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| Interix / SUA | XFreeGC.3 | Interix / SUA |
XCreateGC(3) XLIB FUNCTIONS XCreateGC(3)
NAME
XCreateGC, XCopyGC, XChangeGC, XGetGCValues, XFreeGC,
XGContextFromGC, XGCValues - create or free graphics con-
texts and graphics context structure
SYNTAX
GC XCreateGC(Display *display, Drawable d, unsigned long
valuemask, XGCValues *values);
int XCopyGC(Display *display, GC src, GC dest, unsigned
long valuemask);
int XChangeGC(Display *display, GC gc, unsigned long val-
uemask, XGCValues *values);
Status XGetGCValues(Display *display, GC gc, unsigned long
valuemask, XGCValues *values_return);
int XFreeGC(Display *display, GC gc);
GContext XGContextFromGC(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 argu-
ment is the bitwise inclusive OR of zero or more
of the valid GC component mask bits.
values Specifies any values as specified by the value-
mask.
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 des-
tination GCs must have the same root and depth, or a Bad-
Match error results. The valuemask specifies which compo-
nent 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 components are verified and altered is server
dependent. If an error is generated, a subset of the com-
ponents 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 con-
tains a valid set of GC mask bits (GCFunction, GCPlane-
Mask, GCForeground, GCBackground, GCLineWidth,
GCLineStyle, GCCapStyle, GCJoinStyle, GCFillStyle, GCFill-
Rule, GCTile, GCStipple, GCTileStipXOrigin, GCTileStipYO-
rigin, GCFont, GCSubwindowMode, 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 (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 */ lw(.5i) lw(2.5i)
lw(.75i). T{ #define T} T{ GCFunction T} T{ (1L<<0)
T} T{ #define T} T{ GCPlaneMask T} T{ (1L<<1) T} T{
#define T} T{ GCForeground T} T{ (1L<<2) T} T{ #define
T} T{ GCBackground T} T{ (1L<<3) T} T{ #define T} T{
GCLineWidth T} T{ (1L<<4) T} T{ #define T} T{
GCLineStyle T} T{ (1L<<5) T} T{ #define T} T{ GCCap-
Style T} T{ (1L<<6) T} T{ #define T} T{ GCJoinStyle
T} T{ (1L<<7) T} T{ #define T} T{ GCFillStyle T} T{
(1L<<8) T} T{ #define T} T{ GCFillRule T} T{ (1L<<9)
T} T{ #define T} T{ GCTile T} T{ (1L<<10) T} T{
#define T} T{ GCStipple T} T{ (1L<<11) T} T{ #define
T} T{ GCTileStipXOrigin T} T{ (1L<<12) T} T{ #define
T} T{ GCTileStipYOrigin T} T{ (1L<<13) T} T{ #define
T} T{ GCFont T} T{ (1L<<14) T} T{ #define T} T{
GCSubwindowMode T} T{ (1L<<15) T} T{ #define T} T{
GCGraphicsExposures T} T{ (1L<<16) T} T{ #define T} T{
GCClipXOrigin T} T{ (1L<<17) T} T{ #define T} T{
GCClipYOrigin T} T{ (1L<<18) T} T{ #define T} T{
GCClipMask T} T{ (1L<<19) T} T{ #define T} T{
GCDashOffset T} T{ (1L<<20) T} T{ #define T} T{
GCDashList T} T{ (1L<<21) T} T{ #define T} T{ GCArc-
Mode T} T{ (1L<<22) T}
/* 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 typi-
cally the most useful because it will work on a color dis-
play, but special applications may use other functions,
particularly in concert with particular planes of a color
display. The 16 GC functions, defined in , are:
lw(1.5i) cw(.5i) lw(2i). _
Function Name ValueOperation
_
() ()
T{ GXclear T} T{ 0x0 T} T{ 0 T} T{ GXand T} T{ 0x1
T} T{ src AND dst T} T{ GXandReverse T} T{ 0x2 T} T{
src AND NOT dst T} T{ GXcopy T} T{ 0x3 T} T{ src T} T{
GXandInverted T} T{ 0x4 T} T{ (NOT src) AND dst T} T{
GXnoop T} T{ 0x5 T} T{ dst T} T{ GXxor T} T{ 0x6
T} T{ src XOR dst T} T{ GXor T} T{ 0x7 T} T{ src OR
dst T} T{ GXnor T} T{ 0x8 T} T{ (NOT src) AND (NOT
dst) T} T{ GXequiv T} T{ 0x9 T} T{ (NOT src) XOR dst
T} T{ GXinvert T} T{ 0xa T} T{ NOT dst T} T{ GXorRe-
verse T} T{ 0xb T} T{ src OR (NOT dst) T} T{ GXcopyIn-
verted T} T{ 0xc T} T{ NOT src T} T{ GXorInverted
T} T{ 0xd T} T{ (NOT src) OR dst T} T{ GXnand T} T{
0xe T} T{ (NOT src) OR (NOT dst) T} T{ GXset T} T{ 0xf
T} T{ 1 T}
_
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 per-
formed 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 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 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 horizon-
tal 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 direc-
tion).
