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<HTML><HEAD><TITLE>Xlib Programming Manual: Manipulating Graphics Context/State</TITLE></HEAD><BODY><H1 ALIGN=center>7.1 Manipulating Graphics Context/State</H1>Most attributes of graphics operations are stored in Graphic Contexts (GCs).These include line width, line style, plane mask, foreground, background,tile, stipple, clipping region, end style, join style, and so on.Graphics operations (for example, drawing lines) use these valuesto determine the actual drawing operation.Extensions to X may add additional components to GCs.The contents of a GC are private to Xlib.<P>Xlib implements a write-back cache for all elements of a GC that are notresource IDs to allow Xlib to implement the transparent coalescing of changes to GCs.For example,a call to<B><A HREF="convenience-functions/XSetForeground.html">XSetForeground()</A></B>of a GC followed by a call to<B><A HREF="convenience-functions/XSetLineAttributes.html">XSetLineAttributes()</A></B>results in only a single-change GC protocol request to the server.GCs are neither expected nor encouraged to be shared between client applications, so this write-back caching should present no problems.Applications cannot share GCs without external synchronization.Therefore,sharing GCs between applications is highly discouraged. <A NAME="XGCValues"></A><P>To set an attribute of a GC,set the appropriate member of the<B>XGCValues</B>structure and OR in the corresponding value bitmask in your subsequent calls to<B><A HREF="XCreateGC.html">XCreateGC()</A></B>.The symbols for the value mask bits and the<B>XGCValues</B><P><CODE><PRE>/* GC attribute value mask bits */#define <B>GCFunction</B> (1L<<0)#define <B>GCPlaneMask</B> (1L<<1)#define <B>GCForeground</B> (1L<<2)#define <B>GCBackground</B> (1L<<3)#define <B>GCLineWidth</B> (1L<<4)#define <B>GCLineStyle</B> (1L<<5)#define <B>GCCapStyle</B> (1L<<6)#define <B>GCJoinStyle</B> (1L<<7)#define <B>GCFillStyle</B> (1L<<8)#define <B>GCFillRule</B> (1L<<9)#define <B>GCTile</B> (1L<<10)#define <B>GCStipple</B> (1L<<11)#define <B>GCTileStipXOrigin</B> (1L<<12)#define <B>GCTileStipYOrigin</B> (1L<<13)#define <B>GCFont</B> (1L<<14)#define <B>GCSubwindowMode</B> (1L<<15)#define <B>GCGraphicsExposures</B> (1L<<16)#define <B>GCClipXOrigin</B> (1L<<17)#define <B>GCClipYOrigin</B> (1L<<18)#define <B>GCClipMask</B> (1L<<19)#define <B>GCDashOffset</B> (1L<<20)#define <B>GCDashList</B> (1L<<21)#define <B>GCArcMode</B> (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;</PRE></CODE><P>The default GC values are:<TABLE><TR><TD COLSPAN=2><HR><TR><TD><B>Component <TD><B>Default<TR><TD COLSPAN=2><HR><TR><TD>function <TD><B>GXcopy</B><TR><TD>plane_mask <TD>All ones<TR><TD>foreground <TD>0<TR><TD>background <TD>1<TR><TD>line_width <TD>0<TR><TD>line_style <TD><B>LineSolid</B><TR><TD>cap_style <TD><B>CapButt</B><TR><TD>join_style <TD><B>JoinMiter</B><TR><TD>fill_style <TD><B>FillSolid</B><TR><TD>fill_rule <TD><B>EvenOddRule</B><TR><TD>arc_mode <TD><B>ArcPieSlice</B><TR><TD>tile <TD>Pixmap of unspecified size filled with foreground pixel (that is, client specified pixel if any, else 0) (subsequent changes to foreground do not affect this pixmap)<TR><TD>stipple <TD>Pixmap of unspecified size filled with ones<TR><TD>ts_x_origin <TD>0<TR><TD>ts_y_origin <TD>0<TR><TD>font <TD><implementation dependent><TR><TD>subwindow_mode <TD><B>ClipByChildren</B><TR><TD>graphics_exposures <TD><B>True</B><TR><TD>clip_x_origin <TD>0<TR><TD>clip_y_origin <TD>0<TR><TD>clip_mask <TD><B>None</B><TR><TD>dash_offset <TD>0<TR><TD>dashes <TD>4 (that is, the list [4, 4])<TR><TD COLSPAN=2><HR></TABLE><P>Note that foreground and background are not set to any values likelyto be useful in a window.<A NAME="function"></A><P><!.IN "Display Functions" "" "@DEF@"><!.IN "Source" "" "@DEF@"><!.IN "Destination" "" "@DEF@">The function attributes of a GC are used when you update a section ofa drawable (the destination) with bits from somewhere else (the source). The function in a GC defines how the new destination bits are to becomputed from the source bits and the old destination bits.