Provided by: libx11-doc_1.8.7-1build1_all 
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, unsigned long valuemask, GC dest);
int XChangeGC(Display *display, GC gc, unsigned long valuemask, XGCValues *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 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 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, BadPixmap, 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
specifies 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 clip-mask overrides any previous XSetClipRectangles request on the context. Changing the
dash-offset or dash-list overrides 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, GCSubwindowMode, GCGraphicsExposures, GCClipXOrigin,
GCClipYOrigin, GCDashOffset, or GCArcMode) and no error occurs, XGetGCValues 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 GCClipMask and GCDashList bits, respectively, 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 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 associated storage.
XFreeGC can generate a BadGC error.
STRUCTURES
The XGCValues structure contains:
/* GC attribute value mask bits */
#define GCFunction (1L<<0)
#define GCPlaneMask (1L<<1)
#define GCForeground (1L<<2)
#define GCBackground (1L<<3)
#define GCLineWidth (1L<<4)
#define GCLineStyle (1L<<5)
#define GCCapStyle (1L<<6)
#define GCJoinStyle (1L<<7)
#define GCFillStyle (1L<<8)
#define GCFillRule (1L<<9)
#define GCTile (1L<<10)
#define GCStipple (1L<<11)
#define GCTileStipXOrigin (1L<<12)
#define GCTileStipYOrigin (1L<<13)
#define GCFont (1L<<14)
#define GCSubwindowMode (1L<<15)
#define GCGraphicsExposures (1L<<16)
#define GCClipXOrigin (1L<<17)
#define GCClipYOrigin (1L<<18)
#define GCClipMask (1L<<19)
#define GCDashOffset (1L<<20)
#define GCDashList (1L<<21)
#define GCArcMode (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. 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:
───────────────────────────────────────────────
Function Name Value Operation
───────────────────────────────────────────────
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 AllPlanes 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, background, 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 constraints 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:
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 from the
odd dashes (see fill-style) with CapButt
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 CapNotLast is
treated as CapButt.
The cap-style defines how the endpoints of a path are drawn:
CapNotLast This is equivalent to CapButt 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 CapButt for
line-width of zero).
CapProjecting 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 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. However, if the angle is less
than 11 degrees, then a JoinBevel 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 CapButt 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
CapButt/thin.
CapProjecting thin The results are the same as for
CapButt/thin.
CapButt wide Nothing is drawn.
CapRound wide The closed path is a circle, centered at
the endpoint, and with the diameter
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 represents an infinite two-dimensional 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 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 fill-style
is FillStippled but not FillOpaqueStippled, 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, XDrawText, XDrawText16, XFillRectangle, XFillPolygon, and XFillArc); for line
requests with line-style LineSolid (for example, XDrawLine, XDrawSegments, XDrawRectangle, XDrawArc); and
for the even dashes for line requests with line-style LineOnOffDash or LineDoubleDash, 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 LineDoubleDash, 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 destination 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
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
XSetDashes. Specifying a value of N is equivalent to specifying the two-element list [N, N] in
XSetDashes. The value must be nonzero, or a BadValue 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 BadMatch error results. If clip-mask is set to
None, the pixels are always drawn regardless of the clip origin. The clip-mask also can be set by
calling the XSetClipRectangles or XSetRegion 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 ClipByChildren or IncludeInferiors. For ClipByChildren, both source
and destination windows are additionally clipped by all viewable InputOutput children. For
IncludeInferiors, 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 differing depth is not
illegal, but the semantics are undefined by the core protocol.
The fill-rule 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
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 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
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 XFillArcs function and can be set to ArcPieSlice or ArcChord. For
ArcPieSlice, the arcs are pie-slice filled. For ArcChord, the arcs are chord filled.
The graphics-exposure 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 Window 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 GContext.
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(3), XCopyArea(3), XCreateRegion(3), XDrawArc(3), XDrawLine(3), XDrawRectangle(3), XDrawText(3),
XFillRectangle(3), XQueryBestSize(3), XSetArcMode(3), XSetClipOrigin(3), XSetFillStyle(3), XSetFont(3),
XSetLineAttributes(3), XSetState(3), XSetTile(3)
Xlib - C Language X Interface