----GtkWidget
                         +----GtkContainer
                               +----GtkLayout
                                     +----GnomeCanvas

## Implemented Interfaces

GnomeCanvas implements AtkImplementorIface.

## Properties

      "aa"                       gboolean              : Read / Write / Construct Only
      "focused-item"             GnomeCanvasItem       : Read / Write

## Signals

      "draw-background"                                : Run Last
      "render-background"                              : Run Last

## Description

The GnomeCanvas is an engine for structured graphics that offers a rich imaging model, high performance rendering, and a powerful, high level API. It offers a choice of two rendering back-ends, one based on Xlib for extremely fast display, and another based on Libart, a sophisticated, antialiased, alpha-compositing engine. This widget can be used for flexible display of graphics and for creating interactive user interface elements.

To create a new GnomeCanvas widget call `gnome_canvas_new()` or `gnome_canvas_new_aa()` for an anti-aliased mode canvas.

A GnomeCanvas widget contains one or more GnomeCanvasItem objects. Items consist of graphing elements like lines, ellipses, polygons, images, text, and curves. These items are organized using GnomeCanvasGroup objects, which are themselves derived from GnomeCanvasItem. Since a group is an item it can be contained within other groups, forming a tree of canvas items. Certain operations, like translating and scaling, can be performed on all items in a group.

There is a special root group created by a GnomeCanvas. This is the top level group under which all items in a canvas are contained. To get the root group from a canvas call `gnome_canvas_root()`. To clear a canvas you can simply walk through the item_list member of the GnomeCanvasGroup and call `gtk_object_destroy()` on each one.

There are several different coordinate systems used by GnomeCanvas widgets. The primary system is a logical, abstract coordinate space called world coordinates. World coordinates are expressed as unbounded double floating point numbers. When it comes to rendering to a screen the canvas pixel coordinate system (also referred to as just canvas coordinates) is used. This system uses integers to specify screen pixel positions. A user defined scaling factor and offset are used to convert between world coordinates and canvas coordinates. Each item in a canvas has its own coordinate system called item coordinates. This system is specified in world coordinates but they are relative to an item (0.0, 0.0 would be the top left corner of the item). The final coordinate system of interest is window coordinates. These are like canvas coordinates but are offsets from within a window a canvas is displayed in. This last system is rarely used, but is useful when manually handling GDK events (such as drag and drop) which are specified in window coordinates (the events processed by the canvas are already converted for you).

Along with different coordinate systems comes functions to convert between them. `gnome_canvas_w2c()` converts world to canvas pixel coordinates and `gnome_canvas_c2w()` converts from canvas to world. `gnome_canvas_w2c_d()` is like `gnome_canvas_w2c()` but returns the pixel coordinates as doubles which is useful to avoid precision loss from integer rounding. To get the affine transform matrix for converting from world coordinates to canvas coordinates call `gnome_canvas_w2c_affine()`. `gnome_canvas_window_to_world()` converts from window to world coordinates and `gnome_canvas_world_to_window()` converts in the other direction. There are no functions for converting between canvas and window coordinates, since this is just a matter of subtracting the canvas scrolling offset. To convert to/from item coordinates use the functions defined for GnomeCanvasItem objects.

To set the canvas zoom factor (canvas pixels per world unit, the scaling factor) call `gnome_canvas_set_pixels_per_unit()`, setting this to 1.0 will cause the two coordinate systems to correspond (e.g., [5, 6] in pixel units would be [5.0, 6.0] in world units).

Defining the scrollable area of a canvas widget is done by calling `gnome_canvas_set_scroll_region()` and to get the current region `gnome_canvas_get_scroll_region()` can be used. If the window is larger than the canvas scrolling region it can optionally be centered in the window. Use `gnome_canvas_set_center_scroll_region()` to enable or disable this behavior. To scroll to a particular canvas pixel coordinate use `gnome_canvas_scroll_to()` (typically not used since scrollbars are usually set up to handle the scrolling), and to get the current canvas pixel scroll offset call `gnome_canvas_get_scroll_offsets()`.

## Details

### GnomeCanvas

    typedef struct _GnomeCanvas GnomeCanvas;

This should not be accessed directly. Use the accessor functions as described below.

### GNOME_CANVAS_EPSILON

    #define GNOME_CANVAS_EPSILON 1e-10

A tiny value used by internal canvas operations.

### GNOME_CANVAS_COLOR()

    #define             GNOME_CANVAS_COLOR(r, g, b)

Macro for creating a canvas 32 bit RGBA color from red, green and blue components.

`r` : | Red value (0-255)
`g` : | Green value (0-255)
`b` : | Blue value (0-255)
Returns : | A 32 bit integer of the format 0xRRGGBBAA with the alpha "AA" component set to 0xFF (fully visible, no transparency) and the other bytes set to the input parameters.

### GNOME_CANVAS_COLOR_A()

    #define             GNOME_CANVAS_COLOR_A(r, g, b, a)

Macro for creating a canvas 32 bit RGBA color from red, green, blue and alpha components.

