GDK Reference Manual
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Graphics Contexts

Name

Graphics Contexts -- objects to encapsulate drawing properties.

Synopsis


#include <gdk/gdk.h>


struct      GdkGC;
struct      GdkGCValues;
enum        GdkGCValuesMask;
enum        GdkFunction;
GdkGC*      gdk_gc_new                      (GdkWindow *window);
GdkGC*      gdk_gc_new_with_values          (GdkWindow *window,
                                             GdkGCValues *values,
                                             GdkGCValuesMask values_mask);
GdkGC*      gdk_gc_ref                      (GdkGC *gc);
void        gdk_gc_unref                    (GdkGC *gc);
void        gdk_gc_destroy                  (GdkGC *gc);
void        gdk_gc_get_values               (GdkGC *gc,
                                             GdkGCValues *values);
void        gdk_gc_set_foreground           (GdkGC *gc,
                                             GdkColor *color);
void        gdk_gc_set_background           (GdkGC *gc,
                                             GdkColor *color);
void        gdk_gc_set_font                 (GdkGC *gc,
                                             GdkFont *font);
void        gdk_gc_set_function             (GdkGC *gc,
                                             GdkFunction function);
void        gdk_gc_set_fill                 (GdkGC *gc,
                                             GdkFill fill);
enum        GdkFill;
void        gdk_gc_set_tile                 (GdkGC *gc,
                                             GdkPixmap *tile);
void        gdk_gc_set_stipple              (GdkGC *gc,
                                             GdkPixmap *stipple);
void        gdk_gc_set_ts_origin            (GdkGC *gc,
                                             gint x,
                                             gint y);
void        gdk_gc_set_clip_origin          (GdkGC *gc,
                                             gint x,
                                             gint y);
void        gdk_gc_set_clip_mask            (GdkGC *gc,
                                             GdkBitmap *mask);
void        gdk_gc_set_clip_rectangle       (GdkGC *gc,
                                             GdkRectangle *rectangle);
void        gdk_gc_set_clip_region          (GdkGC *gc,
                                             GdkRegion *region);
void        gdk_gc_set_subwindow            (GdkGC *gc,
                                             GdkSubwindowMode mode);
enum        GdkSubwindowMode;
void        gdk_gc_set_exposures            (GdkGC *gc,
                                             gint exposures);
void        gdk_gc_set_line_attributes      (GdkGC *gc,
                                             gint line_width,
                                             GdkLineStyle line_style,
                                             GdkCapStyle cap_style,
                                             GdkJoinStyle join_style);
enum        GdkLineStyle;
enum        GdkCapStyle;
enum        GdkJoinStyle;
void        gdk_gc_set_dashes               (GdkGC *gc,
                                             gint dash_offset,
                                             gchar dash_list[],
                                             gint n);
void        gdk_gc_copy                     (GdkGC *dst_gc,
                                             GdkGC *src_gc);

Description

All drawing operations in GDK take a graphics context (GC) argument. A graphics context encapsulates information about the way things are drawn, such as the foreground color or line width. By using graphics contexts, the number of arguments to each drawing call is greatly reduced, and communication overhead is minimized, since identical arguments do not need to be passed repeatedly.

Most values of a graphics context can be set at creation time by using gdk_gc_new_with_values(), or can be set one-by-one using functions such as gdk_gc_set_foreground(). A few of the values in the GC, such as the dash pattern, can only be set by the latter method.

Details

struct GdkGC

struct GdkGC
{
  gint dummy_var;
};

The GdkGC structure represents a graphics context. It is an opaque structure with no user-visible elements.

struct GdkGCValues

struct GdkGCValues
{
  GdkColor	    foreground;
  GdkColor	    background;
  GdkFont	   *font;
  GdkFunction	    function;
  GdkFill	    fill;
  GdkPixmap	   *tile;
  GdkPixmap	   *stipple;
  GdkPixmap	   *clip_mask;
  GdkSubwindowMode  subwindow_mode;
  gint		    ts_x_origin;
  gint		    ts_y_origin;
  gint		    clip_x_origin;
  gint		    clip_y_origin;
  gint		    graphics_exposures;
  gint		    line_width;
  GdkLineStyle	    line_style;
  GdkCapStyle	    cap_style;
  GdkJoinStyle	    join_style;
};

The GdkGCValues structure holds a set of values used to create or modify a graphics context.