<B>GXcopy</B>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 <B><TT>X11/X.h</TT></B>,are:<TABLE><TR><TD COLSPAN=3><HR><TR><TD><B>Function Name</B> <TD><B>Value</B> <TD><B>Operation</B><TR><TD COLSPAN=3><HR><TR><TD><B>GXclear</B> <TD>0x0 <TD>0<TR><TD><B>GXand</B> <TD>0x1 <TD>src AND dst<TR><TD><B>GXandReverse</B> <TD>0x2 <TD>src AND NOT dst<TR><TD><B>GXcopy</B> <TD>0x3 <TD>src<TR><TD><B>GXandInverted</B> <TD>0x4 <TD>(NOT src) AND dst<TR><TD><B>GXnoop</B> <TD>0x5 <TD>dst<TR><TD><B>GXxor</B> <TD>0x6 <TD>src XOR dst<TR><TD><B>GXor</B> <TD>0x7 <TD>src OR dst<TR><TD><B>GXnor</B> <TD>0x8 <TD>(NOT src) AND (NOT dst)<TR><TD><B>GXequiv</B> <TD>0x9 <TD>(NOT src) XOR dst<TR><TD><B>GXinvert</B> <TD>0xa <TD>NOT dst<TR><TD><B>GXorReverse</B> <TD>0xb <TD>src OR (NOT dst)<TR><TD><B>GXcopyInverted</B> <TD>0xc <TD>NOT src<TR><TD><B>GXorInverted</B> <TD>0xd <TD>(NOT src) OR dst<TR><TD><B>GXnand</B> <TD>0xe <TD>(NOT src) OR (NOT dst)<TR><TD><B>GXset</B> <TD>0xf <TD>1<TR><TD COLSPAN=3><HR></TABLE><P>Many graphics operations depend on either pixel values or planes in a GC.<!.IN "Pixel value">The planes attribute is of type long, and it specifies which planes of thedestination are to be modified, one bit per plane.<!.IN "Plane" "mask">A monochrome display has only one plane andwill be the least-significant bit of the word.As planes are added to the display hardware, they will occupy moresignificant bits in the plane mask.<P>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<B>AllPlanes()</B>can be used to refer to all planes of the screen simultaneously.The result is computed by the following:<P><CODE><PRE>((src FUNC dst) AND plane-mask) OR (dst AND (NOT plane-mask))</PRE></CODE><A NAME="foreground"></A><A NAME="background"></A><A NAME="foreground"></A><A NAME="plane-mask"></A><A NAME="line-width"></A><P>Range checking is not performed on the values for foreground,background, or plane_mask.They are simply truncated to the appropriatenumber of bits.The line-width is measured in pixels and either can be greater than or equal toone (wide line) or can be the special value zero (thin line).<P>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] andwidth w is a rectangle with vertices at the following real coordinates:<P><CODE><PRE>[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)]</PRE></CODE><P>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 drawnif 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 ofthe line if and only if the interior or the boundary is immediately below (y increasing direction) and the interior or the boundary is immediatelyto the right (x increasing direction).<P>Thin lines (zero line-width) are one-pixel-wide lines drawn using anunspecified, device-dependent algorithm.There are only two constraints on this algorithm. <OL><LI>If a line is drawn unclipped from [x1,y1] to [x2,y2] andif 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.<P><LI>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 touchedby the line when drawn unclipped.</OL><P>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 aredrawn.This permits the use of many manufacturers' line drawing hardware,which may run many times faster than the more precisely specifiedwide lines.<P>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.<A NAME="line-style"></A><P>The line-style defines which sections of a line are drawn:<TABLE><TR><TD><B>LineSolid</B><TD>The full path of the line is drawn.<TR><TD><B>LineDoubleDash</B><TD>The full path of the line is drawn, but the even dashes are filled differently than the odd dashes (see fill-style) with<TR><TD><B>CapButt</B><TD>style used where even and odd dashes meet.<TR><TD><B>LineOnOffDash</B><TD>Only the even dashes are drawn,and cap-style applies to all internal ends of the individual dashes,except <B>CapNotLast</B>is treated as <B>CapButt</B>. </TABLE><A NAME="cap-style"></A><P>The cap-style defines how the endpoints of a path are drawn:<!.IN "Graphics context" "path"><TABLE><TR><TD><B>CapNotLast</B><TD>This is equivalent to <B>CapButt</B>except that for a line-width of zero the final endpoint is not drawn.<TR><TD><B>CapButt</B><TD>The line is square at the endpoint (perpendicular to the slope of the line)with no projection beyond.<TR><TD><B>CapRound</B><TD>The line has a circular arc with the diameter equal to the line-width,centered on the endpoint.(This is equivalent to <B>CapButt</B>for line-width of zero).<TR><TD><B>CapProjecting</B><TD>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 <B>CapButt</B>for line-width of zero).</TABLE><A NAME="join-style"></A><P>The join-style defines how corners are drawn for wide lines:<TABLE><TR><TD><B>JoinMiter</B><TD>The outer edges of two lines extend to meet at an angle.However, if the angle is less than 11 degrees,then a⌨️ 快捷键说明
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