XCreateGC(3X11) RISC/os Reference Manual XCreateGC(3X11)
NAME
XCreateGC, XCopyGC, XChangeGC, XGetGCValues, XFreeGC, XGCon-
textFromGC, 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 one 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 draw-
able having the same root and depth as the specified draw-
able. Use with other drawables results in a BadMatch error.
XCreateGC can generate BadAlloc, BadDrawable, BadFont, Bad-
Match, BadPixmap, and BadValue errors.
The XCopyGC function copies the specified components from
the source GC to the destination GC. The source and desti-
nation 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 con-
tains 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 com-
ponents 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. Note that the clip mask
and dash list (represented by the GCClipMask and GCDashList
bits, respectively, in the valuemask) cannot be requested.
If the valuemask contains a valid set of GC mask bits
(GCFunction, GCPlaneMask, GCForeground, GCBackground,
GCLineWidth, GCLineStyle, GCCapStyle, GCJoinStyle,
GCFillStyle, GCFillRule, GCTile, GCStipple, GCTileStipXOri-
gin, GCTileStipYOrigin, GCFont, GCSubwindowMode, GCGra-
phicsExposures, 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.
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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 */
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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 some-
where 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 Hex Code Operation
_________________________________________________
(Reg.).nr 32 0
GXclear 0x0 0
GXand 0x1 src AND dst
GXandReverse 0x2 src AND NOT dst
GXcopy
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src
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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 modi-
fied, 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:
(Reg.)((src FUNC dst) AND plane-mask) OR (dst AND (NOT plane-mask))
Range checking is not performed on the values for fore-
ground, background, or plane_mask. They are simply trun-
cated 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
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line and so is drawn if the center of the pixel is fully
inside the bounding box (which is viewed as having infin-
itely 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 direc-
tion). Pixels with centers on a horizontal edge are a spe-
cial case and are part of the line if and only if the inte-
rior 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 dif-
ferent 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.
LineDoub- The full path of the line is drawn, but the
leDash even dashes are filled differently than the
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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 indivi-
dual 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).
CapProject- The line is square at the end, but the path
ing 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
CapButt/thin.
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CapProject- thin The results are the same as for
ing 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.
CapProject- wide The closed path is a square, aligned
ing 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 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 Fil-
lOpaqueStippled, 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 stip-
pling.
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, XFillPo-
lygon, and XFillArc); for line requests with line-style
LineSolid (for example, XDrawLine, XDrawSegments, XDrawRec-
tangle, XDrawArc); and for the even dashes for line requests
with line-style LineOnOffDash or LineDoubleDash, the follow-
ing 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
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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 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 imple-
mentation. It is quite likely that some amount of GC infor-
mation 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. Specify-
ing 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 func-
tions. 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 Inclu-
deInferiors. For ClipByChildren, both source and destina-
tion windows are additionally clipped by all viewable Inpu-
tOutput 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 infe-
riors of differing depth is not illegal, but the semantics
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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 inte-
rior 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.
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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 gen-
erate 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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