Provided by: libx11-doc_1.6.4-3_all bug

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

       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(3), XCopyArea(3), XCreateRegion(3), XDrawArc(3), XDrawLine(3),
       XDrawRectangle(3), XDrawText(3), XFillRectangle(3), XQueryBestSize(3), XSetArcMode(3),
       XSetClipOrigin(3), XSetFillStyle(3), XSetFont(3), XSetLineAttributes(3), XSetState(3),
       XSetTile(3)
       Xlib - C Language X Interface