XCreateGC(3X11) X Version 11 (Release 5) 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, 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
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.
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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 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
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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.
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 */
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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
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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.
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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 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
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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 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
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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 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.
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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
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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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