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 draw-
ing 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:
lw(1.3i) lw(4.5i). T{ LineSolid T} T{ The full path of
the line is drawn. T}
T{ LineDoubleDash T} T{ The full path of the line is
drawn, but the even dashes are filled differently from the
odd dashes (see fill-style) with CapButt style used where
even and odd dashes meet. T}
T{ LineOnOffDash T} T{ Only the even dashes are drawn,
and cap-style applies to all internal ends of the individ-
ual dashes, except CapNotLast is treated as CapButt. T}
The cap-style defines how the endpoints of a path are
drawn: lw(1.3i) lw(4.5i). T{ CapNotLast T} T{ This is
equivalent to CapButt except that for a line-width of zero
the final endpoint is not drawn. T}
T{ CapButt T} T{ The line is square at the endpoint
(perpendicular to the slope of the line) with no projec-
tion beyond. T}
T{ CapRound T} T{ 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). T}
T{ CapProjecting T} T{ 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 Cap-
Butt for line-width of zero). T}
The join-style defines how corners are drawn for wide
lines: lw(1.3i) lw(4.5i). T{ JoinMiter T} T{ 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. T}
T{ JoinRound T} T{ The corner is a circular arc with the
diameter equal to the line-width, centered on the join-
point. T}
T{ JoinBevel T} T{ The corner has CapButt endpoint
styles with the triangular notch filled. T}
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: lw(1.3i)
lw(.5i) lw(4i). T{ CapNotLast T} T{ thin T} T{ The
results are device dependent, but the desired effect is
that nothing is drawn. T}
T{ CapButt T} T{ thin T} T{ The results are device
dependent, but the desired effect is that a single pixel
is drawn. T}
T{ CapRound T} T{ thin T} T{ The results are the same
as for CapButt/thin. T}
T{ CapProjecting T} T{ thin T} T{ The results are the
same as for CapButt/thin. T}
T{ CapButt T} T{ wide T} T{ Nothing is drawn. T}
T{ CapRound T} T{ wide T} T{ The closed path is a cir-
cle, centered at the endpoint, and with the diameter equal
to the line-width. T}
T{ CapProjecting T} T{ wide T} T{ The closed path is a
square, aligned with the coordinate axes, centered at the
endpoint, and with the sides equal to the line-width. T}
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 end-
points.
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, XFillRect-
angle, 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 Line-
DoubleDash, the following apply: lw(1.8i) lw(4i). T{
FillSolid T} T{ Foreground T}
T{ FillTiled T} T{ Tile T}
T{ FillOpaqueStippled T} T{ 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 T}
T{ FillStippled T} T{ Foreground masked by stipple T}
When drawing lines with line-style LineDoubleDash, the odd
dashes are controlled by the fill-style in the following
manner: lw(1.8i) lw(4i). T{ FillSolid T} T{ Background
T}
T{ FillTiled T} T{ Same as for even dashes T}
T{ FillOpaqueStippled T} T{ Same as for even dashes T}
T{ FillStippled T} T{ Background masked by stipple T}
Storing a pixmap in a GC might or might not result in a
copy being made. If the pixmap is later used as the des-
tination for a graphics request, the change might or might
not be reflected in the GC. If the pixmap is used simul-
taneously 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 possi-
ble with the same GC (without changing its components).
The costs of changing GC components relative to using dif-
ferent 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. Speci-
fying 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 draw-
able. 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 what-
ever 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 view-
able 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 pro-
tocol.
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 counterclock-
wise 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 ArcPieS-
lice, 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 cor-
rect 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 spe-
cific 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), XQueryBest-
Size(3X11), XSetArcMode(3X11), XSetClipOrigin(3X11), XSet-
FillStyle(3X11), XSetFont(3X11), XSetLineAttributes(3X11),
XSetState(3X11), XSetTile(3X11)
Xlib - C Language X Interface
()