`r` : | Red value (0-255)
`g` : | Green value (0-255)
`b` : | Blue value (0-255)
`a` : | Alpha value (0-255, 0=completely transparent, 255=opaque)
Returns : | A 32 bit integer of the format 0xRRGGBBAA created from the input parameters.

### GnomeCanvasBuf

    typedef struct {
    	/* 24-bit RGB buffer for rendering */
    	guchar *buf;

    	/* Rectangle describing the rendering area */
    	ArtIRect rect;

    	/* Rowstride for the buffer */
    	int buf_rowstride;

    	/* 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;

A buffer used for rendering for antialiased mode canvas widgets.

### gnome_canvas_new ()

    GtkWidget*          gnome_canvas_new                    (void);

Creates a new empty canvas in non-antialiased mode.

Returns : | A newly-created canvas.

### gnome_canvas_new_aa ()

    GtkWidget*          gnome_canvas_new_aa                 (void);

Creates a new empty canvas in antialiased mode.

Returns : | A newly-created antialiased canvas.

### gnome_canvas_root ()

    GnomeCanvasGroup*   gnome_canvas_root                   (GnomeCanvas *canvas);

Queries the root group of a canvas.

`canvas` : | A canvas.
Returns : | The root group of the specified canvas.

### gnome_canvas_set_scroll_region ()

    void                gnome_canvas_set_scroll_region      (GnomeCanvas *canvas,
                                                             double x1,
                                                             double y1,
                                                             double x2,
                                                             double y2);

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.

`canvas` : | A canvas.
`x1` : | Leftmost limit of the scrolling region.
`y1` : | Upper limit of the scrolling region.
`x2` : | Rightmost limit of the scrolling region.
`y2` : | Lower limit of the scrolling region.

### gnome_canvas_get_scroll_region ()

    void                gnome_canvas_get_scroll_region      (GnomeCanvas *canvas,
                                                             double *x1,
                                                             double *y1,
                                                             double *x2,
                                                             double *y2);

Queries the scrolling region of a canvas.

`canvas` : | A canvas.
`x1` : | Leftmost limit of the scrolling region (return value).
`y1` : | Upper limit of the scrolling region (return value).
`x2` : | Rightmost limit of the scrolling region (return value).
`y2` : | Lower limit of the scrolling region (return value).

### gnome_canvas_set_center_scroll_region ()

    void                gnome_canvas_set_center_scroll_region
                                                            (GnomeCanvas *canvas,
                                                             gboolean center_scroll_region);

When the scrolling region of the canvas is smaller than the canvas window, e.g. the allocation of the canvas, it can be either centered on the window or simply made to be on the upper-left corner on the window. This function lets you configure this property.

`canvas` : | A canvas.
`center_scroll_region` : | Whether to center the scrolling region in the canvas window when it is smaller than the canvas' allocation.

### gnome_canvas_get_center_scroll_region ()

    gboolean            gnome_canvas_get_center_scroll_region
                                                            (GnomeCanvas *canvas);

Returns whether the canvas is set to center the scrolling region in the window if the former is smaller than the canvas' allocation.

`canvas` : | A canvas.
Returns : | Whether the scroll region is being centered in the canvas window.

### gnome_canvas_set_pixels_per_unit ()

    void                gnome_canvas_set_pixels_per_unit    (GnomeCanvas *canvas,
                                                             double n);

Sets the zooming factor of a canvas by specifying the number of pixels that correspond to one canvas unit.

The anchor point for zooming, i.e. the point that stays fixed and all others zoom inwards or outwards from it, depends on whether the canvas is set to center the scrolling region or not. You can control this using the `gnome_canvas_set_center_scroll_region()` function. If the canvas is set to center the scroll region, then the center of the canvas window is used as the anchor point for zooming. Otherwise, the upper-left corner of the canvas window is used as the anchor point.

`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.

`canvas` : | A canvas.
`cx` : | Horizontal scrolling offset (return value).
`cy` : | Vertical scrolling offset (return value).

### gnome_canvas_update_now ()

    void                gnome_canvas_update_now             (GnomeCanvas *canvas);

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.

`canvas` : | A canvas.

### gnome_canvas_get_item_at ()

    GnomeCanvasItem*    gnome_canvas_get_item_at            (GnomeCanvas *canvas,
                                                             double x,
                                                             double y);

Looks for the item that is under the specified position, which must be specified in world coordinates.

`canvas` : | A canvas.
`x` : | X position in world coordinates.
`y` : | Y position in world coordinates.
Returns : | The sought item, or NULL if no item is at the specified coordinates.

### gnome_canvas_request_redraw_uta ()

    void                gnome_canvas_request_redraw_uta     (GnomeCanvas *canvas,
                                                             ArtUta *uta);

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. It will be freed by this function, so the argument you pass will be invalid after you call this function.

### 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.

### gnome_canvas_w2c_affine ()

    void                gnome_canvas_w2c_affine             (GnomeCanvas *canvas,
                                                             double affine[6]);

Gets the affine transform that converts from world coordinates to canvas pixel coordinates.

`canvas` : | A canvas.
`affine` : | An affine transformation matrix (return value).

