XCreateGC(3X11) MIT X11R4 XCreateGC(3X11)
Name
XCreateGC, XCopyGC, XChangeGC, XGetGCValues, XFreeGC, XGContextFromGC, XGCValues - create or free graphics contexts and graphics context
structure
Syntax
GC XCreateGC(display, d, valuemask, values)
Display *display;
Drawable d;
unsigned long valuemask;
XGCValues *values;
XCopyGC(display, src, valuemask, dest)
Display *display;
GC src, dest;
unsigned long valuemask;
XChangeGC(display, gc, valuemask, values)
Display *display;
GC gc;
unsigned long valuemask;
XGCValues *values;
Status XGetGCValues(display, gc, valuemask, values_return)
Display *display;
GC gc;
unsigned long valuemask;
XGCValues *values_return;
XFreeGC(display, gc)
Display *display;
GC gc;
GContext XGContextFromGC(gc)
GC gc;
Arguments
d Specifies the drawable.
dest Specifies the destination GC.
display Specifies the connection to the X server.
gc Specifies the GC.
src Specifies the components of the source GC.
valuemask Specifies which components in the GC are to be set, copied, changed, or returned . This argument is the bitwise inclusive OR of
one or more of the valid GC component mask bits.
values Specifies any values as specified by the valuemask.
values_return
Returns the GC values in the specified structure.
Description
The function creates a graphics context and returns a GC. The GC can be used with any destination drawable having the same root and depth
as the specified drawable. Use with other drawables results in a error.
can generate and errors.
The function copies the specified components from the source GC to the destination GC. The source and destination GCs must have the same
root and depth, or a error results. The valuemask specifies which component to copy, as for
can generate and errors.
The function changes the components specified by valuemask for the specified GC. The values argument contains the values to be set. The
values and restrictions are the same as for Changing the clip-mask overrides any previous request on the context. Changing the dash-offset
or dash-list overrides any previous request on the context. The order in which components are verified and altered is server-dependent.
If an error is generated, a subset of the components may have been altered.
can generate and errors.
The function returns the components specified by valuemask for the specified GC. Note that the clip mask and dash list (represented by the
and bits, respectively, in the valuemask) cannot be requested. If the valuemask contains a valid set of GC mask bits or and no error
occur, sets the requested components in values_return and returns a nonzero status. Otherwise, it returns a zero status.
The function destroys the specified GC as well as all the associated storage.
can generate a error.
Structures
The structure contains:
/* GC attribute value mask bits */
#define (1L<<0)
#define (1L<<1)
#define (1L<<2)
#define (1L<<3)
#define (1L<<4)
#define (1L<<5)
#define (1L<<6)
#define (1L<<7)
#define (1L<<8)
#define (1L<<9)
#define (1L<<10)
#define (1L<<11)
#define (1L<<12)
#define (1L<<13)
#define (1L<<14)
#define (1L<<15)
#define (1L<<16)
#define (1L<<17)
#define (1L<<18)
#define (1L<<19)
#define (1L<<20)
#define (1L<<21)
#define (1L<<22)
/* Values */
typedef struct {
int function; /* logical operation */
unsigned long plane_mask; /* plane mask */
unsigned long foreground; /* foreground pixel */
unsigned long background; /* background pixel */
int line_width; /* line width (in pixels) */
int line_style; /* LineSolid, LineOnOffDash,
LineDoubleDash */
int cap_style; /* CapNotLast, CapButt,
CapRound, CapProjecting */
int join_style; /* JoinMiter, JoinRound,
JoinBevel */
int fill_style; /* FillSolid, FillTiled,
FillStippled,
FillOpaqueStippled*/
int fill_rule; /* EvenOddRule, WindingRule */
int arc_mode; /* ArcChord, ArcPieSlice */
Pixmap tile; /* tile pixmap for tiling
operations */
Pixmap stipple; /* stipple 1 plane pixmap for
stippling */
int ts_x_origin; /* offset for tile or stipple
operations */
int ts_y_origin;
Font font; /* default text font for text
operations */
int subwindow_mode; /* ClipByChildren,
IncludeInferiors */
Bool graphics_exposures; /* boolean, should exposures be
generated */
int clip_x_origin; /* origin for clipping */
int clip_y_origin;
Pixmap clip_mask; /* bitmap clipping; other calls
for rects */
int dash_offset; /* patterned/dashed line
information */
char dashes;
} XGCValues;
The function attributes of a GC are used when you update a section of a drawable (the destination) with bits from somewhere else (the
source). The function in a GC defines how the new destination bits are to be computed from the source bits and the old destination bits.
