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, unsigned long valuemask, GC
dest);
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 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, 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 clip-mask overrides any previous XSetClipRectangles re-
quest 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, 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 clip-mask and dash-list (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 */
#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, de-
fined 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 re-
sult is computed bitwise on corresponding bits of the pixels. That is,
a Boolean operation is performed in each bit plane. The plane_mask re-
stricts 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 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 re-
quest. 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 be-
low (y increasing direction) and the interior or the boundary is imme-
diately 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 an-
other 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.
LineDou- The full path of the line is drawn, but the
bleDash even dashes are filled differently from the
odd dashes (see fill-style) with CapButt
style used where even and odd dashes meet.
LineOnOff- Only the even dashes are drawn, and cap-style
Dash applies to all internal ends of the individ-
ual 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 (perpen-
dicular 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 end-
point. (This is equivalent to CapButt for
line-width of zero).
CapPro- The line is square at the end, but the path
jecting 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 diame-
ter 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 Cap-
Butt/thin.
CapPro- thin The results are the same as for Cap-
jecting Butt/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.
CapPro- wide The closed path is a square, aligned
jecting 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 superim-
posed 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 op-
erations where the fill-style is FillStippled but not FillOpaqueStip-
pled, the stipple pattern is tiled in a single plane and acts as an ad-
ditional 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 re-
quests with line-style LineSolid (for example, XDrawLine, XDrawSeg-
ments, XDrawRectangle, XDrawArc); and for the even dashes for line re-
quests with line-style LineOnOffDash or LineDoubleDash, the following
apply:
FillSolid Foreground
FillTiled Tile
FillOpaqueStip- A tile with the same width and height as
pled 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
FillOpaqueStip- Same as for even dashes
pled
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 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 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 XSetRe-
gion 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 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 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 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 in-
creasing direction). Pixels with centers on a horizontal edge are a
special case and are inside if and only if the polygon interior is im-
mediately 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 Win-
dow or Pixmap.
BadFont A value for a Font or GContext argument does not name a de-
fined 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(3), XCopyArea(3), XCreateRegion(3), XDrawArc(3), XDraw-
Line(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
X Version 11 libX11 1.8.4 XCreateGC(3)
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