### gnome_canvas_w2c ()

    void                gnome_canvas_w2c                    (GnomeCanvas *canvas,
                                                             double wx,
                                                             double wy,
                                                             int *cx,
                                                             int *cy);

Converts world coordinates into canvas pixel coordinates.

`canvas` : | A canvas.
`wx` : | World X coordinate.
`wy` : | World Y coordinate.
`cx` : | X pixel coordinate (return value).
`cy` : | Y pixel coordinate (return value).

### gnome_canvas_w2c_d ()

    void                gnome_canvas_w2c_d                  (GnomeCanvas *canvas,
                                                             double wx,
                                                             double wy,
                                                             double *cx,
                                                             double *cy);

Converts world coordinates into canvas pixel coordinates. This version returns coordinates in floating point coordinates, for greater precision.

`canvas` : | A canvas.
`wx` : | World X coordinate.
`wy` : | World Y coordinate.
`cx` : | X pixel coordinate (return value).
`cy` : | Y pixel coordinate (return value).

### gnome_canvas_c2w ()

    void                gnome_canvas_c2w                    (GnomeCanvas *canvas,
                                                             int cx,
                                                             int cy,
                                                             double *wx,
                                                             double *wy);

Converts canvas pixel coordinates to world coordinates.

`canvas` : | A canvas.
`cx` : | Canvas pixel X coordinate.
`cy` : | Canvas pixel Y coordinate.
`wx` : | X world coordinate (return value).
`wy` : | Y world coordinate (return value).

### gnome_canvas_window_to_world ()

    void                gnome_canvas_window_to_world        (GnomeCanvas *canvas,
                                                             double winx,
                                                             double winy,
                                                             double *worldx,
                                                             double *worldy);

Converts window-relative coordinates into world coordinates. You can use this when you need to convert mouse coordinates into world coordinates, for example.

`canvas` : | A canvas.
`winx` : | Window-relative X coordinate.
`winy` : | Window-relative Y coordinate.
`worldx` : | X world coordinate (return value).
`worldy` : | Y world coordinate (return value).

### gnome_canvas_world_to_window ()

    void                gnome_canvas_world_to_window        (GnomeCanvas *canvas,
                                                             double worldx,
                                                             double worldy,
                                                             double *winx,
                                                             double *winy);

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,
                                                             const 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.

### gnome_canvas_get_color_pixel ()

    gulong              gnome_canvas_get_color_pixel        (GnomeCanvas *canvas,
                                                             guint rgba);

Allocates a color from the RGBA value passed into this function. The alpha opacity value is discarded, since normal X colors do not support it.

`canvas` : | A canvas.
`rgba` : | RGBA color specification.
Returns : | Allocated pixel value corresponding to the specified color.

### gnome_canvas_set_stipple_origin ()

    void                gnome_canvas_set_stipple_origin     (GnomeCanvas *canvas,
                                                             GdkGC *gc);

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.

`canvas` : | A canvas.
`gc` : | GC on which to set the stipple origin.

### gnome_canvas_set_dither ()

    void                gnome_canvas_set_dither             (GnomeCanvas *canvas,
                                                             GdkRgbDither dither);

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 dithering used to render an antialiased canvas.

### gnome_canvas_get_dither ()

    GdkRgbDither        gnome_canvas_get_dither             (GnomeCanvas *canvas);

Returns the type of dithering used to render an antialiased canvas.

`canvas` : | A canvas.
Returns : | The dither setting.

## Property Details

### The "`aa`" property

      "aa"                       gboolean              : Read / Write / Construct Only

Enable anti-aliasing mode?

Note that this parameter can only be set at the time of object construction. The same effect can be achieved by calling `gnome_canvas_new()` to create new non-aa canvas or `gnome_canvas_new_aa()` for an anti-aliased canvas.

Default value: FALSE

### The "`focused-item`" property

      "focused-item"             GnomeCanvasItem       : Read / Write

## Signal Details

### The "draw-background" signal

    void                user_function                      (GnomeCanvas *canvas,
                                                            GdkDrawable *arg1,
                                                            gint         arg2,
                                                            gint         arg3,
                                                            gint         arg4,
                                                            gint         arg5,
                                                            gpointer     user_data)      : Run Last

This signal is emitted to draw the background for non-antialiased mode canvas widgets. The default method uses the canvas widget's style to draw the background.

`canvas` : | the object which received the signal.
`arg1` : | GdkDrawable to draw to
`arg2` : | Leftmost X coordinate of area to draw to
`arg3` : | Top Y coordinate of area to draw to
`arg4` : | Width of area
`arg5` : | Height of area
`user_data` : | user data set when the signal handler was connected.

### The "render-background" signal

    void                user_function                      (GnomeCanvas *canvas,
                                                            gpointer     arg1,
                                                            gpointer     user_data)      : Run Last

This signal is emitted for antialiased mode canvas widgets to render the background. The buf data structure contains both a pointer to a packed 24-bit RGB array and the coordinates.

`canvas` : | the object which received the signal.
`arg1` : | GnomeCanvasBuf to render to
`user_data` : | user data set when the signal handler was connected.