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XCreateGC(3)                    XLIB FUNCTIONS                    XCreateGC(3)



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, GC dest, unsigned long value-
              mask);

       int XChangeGC(Display *display, GC gc, unsigned long valuemask, XGCVal-
              ues *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 inclu-
                 sive 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, Bad-
       Pixmap, 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 speci-
       fies 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 over-
       rides 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, GCSubwindow-
       Mode, 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 (repre-
       sented by the GCClipMask and GCDashList bits, respectively, in the val-
       uemask) 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 asso-
       ciated 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{
       GCCapStyle 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{ GCClipYO-
       rigin 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{ GCAr-
       cMode 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
       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:

       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{ GXe-
       quiv T}   T{ 0x9 T}   T{ (NOT src) XOR dst T} T{ GXinvert T}   T{ 0xa
       T}   T{ NOT dst T} T{ GXorReverse T}   T{ 0xb T}   T{ src OR (NOT dst)
       T} T{ GXcopyInverted 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 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 simultane-
       ously.  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, back-
       ground, 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 con-
       straints 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:

       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 individual dashes, except CapNot-
       Last 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 projection beyond.  T}
       T{ CapRound T}   T{ 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).  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 CapButt 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 joinpoint.  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 Cap-
       Butt/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 circle, 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 endpoints.

       The tile/stipple represents an infinite two-dimensional plane, with the
       tile/stipple replicated in all dimensions.  When that plane is superim-
       posed 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 FillOpaqueStip-
       pled, 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, XDrawSeg-
       ments, XDrawRectangle, XDrawArc); and for the even dashes for line
       requests with line-style LineOnOffDash or LineDoubleDash, 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 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 destina-
       tion 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 hard-
       ware 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 pat-
       terns 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 XSetRe-
       gion 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 addition-
       ally clipped by all viewable InputOutput children.  For IncludeInferi-
       ors, 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 dif-
       fering 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 seg-
       ments.  A clockwise directed path segment is one that crosses the ray
       from left to right as observed from the point.  A counterclockwise seg-
       ment 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 bound-
       ary.  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 Win-
                 dow 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 GCon-
                 text.

       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), XFillRectan-
       gle(3X11), XQueryBestSize(3X11), XSetArcMode(3X11), XSetClipOri-
       gin(3X11), XSetFillStyle(3X11), XSetFont(3X11), XSetLineAt-
       tributes(3X11), XSetState(3X11), XSetTile(3X11)
       Xlib - C Language X Interface



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