Graphics Contexts

  • a graphics context or GC contains the attributes that are used in drawing commands
  • these attributes include background and foreground colours, function, line style, fill pattern, line width, text font, etc
  • the graphics context itself is stored in the server, the client program only has its ID, the client program can change the value of a GC by calling procedures in the X library
  • there are two ways of doing this, the most general technique allows the programmer to change most of the values in the GC is a single call, this approach is based on a data structure called XGCValues, this data structure mirrors the actual GC data structure
  • there is a field in the XGCValues data structure for most of the fields in the graphics context, the programmer can change the fields in this data structure and then have the result sent to the X server, a mask is used to specify the fields that are to be changed
  • the other approach is based on a set of convenience functions that allow the programmer to change each of the GC fields individually
  • this is easier when there are only a few fields that need to be changed
  • the declaration of the XGCValues data structure is:
      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 */
           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, FillOpaeueStippled */
           int fill_rule;             /* EvenOddRule, WindingRule */
           int arc_mode;              /* ArcChord, ArcPieSlice */
           Pixmap tile;               /* tile pixmap for tiling operations */
           Pixmap stipple;            /* stipple 1 plane pixmap for stipping */
           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 mask values that can be used to specify the XGCValues field to use are:
      #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)
  • these values are or’ed together to specify the fields in the GC that will be changed
  • the function field specifies how the pixels are drawn in the drawable, this is like the function that is used in the bitblt operation
  • for example if GXand is specified, then when a line is drawn the value of the pixels covered by the line will be the and of the current value of the pixel and the foreground colour in the GC
  • all the functions are specified in terms of the value of the source (the foreground colour in the GC) and the destination (the pixel value in the drawable)
  • the possible values for the function field are:
      #define GXclear          0x0       /* 0 */
      #define GXand            0x1       /* src AND dst */
      #define GXandReverse     0x2       /* src AND NOT dst */
      #define GXcopy           0x3       /* src */
      #define GXandInverted    0x4       /* NOT src AND dst */
      #define GXnoop           0x5       /* dst */
      #define GXxor            0x6       /* src XOR dst */
      #define GXor             0x7       /* src OR dst */
      #define GXnor            0x8       /* NOT src AND NOT dst */
      #define GXequiv          0x9       /* NOT src XOR dst */
      #define GXinvert         0xa       /* NOT dst */
      #define GXorReverse      0xb       /* src OR NOT dst */
      #define GXcopyInverted   0xc       /* NOT src */
      #define GXorInverted     0xd       /* NOT src OR dst */
      #define GXnand           0xe       /* NOT src OR NOT dst */
      #define GXset            0xf       /* 1 */
  • the default values for the fields in a graphics context are:
      function            GXcopy
      plane_mask          all ones
      foreground          0
      background          1
      line_width          0
      line_style          LineSolid
      cap_style           CapButt
      join_style          JoinMiter
      fill_style          FillSolid
      fill_rule           EvenOddRule
      arc_mode            ArcPieSlice
      tile                Pixmap of unspecified size filled with foreground
      stipple             Pixmap of unspecified size filled with ones
      ts_x_origin         0
      tx_y_origin         0
      font                implementation dependent
      subwindow_mode      ClipByChildren
      graphics_exposures  True
      clip_x_origin       0
      clip_y_origin       0
      clip_mask           None
      dash_offset         0
      dashes              4
  • a new GC can be created by calling the XCreateGC procedure, which has the following declaration:
        GC XCreateGC(display, drawable, valuemask, values)
        Display        *display;
        Drawable  drawable;
        unsigned long  valuemask;
        XGCValues *values;
  • this procedure first creates a new graphics context using the default values, it then substitutes in the new values in the values parameter, the valuemask parameter specifies the fields that should be changed
  • the resulting GC can be used on any drawable that has the same root window and depth as the drawable passed as a parameter
  • the values in one GC can be copied to another GC by using the XCopyGC procedure:
        XCopyGC(display, source, valuemask, destination)
        Display        *display;
        GC        source;
        unsigned long  valuemask;
        GC        destination;
  • the valuemask parameter specifies the fields in the source GC that are to be copied to the destination GC
  • the fields in a GC can be changed by calling the XChangeGC procedure, which has the following declaration:
        XChangeGC(display, gc, valuemask, values)
        Display        *display;
        GC        gc;
        unsigned long  valuemask;
        XGCValues *values;
  • the valuemask parameter specifies the fields in the GC parameter that are to be changed, the new values for these fields are stored in the values parameter
  • the values of the fields of a GC can be retrieved using the XGetGCValues procedure, the clip_mask and dashes fields cannot be retrieved
        status XGetGCValues(display, gc, value_mask, values_return)
        Display        *display;
        GC        gc;
        unsigned long  value_mask;
        XGCValues *values_return
  • the value_mask parameter specifies the fields that are to be retrieved from the gc, these field values are returned in the values_returned parameter
  • the procedure returns a non-zero value if its was able to retrieve all the requested fields, otherwise a zero value is returned
  • a graphics context can be freed by calling the XFreeGC procedure, the declaration of this procedure is:
        XFreeGC(display, gc)
        Display   *display;
        GC   gc;
  • there are also a number of convenience routines that make it easier to set individual fields of the graphics context
  • the XSetState function sets some of the more common GC fields, its declaration is:
        XSetState(display, gc, foreground, background, function,
        Display        *display;
        GC        gc;
        unsigned long  foreground;
        unsigned long  background;
        int       function;
        unsigned long  plane_mask;
  • the following four functions can be used to independently set the foreground colour, background colour, function and plane mask
        XSetForeground(display, gc, foreground)
        Display        *display;
        GC        gc;
        unsigned long  foreground;
        XSetBackground(display, gc, background)
        Display        *display;
        GC        gc;
        unsigned long  background;
        XSetFunction(display, gc, function)
        Display        *display;
        GC        gc;
        int       function;
        XSetPlaneMask(display, gc, plane_mask)
        Display        *display;
        GC        gc;
        unsigned long  plane_mask;