XCreateGC(3) XLIB FUNCTIONS XCreateGC(3)
NAME
XCreateGC, XCopyGC, XChangeGC, XGetGCValues, XFreeGC, XGContextFromGC,
XGCValues  create or free graphics contexts and graphics context
structure
SYNTAX
GC XCreateGC(Display *display, Drawable d, unsigned long valuemask,
XGCValues *values);
int XCopyGC(Display *display, GC src, GC dest, unsigned long value
mask);
int XChangeGC(Display *display, GC gc, unsigned long valuemask, XGCVal
ues *values);
Status XGetGCValues(Display *display, GC gc, unsigned long valuemask,
XGCValues *values_return);
int XFreeGC(Display *display, GC gc);
GContext XGContextFromGC(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 inclu
sive 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 XGCValues structure.
DESCRIPTION
The XCreateGC 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 BadMatch error.
XCreateGC can generate BadAlloc, BadDrawable, BadFont, BadMatch, Bad
Pixmap, and BadValue errors.
The XCopyGC 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 BadMatch error results. The valuemask speci
fies which component to copy, as for XCreateGC.
XCopyGC can generate BadAlloc, BadGC, and BadMatch errors.
The XChangeGC 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 XCreateGC.
Changing the clipmask overrides any previous XSetClipRectangles
request on the context. Changing the dashoffset or dashlist over
rides any previous XSetDashes 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.
XChangeGC can generate BadAlloc, BadFont, BadGC, BadMatch, BadPixmap,
and BadValue errors.
The XGetGCValues function returns the components specified by valuemask
for the specified GC. If the valuemask contains a valid set of GC mask
bits (GCFunction, GCPlaneMask, GCForeground, GCBackground, GCLineWidth,
GCLineStyle, GCCapStyle, GCJoinStyle, GCFillStyle, GCFillRule, GCTile,
GCStipple, GCTileStipXOrigin, GCTileStipYOrigin, GCFont, GCSubwindow
Mode, GCGraphicsExposures, GCClipXOrigin, GCCLipYOrigin, GCDashOffset,
or GCArcMode) and no error occurs, XGetGCValues sets the requested com
ponents in values_return and returns a nonzero status. Otherwise, it
returns a zero status. Note that the clipmask and dashlist (repre
sented by the GCClipMask and GCDashList bits, respectively, in the val
uemask) cannot be requested. Also note that an invalid resource ID
(with one or more of the three most significant bits set to 1) will be
returned for GCFont, GCTile, and GCStipple if the component has never
been explicitly set by the client.
The XFreeGC function destroys the specified GC as well as all the asso
ciated storage.
XFreeGC can generate a BadGC error.
STRUCTURES
The XGCValues structure contains:
/* GC attribute value mask bits */
lw(.5i) lw(2.5i) lw(.75i). T{ #define T} T{ GCFunction T} T{
(1L<<0) T} T{ #define T} T{ GCPlaneMask T} T{ (1L<<1) T} T{ #define
T} T{ GCForeground T} T{ (1L<<2) T} T{ #define T} T{ GCBackground
T} T{ (1L<<3) T} T{ #define T} T{ GCLineWidth T} T{ (1L<<4) T} T{
#define T} T{ GCLineStyle T} T{ (1L<<5) T} T{ #define T} T{
GCCapStyle T} T{ (1L<<6) T} T{ #define T} T{ GCJoinStyle T} T{
(1L<<7) T} T{ #define T} T{ GCFillStyle T} T{ (1L<<8) T} T{ #define
T} T{ GCFillRule T} T{ (1L<<9) T} T{ #define T} T{ GCTile T} T{
(1L<<10) T} T{ #define T} T{ GCStipple T} T{ (1L<<11) T} T{ #define
T} T{ GCTileStipXOrigin T} T{ (1L<<12) T} T{ #define T} T{
GCTileStipYOrigin T} T{ (1L<<13) T} T{ #define T} T{ GCFont T} T{
(1L<<14) T} T{ #define T} T{ GCSubwindowMode T} T{ (1L<<15) T} T{
#define T} T{ GCGraphicsExposures T} T{ (1L<<16) T} T{ #define
T} T{ GCClipXOrigin T} T{ (1L<<17) T} T{ #define T} T{ GCClipYO
rigin T} T{ (1L<<18) T} T{ #define T} T{ GCClipMask T} T{
(1L<<19) T} T{ #define T} T{ GCDashOffset T} T{ (1L<<20) T} T{
#define T} T{ GCDashList T} T{ (1L<<21) T} T{ #define T} T{ GCAr
cMode T} T{ (1L<<22) T}
/* 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. GXcopy 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 <X11/X.h>, are:
lw(1.5i) cw(.5i) lw(2i). _
Function Name ValueOperation
_
() ()
T{ GXclear T} T{ 0x0 T} T{ 0 T} T{ GXand T} T{ 0x1 T} T{ src
AND dst T} T{ GXandReverse T} T{ 0x2 T} T{ src AND NOT dst T} T{
