XCreateGC(3X11)XCreateGC(3X11)NAME
XCreateGC, XCopyGC, XChangeGC, XGetGCValues, XFreeGC, XGContextFromGC,
XGCValues - create or free graphics contexts and graphics context
structure
SYNOPSIS
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
Specifies the drawable. Specifies the destination GC. Specifies the
connection to the X server. Specifies the GC. Specifies the compo‐
nents of the source GC. 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. Specifies
any values as specified by the valuemask. 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. Chang‐
ing the clip-mask overrides any previous XSetClipRectangles request on
the context. Changing the dash-offset or dash-list overrides any previ‐
ous 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, 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 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
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. 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. 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 differently than the odd dashes (see fill-style)
with CapButt 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 CapNotLast is treated as CapButt.
The cap-style defines how the endpoints of a path are drawn: This is
equivalent to CapButt except that for a line-width of zero the final
endpoint is not drawn. The line is square at the endpoint (perpendicu‐
lar 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 CapButt for line-width of zero).
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 equivalent
to CapButt 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 JoinBevel 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 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 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 opera‐
tions where the fill-style is FillStippled but not FillOpaqueStippled,
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, XDrawText,
XDrawText16, XFillRectangle, XFillPolygon, and XFillArc); for line
requests with line-style LineSolid (for example, XDrawLine, XDrawSeg‐
ments, 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 stip‐
ple, 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 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 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
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 InputOnly 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)