is typically the most useful because it will work on a color display, but special applications may use other functions, particularly in
concert with particular planes of a color display. The 16 GC functions, defined in are:
-----------------------------------------------------
Function Name Hex Code Operation
-----------------------------------------------------
(R)GXclear 0x0 0
GXand 0x1 src AND dst
GXandReverse 0x2 src AND NOT dst
GXcopy 0x3 src
GXandInverted 0x4 (NOT src) AND dst
GXnoop 0x5 dst
GXxor 0x6 src XOR dst
GXor 0x7 src OR dst
GXnor 0x8 (NOT src) AND (NOT dst)
GXequiv 0x9 (NOT src) XOR dst
GXinvert 0xa NOT dst
GXorReverse 0xb src OR (NOT dst)
GXcopyInverted 0xc NOT src
GXorInverted 0xd (NOT src) OR dst
GXnand 0xe (NOT src) OR (NOT dst)
GXset 0xf 1
-----------------------------------------------------
Many graphics operations depend on either pixel values or planes in a GC. The planes attribute is of type long, and it specifies which
planes of the destination are to be modified, one bit per plane. A monochrome display has only one plane and will be the least-significant
bit of the word. As planes are added to the display hardware, they will occupy more significant bits in the plane mask.
In graphics operations, given a source and destination pixel, the result is computed bitwise on corresponding bits of the pixels. That is,
a Boolean operation is performed in each bit plane. The plane_mask restricts the operation to a subset of planes. A macro constant can be
used to refer to all planes of the screen simultaneously. The result is computed by the following:
(R)((src FUNC dst) AND plane-mask) OR (dst AND (NOT plane-mask))
Range checking is not performed on the values for foreground, background, or plane_mask. They are simply truncated to the appropriate num-
ber of bits. The line-width is measured in pixels and either can be greater than or equal to one (wide line) or can be the special value
zero (thin line).
Wide lines are drawn centered on the path described by the graphics request. Unless otherwise specified by the join-style or cap-style,
the bounding box of a wide line with endpoints [x1, y1], [x2, y2] and width w is a rectangle with vertices at the following real coordi-
nates:
[x1-(w*sn/2), y1+(w*cs/2)], [x1+(w*sn/2), y1-(w*cs/2)],
[x2-(w*sn/2), y2+(w*cs/2)], [x2+(w*sn/2), y2-(w*cs/2)]
Here sn is the sine of the angle of the line, and cs is the cosine of the angle of the line. A pixel is part of the line and so is drawn
if the center of the pixel is fully inside the bounding box (which is viewed as having infinitely thin edges). If the center of the pixel
is exactly on the bounding box, it is part of the line if and only if the interior is immediately to its right (x increasing direction).
Pixels with centers on a horizontal edge are a special case and are part of the line if and only if the interior or the boundary is immedi-
ately below (y increasing direction) and the interior or the boundary is immediately to the right (x increasing direction).
Thin lines (zero line-width) are one-pixel-wide lines drawn using an unspecified, device-dependent algorithm. There are only two con-
straints on this algorithm.
1. If a line is drawn unclipped from [x1,y1] to [x2,y2] and if another line is drawn unclipped from [x1+dx,y1+dy] to [x2+dx,y2+dy], a
point [x,y] is touched by drawing the first line if and only if the point [x+dx,y+dy] is touched by drawing the second line.
2. The effective set of points comprising a line cannot be affected by clipping. That is, a point is touched in a clipped line if and
only if the point lies inside the clipping region and the point would be touched by the line when drawn unclipped.