GXcopy T} T{ 0x3 T} T{ src T} T{ GXandInverted T} T{ 0x4 T} T{
(NOT src) AND dst T} T{ GXnoop T} T{ 0x5 T} T{ dst T} T{ GXxor
T} T{ 0x6 T} T{ src XOR dst T} T{ GXor T} T{ 0x7 T} T{ src OR
dst T} T{ GXnor T} T{ 0x8 T} T{ (NOT src) AND (NOT dst) T} T{ GXe
quiv T} T{ 0x9 T} T{ (NOT src) XOR dst T} T{ GXinvert T} T{ 0xa
T} T{ NOT dst T} T{ GXorReverse T} T{ 0xb T} T{ src OR (NOT dst)
T} T{ GXcopyInverted T} T{ 0xc T} T{ NOT src T} T{ GXorInverted
T} T{ 0xd T} T{ (NOT src) OR dst T} T{ GXnand T} T{ 0xe T} T{
(NOT src) OR (NOT dst) T} T{ GXset T} T{ 0xf T} T{ 1 T}
_
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
AllPlanes can be used to refer to all planes of the screen simultane
ously. The result is computed by the following:
((src FUNC dst) AND planemask) OR (dst AND (NOT planemask))
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 linewidth 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 joinstyle or capstyle,
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 linewidth) are onepixelwide lines drawn using an
unspecified, devicedependent 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 capstyle
and joinstyle. It is recommended that this property be true for thin
lines, but this is not required. A linewidth of zero may differ from
a linewidth 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 linewidth of one rather than a linewidth
of zero.
The linestyle defines which sections of a line are drawn:
lw(1.3i) lw(4.5i). T{ LineSolid T} T{ The full path of the line is
drawn. T}
T{ LineDoubleDash T} T{ The full path of the line is drawn, but the
even dashes are filled differently from the odd dashes (see fillstyle)
with CapButt style used where even and odd dashes meet. T}
T{ LineOnOffDash T} T{ Only the even dashes are drawn, and capstyle
applies to all internal ends of the individual dashes, except CapNot
Last is treated as CapButt. T}
The capstyle defines how the endpoints of a path are drawn:
lw(1.3i) lw(4.5i). T{ CapNotLast T} T{ This is equivalent to CapButt
except that for a linewidth of zero the final endpoint is not drawn.
T}
T{ CapButt T} T{ The line is square at the endpoint (perpendicular to
the slope of the line) with no projection beyond. T}
T{ CapRound T} T{ The line has a circular arc with the diameter equal
to the linewidth, centered on the endpoint. (This is equivalent to
CapButt for linewidth of zero). T}
T{ CapProjecting T} T{ The line is square at the end, but the path
continues beyond the endpoint for a distance equal to half the line
width. (This is equivalent to CapButt for linewidth of zero). T}
The joinstyle defines how corners are drawn for wide lines:
lw(1.3i) lw(4.5i). T{ JoinMiter T} T{ The outer edges of two lines
extend to meet at an angle. However, if the angle is less than 11
degrees, then a JoinBevel joinstyle is used instead. T}
T{ JoinRound T} T{ The corner is a circular arc with the diameter
equal to the linewidth, centered on the joinpoint. T}
T{ JoinBevel T} T{ The corner has CapButt endpoint styles with the
triangular notch filled. T}
For a line with coincident endpoints (x1=x2, y1=y2), when the capstyle
is applied to both endpoints, the semantics depends on the linewidth
and the capstyle:
lw(1.3i) lw(.5i) lw(4i). T{ CapNotLast T} T{ thin T} T{ The
results are device dependent, but the desired effect is that nothing is
drawn. T}
T{ CapButt T} T{ thin T} T{ The results are device dependent, but
the desired effect is that a single pixel is drawn. T}
T{ CapRound T} T{ thin T} T{ The results are the same as for Cap
Butt/thin. T}
T{ CapProjecting T} T{ thin T} T{ The results are the same as for
CapButt/thin. T}
T{ CapButt T} T{ wide T} T{ Nothing is drawn. T}
T{ CapRound T} T{ wide T} T{ The closed path is a circle, centered
at the endpoint, and with the diameter equal to the linewidth. T}
T{ CapProjecting T} T{ wide T} T{ The closed path is a square,
aligned with the coordinate axes, centered at the endpoint, and with
the sides equal to the linewidth. T}
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 capstyle is applied at both endpoints.
