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XCreateGC(3X11)
X11R6
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
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, 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 occurs, 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 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
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 */
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 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.
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 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 two-dimensional 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 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 on 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, 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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Index for
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Alphabetical
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