A wide line drawn from [x1,y1] to [x2,y2] always draws the same pixels as a wide line drawn from [x2,y2] to [x1,y1], not counting cap-style
and join-style. It is recommended that this property be true for thin lines, but this is not required. A line-width of zero may differ
from a line-width of one in which pixels are drawn. This permits the use of many manufacturers' line drawing hardware, which may run many
times faster than the more precisely specified wide lines.
In general, drawing a thin line will be faster than drawing a wide line of width one. However, because of their different drawing algo-
rithms, thin lines may not mix well aesthetically with wide lines. If it is desirable to obtain precise and uniform results across all
displays, a client should always use a line-width of one rather than a line-width of zero.
The line-style defines which sections of a line are drawn:
LineSolid The full path of the line is
drawn.
LineDoubleDash The full path of the line is
drawn, but the even dashes are
filled differently than the odd
dashes (see fill-style) with style
used where even and odd dashes
meet.
LineOnOffDash Only the even dashes are drawn,
and cap-style applies to all
internal ends of the individual
dashes, except is treated as
The cap-style defines how the endpoints of a path are drawn:
CapNotLast This is equivalent to except that
for a line-width of zero the final
endpoint is not drawn.
CapButt The line is square at the endpoint
(perpendicular to the slope of the
line) with no projection beyond.
CapRound The line has a circular arc with
the diameter equal to the line-
width, centered on the endpoint.
(This is equivalent to for line-
width of zero).
CapProjecting The line is square at the end, but
the path continues beyond the end-
point for a distance equal to half
the line-width. (This is equiva-
lent to for line-width of zero).
The join-style defines how corners are drawn for wide lines:
JoinMiter The outer edges of two lines
extend to meet at an angle. How-
ever, if the angle is less than 11
degrees, then a join-style is used
instead.
JoinRound The corner is a circular arc with
the diameter equal to the line-
width, centered on the joinpoint.
JoinBevel The corner has endpoint styles
with the triangular notch filled.
For a line with coincident endpoints (x1=x2, y1=y2), when the cap-style is applied to both endpoints, the semantics depends on the line-
width and the cap-style:
CapNotLast thin The results are device-
dependent, but the
desired effect is that
nothing is drawn.
CapButt thin The results are device-
dependent, but the
desired effect is that a
single pixel is drawn.
CapRound thin The results are the same
as for
CapProjecting thin The results are the same
as for
CapButt wide Nothing is drawn.
CapRound wide The closed path is a cir-
cle, centered at the end-
point, and with the diam-
eter equal to the line-
width.
CapProjecting wide The closed path is a
square, aligned with the
coordinate axes, centered
at the endpoint, and with
the sides equal to the
line-width.
For a line with coincident endpoints (x1=x2, y1=y2), when the join-style is applied at one or both endpoints, the effect is as if the line
was removed from the overall path. However, if the total path consists of or is reduced to a single point joined with itself, the effect
is the same as when the cap-style is applied at both endpoints.
The tile/stipple and clip origins are interpreted relative to the origin of whatever destination drawable is specified in a graphics
request. The tile pixmap must have the same root and depth as the GC, or a error results. The stipple pixmap must have depth one and must
have the same root as the GC, or a error results. For stipple operations where the fill-style is but not the stipple pattern is tiled in a
single plane and acts as an additional clip mask to be ANDed with the clip-mask. Although some sizes may be faster to use than others, any
size pixmap can be used for tiling or stippling.
The fill-style defines the contents of the source for line, text, and fill requests. For all text and fill requests (for example, and for
line requests with line-style (for example, and for the even dashes for line requests with line-style or the following apply:
FillSolid Foreground
FillTiled Tile
FillOpaqueStippled A tile with the same width and
height as stipple, but with
background everywhere stipple
has a zero and with foreground
everywhere stipple has a one
FillStippled Foreground masked by stipple
When drawing lines with line-style the odd dashes are controlled by the fill-style in the following manner:
FillSolid Background
FillTiled Same as for even dashes
FillOpaqueStippled Same as for even dashes
FillStippled Background masked by stipple
Storing a pixmap in a GC might or might not result in a copy being made. If the pixmap is later used as the destination for a graphics
request, the change might or might not be reflected in the GC. If the pixmap is used simultaneously in a graphics request both as a desti-
nation and as a tile or stipple, the results are undefined.
