XCreateGC(3X11) X11 SDE 5.4R3.00 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, valuesreturn)
Display *display;
GC gc;
unsigned long valuemask;
XGCValues *valuesreturn;
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.
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valuesreturn
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 occur, 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 one) 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.
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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;
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} 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).
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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.
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LineDoubleDash 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.
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.
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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 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 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
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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 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
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,
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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(3X11), XCopyArea(3X11), XCreateRegion(3X11),
XDrawArc(3X11), XDrawLine(3X11), XDrawRectangle(3X11),
XDrawText(3X11), XFillRectangle(3X11), XQueryBestSize(3X11),
XSetArcMode(3X11), XSetClipOrigin(3X11), XSetFillStyle(3X11),
XSetFont(3X11), XSetLineAttributes(3X11), XSetState(3X11),
XSetTile(3X11)
Xlib - C Language X Interface
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