XCreateGC(3X11) X11R5 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 zero 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-off‐
set 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. 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. Note
that the clip-mask and dash-list (represented by the and bits, respec‐
tively, in the valuemask) cannot be requested. Also note that an
invalid resource ID (with one or more of the three most-significant
bits set to one) will be returned for and if the component has never
been explicitly set by the client.
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 typi‐
cally the most useful because it will work on a color display, but spe‐
cial applications may use other functions, particularly in concert with
particular planes of a color display. The 16 GC functions, defined in
are:
─────────────────────────────────────────────────────────────
Function Name Value Operation
─────────────────────────────────────────────────────────────
0x0 0
0x1 src AND dst
0x2 src AND NOT dst
0x3 src
0x4 (NOT src) AND dst
0x5 dst
0x6 src XOR dst
0x7 src OR dst
0x8 (NOT src) AND (NOT
dst)
0x9 (NOT src) XOR dst
0xa NOT dst
0xb src OR (NOT dst)
0xc NOT src
0xd (NOT src) OR dst
0xe (NOT src) OR (NOT
dst)
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:
((src FUNC dst) AND plane-mask) OR (dst AND (NOT plane-mask))
Range checking is not performed on the values for foreground, back‐
ground, or plane_mask. They are simply truncated to the appropriate
number 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 coordinates:
[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 immediately
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 algorithms,
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:
The full path of the line
is drawn.
The full path of the line
is drawn, but the even
dashes are filled differ‐
ently than the odd dashes
(see fill-style) with
style used where even and
odd dashes meet.
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:
This is equivalent to
except that for a line-
width of zero the final
endpoint is not drawn.
The line is square at the
endpoint (perpendicular to
the slope of the line)
with no projection beyond.
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).
The line is square at the
end, but the path contin‐
ues beyond the endpoint
for a distance equal to
half the line-width.
(This is equivalent to for
line-width of zero).
The join-style defines how corners are drawn for wide lines:
The outer edges of two
lines extend to meet at an
angle. However, if the
angle is less than 11
degrees, then a join-style
is used instead.
The corner is a circular
arc with the diameter
equal to the line-width,
centered on the joinpoint.
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:
thin The results are
device-dependent,
but the desired
effect is that
nothing is drawn.
thin The results are
device-dependent,
but the desired
effect is that a
single pixel is
drawn.
thin The results are the
same as for
thin The results are the
same as for
wide Nothing is drawn.
wide The closed path is
a circle, centered
at the endpoint,
and with the diame‐
ter equal to the
line-width.
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 represents an infinite 2D plane, with the tile/stipple
replicated in all dimensions. When that plane is superimposed on the
drawable for use in a graphics operation, the upper left corner of some
instance of the tile/stipple is at the coordinates within the drawable
specified by the tile/stipple origin. The tile/stipple and clip ori‐
gins 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 addi‐
tional 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:
Foreground
Tile
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
Foreground masked by stip‐
ple
When drawing lines with line-style the odd dashes are controlled by the
fill-style in the following manner:
Background
Same as for even dashes
Same as for even dashes
Background masked by stip‐
ple
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 destina‐
tion 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 pat‐
terns 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 ori‐
gin 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 neither
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 win‐
dow 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 seg‐
ments. A clockwise directed path segment is one that crosses the ray
from left to right as observed from the point. A counterclockwise seg‐
ment 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 poly‐
gon 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 ALSOAllPlanes(3X11), XCopyArea(3X11), XCreateRegion(3X11), XDrawArc(3X11),
XDrawLine(3X11), XDrawRectangle(3X11), XDrawText(3X11), XFillRectan‐
gle(3X11), XQueryBestSize(3X11), XSetArcMode(3X11), XSetClipOri‐
gin(3X11), XSetFillStyle(3X11), XSetFont(3X11), XSetLineAt‐
tributes(3X11), XSetState(3X11), XSetTile(3X11)
Xlib - C Language X Interface
XCreateGC(3X11)