XCreateGC, XCopyGC, XChangeGC, XGetGCValues, XFreeGC,
       XGContextFromGC, XGCValues - create or free graphics con-
       texts 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 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-
       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,
       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, XGetGC-
       Values 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 (repre-
       sented by the GCClipMask and GCDashList bits, respec-
       tively, 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:

       #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 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 <X11/X.h>, are:
       -----------------------------------------------
       Function Name     Value   Operation
       -----------------------------------------------
       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 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
            [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 speci-
       fied 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:
       LineSolid       The full path of the line is drawn.
       LineDou-        The full path of the line is drawn, but the
       bleDash         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 individ-
                       ual 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).
                       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 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.
                            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 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.
       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
       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 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.

       Xlib - C Language X Interface



                                                                1