GdkColor foreground;the foreground color.
GdkColor background;the background color.
GdkFont *font;the default font..
GdkFunction function;the bitwise operation used when drawing.
GdkFill fill;the fill style.
GdkPixmap *tile;the tile pixmap.
GdkPixmap *stipple;the stipple bitmap.
GdkPixmap *clip_mask;the clip mask bitmap.
GdkSubwindowMode subwindow_mode;the subwindow mode.
gint ts_x_origin;the x origin of the tile or stipple.
gint ts_y_origin;the y origin of the tile or stipple.
gint clip_x_origin;the x origin of the clip mask.
gint clip_y_origin;the y origin of the clip mask.
gint graphics_exposures;whether graphics exposures are enabled.
gint line_width;the line width
GdkLineStyle line_style;the way dashed lines are drawn
GdkCapStyle cap_style;the way the ends of lines are drawn
GdkJoinStyle join_style;the way joins between lines are drawn

enum GdkGCValuesMask

typedef enum
{
  GDK_GC_FOREGROUND    = 1 << 0,
  GDK_GC_BACKGROUND    = 1 << 1,
  GDK_GC_FONT	       = 1 << 2,
  GDK_GC_FUNCTION      = 1 << 3,
  GDK_GC_FILL	       = 1 << 4,
  GDK_GC_TILE	       = 1 << 5,
  GDK_GC_STIPPLE       = 1 << 6,
  GDK_GC_CLIP_MASK     = 1 << 7,
  GDK_GC_SUBWINDOW     = 1 << 8,
  GDK_GC_TS_X_ORIGIN   = 1 << 9,
  GDK_GC_TS_Y_ORIGIN   = 1 << 10,
  GDK_GC_CLIP_X_ORIGIN = 1 << 11,
  GDK_GC_CLIP_Y_ORIGIN = 1 << 12,
  GDK_GC_EXPOSURES     = 1 << 13,
  GDK_GC_LINE_WIDTH    = 1 << 14,
  GDK_GC_LINE_STYLE    = 1 << 15,
  GDK_GC_CAP_STYLE     = 1 << 16,
  GDK_GC_JOIN_STYLE    = 1 << 17
} GdkGCValuesMask;

A set of bit flags used to indicate which fields GdkGCValues structure are set.

enum GdkFunction

typedef enum
{
  GDK_COPY,
  GDK_INVERT,
  GDK_XOR,
  GDK_CLEAR,
  GDK_AND,
  GDK_AND_REVERSE,
  GDK_AND_INVERT,
  GDK_NOOP,
  GDK_OR,
  GDK_EQUIV,
  GDK_OR_REVERSE,
  GDK_COPY_INVERT,
  GDK_OR_INVERT,
  GDK_NAND,
  GDK_SET
} GdkFunction;

Determines how the bit values for the source pixels are combined with the bit values for destination pixels to produce the final result. The sixteen values here correspond to the 16 different possible 2x2 truth tables. Only a couple of these values are usually useful; for colored images, only GDK_COPY, GDK_XOR and GDK_INVERT are generally useful. For bitmaps, GDK_AND and GDK_OR are also useful.

gdk_gc_new ()

GdkGC*      gdk_gc_new                      (GdkWindow *window);

Create a new graphics context with default values.

window :a GdkDrawable. The created GC must always be used with drawables of the same depth as this one.
Returns :the new graphics context.

gdk_gc_new_with_values ()

GdkGC*      gdk_gc_new_with_values          (GdkWindow *window,
                                             GdkGCValues *values,
                                             GdkGCValuesMask values_mask);

Create a new GC with the given initial values.

window :a GdkDrawable. The created GC must always be used with drawables of the same depth as this one.
values :a structure containing initial values for the GC.
values_mask :a bit mask indicating which fields in values are set.
Returns :the new graphics context.

gdk_gc_ref ()

GdkGC*      gdk_gc_ref                      (GdkGC *gc);

Increase the reference count on a graphics context.

gc :a GdkGC.
Returns :gc.

gdk_gc_unref ()

void        gdk_gc_unref                    (GdkGC *gc);

Decrease the reference count on a graphics context. If the resulting reference count is zero, the graphics context will be destroyed.

gc :a GdkGC.

gdk_gc_destroy ()

void        gdk_gc_destroy                  (GdkGC *gc);

Identical to gdk_gc_unref(). This function is obsolete and should not be used.

gc :a GdkGC.

gdk_gc_get_values ()

void        gdk_gc_get_values               (GdkGC *gc,
                                             GdkGCValues *values);