For optimum performance, you should draw as much as possible with the same GC (without changing its components). The costs of changing GC
components relative to using different GCs depend upon the display hardware and the server implementation. It is quite likely that some
amount of GC information will be cached in display hardware and that such hardware can only cache a small number of GCs.
The dashes value is actually a simplified form of the more general patterns that can be set with Specifying a value of N is equivalent to
specifying the two-element list [N, N] in The value must be nonzero, or a error results.
The clip-mask restricts writes to the destination drawable. If the clip-mask is set to a pixmap, it must have depth one and have the same
root as the GC, or a error results. If clip-mask is set to the pixels are always drawn regardless of the clip origin. The clip-mask also
can be set by calling the or functions. Only pixels where the clip-mask has a bit set to 1 are drawn. Pixels are not drawn outside the
area covered by the clip-mask or where the clip-mask has a bit set to 0. The clip-mask affects all graphics requests. The clip-mask does
not clip sources. The clip-mask origin is interpreted relative to the origin of whatever destination drawable is specified in a graphics
request.
You can set the subwindow-mode to or For both source and destination windows are additionally clipped by all viewable children. For nei-
ther source nor destination window is clipped by inferiors. This will result in including subwindow contents in the source and drawing
through subwindow boundaries of the destination. The use of on a window of one depth with mapped inferiors of differing depth is not ille-
gal, but the semantics are undefined by the core protocol.
The fill-rule defines what pixels are inside (drawn) for paths given in requests and can be set to or For a point is inside if an infinite
ray with the point as origin crosses the path an odd number of times. For a point is inside if an infinite ray with the point as origin
crosses an unequal number of clockwise and counterclockwise directed path segments. A clockwise directed path segment is one that crosses
the ray from left to right as observed from the point. A counterclockwise segment is one that crosses the ray from right to left as
observed from the point. The case where a directed line segment is coincident with the ray is uninteresting because you can simply choose
a different ray that is not coincident with a segment.
For both and a point is infinitely small, and the path is an infinitely thin line. A pixel is inside if the center point of the pixel is
inside and the center point is not on the boundary. If the center point is on the boundary, the pixel is inside if and only if the polygon
interior is immediately to its right (x increasing direction). Pixels with centers on a horizontal edge are a special case and are inside
if and only if the polygon interior is immediately below (y increasing direction).
The arc-mode controls filling in the function and can be set to or For the arcs are pie-slice filled. For the arcs are chord filled.
The graphics-exposure flag controls event generation for and requests (and any similar requests defined by extensions).
Diagnostics
The server failed to allocate the requested resource or server memory.
A value for a Drawable argument does not name a defined Window or Pixmap.
A value for a Font or GContext argument does not name a defined Font.
A value for a GContext argument does not name a defined GContext.
An window is used as a Drawable.
Some argument or pair of arguments has the correct type and range but fails
to match in some other way required by the request.
A value for a Pixmap argument does not name a defined Pixmap.
Some numeric value falls outside the range of values accepted by the request.
Unless a specific range is specified for an argument, the full range defined by the argument's type is accepted. Any argument
defined as a set of alternatives can generate this error.
See Also
AllPlanes(3X11), XCopyArea(3X11), XCreateRegion(3X11), XDrawArc(3X11), XDrawLine(3X11), XDrawRectangle(3X11), XDrawText(3X11), XFillRectan-
gle(3X11), XQueryBestSize(3X11), XSetArcMode(3X11), XSetClipOrigin(3X11), XSetFillStyle(3X11), XSetFont(3X11), XSetLineAttributes(3X11),
XSetState(3X11), XSetTile(3X11)
X Window System: The Complete Reference, Second Edition, Robert W. Scheifler and James Gettys
XCreateGC(3X11)