Ubuntu Manpage: XCreateGC, XCopyGC, XChangeGC, XGetGCValues, XFreeGC, XGContextFromGC, XGCValues

NAME

       XCreateGC,  XCopyGC, XChangeGC, XGetGCValues, XFreeGC, XGContextFromGC, XGCValues - create
       or free graphics contexts and graphics context structure

SYNTAX

       GC XCreateGC(Display *display, Drawable d, unsigned long valuemask, XGCValues *values);

       int XCopyGC(Display *display, GC src, unsigned long valuemask, GC dest);

       int XChangeGC(Display *display, GC gc, unsigned long valuemask, 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 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 from 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