Retrieves the current values from a graphics context.

gc :a GdkGC.
values :the GdkGCValues structure in which to store the results.

gdk_gc_set_foreground ()

void        gdk_gc_set_foreground           (GdkGC *gc,
                                             GdkColor *color);

Sets the foreground color for a graphics context.

gc :a GdkGC.
color :the new foreground color.

gdk_gc_set_background ()

void        gdk_gc_set_background           (GdkGC *gc,
                                             GdkColor *color);

Sets the background color for a graphics context.

gc :a GdkGC.
color :the new background color.

gdk_gc_set_font ()

void        gdk_gc_set_font                 (GdkGC *gc,
                                             GdkFont *font);

Sets the font for a graphics context. (Note that all text-drawing functions in GDK take a font argument; the value set here is used when that argument is NULL.)

gc :a GdkGC.
font :the new font.

gdk_gc_set_function ()

void        gdk_gc_set_function             (GdkGC *gc,
                                             GdkFunction function);

Determines how the current pixel values and the pixel values being drawn are combined to produce the final pixel values.

gc :a GdkGC.
function : 

gdk_gc_set_fill ()

void        gdk_gc_set_fill                 (GdkGC *gc,
                                             GdkFill fill);

Set the fill mode for a graphics context.

gc :a GdkGC.
fill :the new fill mode.

enum GdkFill

typedef enum
{
  GDK_SOLID,
  GDK_TILED,
  GDK_STIPPLED,
  GDK_OPAQUE_STIPPLED
} GdkFill;

Determines how primitives are drawn.

GDK_SOLIDdraw with the foreground color.
GDK_TILEDdraw with a tiled pixmap.
GDK_STIPPLEDdraw using the stipple bitmap. Pixels corresponding to bits in the stipple bitmap that are set will be drawn in the foreground color; pixels corresponding to bits that are not set will be left untouched.
GDK_OPAQUE_STIPPLEDdraw using the stipple bitmap. Pixels corresponding to bits in the stipple bitmap that are set will be drawn in the foreground color; pixels corresponding to bits that are not set will be drawn with the background color.

gdk_gc_set_tile ()

void        gdk_gc_set_tile                 (GdkGC *gc,
                                             GdkPixmap *tile);

Set a tile pixmap for a graphics context. This will only be used if the fill mode is GDK_TILED.

gc :a GdkGC.
tile :the new tile pixmap.

gdk_gc_set_stipple ()

void        gdk_gc_set_stipple              (GdkGC *gc,
                                             GdkPixmap *stipple);

Set the stipple bitmap for a graphics context. The stipple will only be used if the fill mode is GDK_STIPPLED or GDK_OPAQUE_STIPPLED.

gc :a GdkGC.
stipple :the new stipple bitmap.

gdk_gc_set_ts_origin ()

void        gdk_gc_set_ts_origin            (GdkGC *gc,
                                             gint x,
                                             gint y);

Set the origin when using tiles or stipples with the GC. The tile or stipple will be aligned such that the upper left corner of the tile or stipple will coincide with this point.

gc :a GdkGC.
x :the x-coordinate of the origin.
y :the y-coordinate of the origin.

gdk_gc_set_clip_origin ()

void        gdk_gc_set_clip_origin          (GdkGC *gc,
                                             gint x,
                                             gint y);

Sets the origin of the clip mask. The coordinates are interpreted relative to the upper-left corner of the destination drawable of the current operation.

gc :a GdkGC.
x :the x-coordinate of the origin.
y :the y-coordinate of the origin.

gdk_gc_set_clip_mask ()

void        gdk_gc_set_clip_mask            (GdkGC *gc,
                                             GdkBitmap *mask);

Sets the clip mask for a graphics context from a bitmap. The clip mask is interpreted relative to the clip origin. (See gdk_gc_set_clip_origin()).

gc :the GdkGC.
mask :a bitmap.

gdk_gc_set_clip_rectangle ()

void        gdk_gc_set_clip_rectangle       (GdkGC *gc,
                                             GdkRectangle *rectangle);

Sets the clip mask for a graphics context from a rectangle. The clip mask is interpreted relative to the clip origin. (See gdk_gc_set_clip_origin()).

gc :a GdkGC.
rectangle :the rectangle to clip to.

gdk_gc_set_clip_region ()

void        gdk_gc_set_clip_region          (GdkGC *gc,
                                             GdkRegion *region);

Sets the clip mask for a graphics context from a region structure. The clip mask is interpreted relative to the clip origin. (See gdk_gc_set_clip_origin()).

gc :a GdkGC.
region :the GdkRegion.

gdk_gc_set_subwindow ()

void        gdk_gc_set_subwindow            (GdkGC *gc,
                                             GdkSubwindowMode mode);

Sets how drawing with this GC on a window will affect child windows of that window.

gc :a GdkGC.
mode :the subwindow mode.

enum GdkSubwindowMode

typedef enum
{
  GDK_CLIP_BY_CHILDREN	= 0,
  GDK_INCLUDE_INFERIORS = 1
} GdkSubwindowMode;

Determines how drawing onto a window will affect child windows of that window.