The tile/stipple represents an infinite twodimensional plane, with the
tile/stipple replicated in all dimensions. When that plane is superim
posed on the drawable for use in a graphics operation, the upperleft
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 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 BadMatch
error results. The stipple pixmap must have depth one and must have
the same root as the GC, or a BadMatch error results. For stipple
operations where the fillstyle is FillStippled but not FillOpaqueStip
pled, the stipple pattern is tiled in a single plane and acts as an
additional clip mask to be ANDed with the clipmask. Although some
sizes may be faster to use than others, any size pixmap can be used for
tiling or stippling.
The fillstyle defines the contents of the source for line, text, and
fill requests. For all text and fill requests (for example, XDrawText,
XDrawText16, XFillRectangle, XFillPolygon, and XFillArc); for line
requests with linestyle LineSolid (for example, XDrawLine, XDrawSeg
ments, XDrawRectangle, XDrawArc); and for the even dashes for line
requests with linestyle LineOnOffDash or LineDoubleDash, the following
apply:
lw(1.8i) lw(4i). T{ FillSolid T} T{ Foreground T}
T{ FillTiled T} T{ Tile T}
T{ FillOpaqueStippled T} T{ 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 T}
T{ FillStippled T} T{ Foreground masked by stipple T}
When drawing lines with linestyle LineDoubleDash, the odd dashes are
controlled by the fillstyle in the following manner:
lw(1.8i) lw(4i). T{ FillSolid T} T{ Background T}
T{ FillTiled T} T{ Same as for even dashes T}
T{ FillOpaqueStippled T} T{ Same as for even dashes T}
T{ FillStippled T} T{ Background masked by stipple T}
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 on the display hard
ware 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 XSetDashes. Specifying a value of N is
equivalent to specifying the twoelement list [N, N] in XSetDashes.
The value must be nonzero, or a BadValue error results.
The clipmask restricts writes to the destination drawable. If the
clipmask is set to a pixmap, it must have depth one and have the same
root as the GC, or a BadMatch error results. If clipmask is set to
None, the pixels are always drawn regardless of the clip origin. The
clipmask also can be set by calling the XSetClipRectangles or XSetRe
gion functions. Only pixels where the clipmask has a bit set to 1 are
drawn. Pixels are not drawn outside the area covered by the clipmask
or where the clipmask has a bit set to 0. The clipmask affects all
graphics requests. The clipmask does not clip sources. The clipmask
origin is interpreted relative to the origin of whatever destination
drawable is specified in a graphics request.
You can set the subwindowmode to ClipByChildren or IncludeInferiors.
For ClipByChildren, both source and destination windows are addition
ally clipped by all viewable InputOutput children. For IncludeInferi
ors, 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
IncludeInferiors on a window of one depth with mapped inferiors of dif
fering depth is not illegal, but the semantics are undefined by the
core protocol.
The fillrule defines what pixels are inside (drawn) for paths given in
XFillPolygon requests and can be set to EvenOddRule or WindingRule.
For EvenOddRule, a point is inside if an infinite ray with the point as
origin crosses the path an odd number of times. For WindingRule, 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 EvenOddRule and WindingRule, 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 bound
ary. 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 arcmode controls filling in the XFillArcs function and can be set
to ArcPieSlice or ArcChord. For ArcPieSlice, the arcs are pieslice
filled. For ArcChord, the arcs are chord filled.
The graphicsexposure flag controls GraphicsExpose event generation for
XCopyArea and XCopyPlane requests (and any similar requests defined by
extensions).
DIAGNOSTICS
BadAlloc The server failed to allocate the requested resource or
server memory.
BadDrawable
A value for a Drawable argument does not name a defined Win
dow or Pixmap.
BadFont A value for a Font or GContext argument does not name a
defined Font.
BadGC A value for a GContext argument does not name a defined GCon
text.
BadMatch An InputOnly window is used as a Drawable.
BadMatch Some argument or pair of arguments has the correct type and
range but fails to match in some other way required by the
request.
BadPixmap A value for a Pixmap argument does not name a defined Pixmap.
BadValue 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), XSetClipOri
gin(3X11), XSetFillStyle(3X11), XSetFont(3X11), XSetLineAt
tributes(3X11), XSetState(3X11), XSetTile(3X11)
Xlib  C Language X Interface
()