GDK_CLIP_BY_CHILDRENonly draw onto the window itself.
GDK_INCLUDE_INFERIORSDraw onto the window and child windows.

gdk_gc_set_exposures ()

void        gdk_gc_set_exposures            (GdkGC *gc,
                                             gint exposures);

Sets whether copying non-visible portions of a drawable using this graphics context generate exposure events for the corresponding regions of the destination drawable. (See gdk_draw_pixmap()).

gc :a GdkGC.
exposures :if TRUE, exposure events will be generated.

gdk_gc_set_line_attributes ()

void        gdk_gc_set_line_attributes      (GdkGC *gc,
                                             gint line_width,
                                             GdkLineStyle line_style,
                                             GdkCapStyle cap_style,
                                             GdkJoinStyle join_style);

Sets various attributes of how lines are drawn. See the corresponding members of GdkGCValues for full explanations of the arguments.

gc :a GdkGC.
line_width :the width of lines.
line_style :the dash-style for lines.
cap_style :the manner in which the ends of lines are drawn.
join_style :the in which lines are joined together.

enum GdkLineStyle

typedef enum
{
  GDK_LINE_SOLID,
  GDK_LINE_ON_OFF_DASH,
  GDK_LINE_DOUBLE_DASH
} GdkLineStyle;

Determines how lines are drawn.

GDK_LINE_SOLIDlines are drawn solid.
GDK_LINE_ON_OFF_DASHeven segments are drawn; odd segments are not drawn.
GDK_LINE_DOUBLE_DASHeven segments are normally. Odd segments are drawn in the background color if the fill style is GDK_SOLID, or in the background color masked by the stipple if the fill style is GDK_STIPPLED.

enum GdkCapStyle

typedef enum
{
  GDK_CAP_NOT_LAST,
  GDK_CAP_BUTT,
  GDK_CAP_ROUND,
  GDK_CAP_PROJECTING
} GdkCapStyle;

Determines how the end of lines are drawn.

GDK_CAP_NOT_LASTthe same as GDK_CAP_BUTT for lines of non-zero width. for zero width lines, the final point on the line will not be drawn.
GDK_CAP_BUTTthe ends of the lines are drawn squared off and extending to the coordinates of the end point.
GDK_CAP_ROUNDthe ends of the lines are drawn as semicircles with the diameter equal to the line width and centered at the end point.
GDK_CAP_PROJECTINGthe ends of the lines are drawn squared off and extending half the width of the line beyond the end point.

enum GdkJoinStyle

typedef enum
{
  GDK_JOIN_MITER,
  GDK_JOIN_ROUND,
  GDK_JOIN_BEVEL
} GdkJoinStyle;

Determines how the joins between segments of a polygon are drawn.

GDK_JOIN_MITERthe sides of each line are extended to meet at an angle.
GDK_JOIN_ROUNDthe sides of the two lines are joined by a circular arc.
GDK_JOIN_BEVELthe sides of the two lines are joined by a straight line which makes an equal angle with each line.

gdk_gc_set_dashes ()

void        gdk_gc_set_dashes               (GdkGC *gc,
                                             gint dash_offset,
                                             gchar dash_list[],
                                             gint n);

Sets the way dashed-lines are drawn. Lines will be drawn with alternating on and off segments of the lengths specified in dash_list. The manner in which the on and off segments are drawn is determined by the line_style value of the GC. (This can be changed with gdk_gc_set_line_attributes)

gc :a GdkGC.
dash_offset :the
dash_list :an array of dash lengths.
n :the number of elements in dash_list.

gdk_gc_copy ()

void        gdk_gc_copy                     (GdkGC *dst_gc,
                                             GdkGC *src_gc);

Copy the set of values from one graphics context onto another graphics context.

dst_gc :the destination graphics context.
src_gc :the source graphics context.



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