Graphics Context Functions
A number of resources are used when performing graphics operations in X. Most information
about performing graphics (for example, foreground color, background color, line style, and so
on) is stored in resources called graphics contexts (GCs). Most graphics operations (see chapter
8) take a GC as an argument. Although in theory the X protocol permits sharing of GCs between
applications, it is expected that applications will use their own GCs when performing operations.
Sharing of GCs is highly discouraged because the library may cache GC state.
Graphics operations can be performed to either windows or pixmaps, which collectively are
called drawables. Each drawable exists on a single screen. A GC is created for a specific screen
and drawable depth and can only be used with drawables of matching screen and depth.
This chapter discusses how to:
Manipulate graphics context/stateUse graphics context convenience functionsManipulating Graphics Context/State
Most attributes of graphics operations are stored in 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 values
to determine the actual drawing operation.
Extensions to X may add additional components to GCs.
The contents of a GC are private to Xlib.
Xlib implements a write-back cache for all elements of a GC that are not
resource IDs to allow Xlib to implement the transparent coalescing of changes
to GCs.
For example,
a call to
of a GC followed by a call to
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.
To set an attribute of a GC,
set the appropriate member of the
XGCValues
structure and OR in the corresponding value bitmask in your subsequent calls to
.
The symbols for the value mask bits and the
XGCValues
structure are:
/* 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 default GC values are:
ComponentDefaultfunctionGXcopyplane_maskAll onesforeground0background1line_width0line_styleLineSolidcap_styleCapButtjoin_styleJoinMiterfill_styleFillSolidfill_ruleEvenOddRulearc_modeArcPieSlicetilePixmap 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)stipplePixmap of unspecified size filled with onests_x_origin0ts_y_origin0font<implementation dependent>subwindow_modeClipByChildrengraphics_exposuresTrueclip_x_origin0clip_y_origin0clip_maskNonedash_offset0dashes4 (that is, the list [4, 4])
Note that foreground and background are not set to any values likely
to be useful in a window.
Display FunctionsSourceDestination
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>,
X11/X.hFiles<X11/X.h>Headers<X11/X.h>
are:
Function NameValueOperationGXclear0x00GXand0x1src AND dstGXandReverse0x2src AND NOT dstGXcopy0x3srcGXandInverted0x4(NOT src) AND dstGXnoop0x5dstGXxor0x6src XOR dstGXor0x7src OR dstGXnor0x8(NOT src) AND (NOT dst)GXequiv0x9(NOT src) XOR dstGXinvert0xaNOT dstGXorReverse0xbsrc OR (NOT dst)GXcopyInverted0xcNOT srcGXorInverted0xd(NOT src) OR dstGXnand0xe(NOT src) OR (NOT dst)GXset0xf1
Many graphics operations depend on either pixel values or planes in a GC.
Pixel value
The planes attribute is of type long, and it specifies which planes of the
destination are to be modified, one bit per plane.
Planemask
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.
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.
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:
LineSolidThe 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:
CapNotLastthinThe results are device dependent,
but the desired effect is that nothing is drawn.CapButtthinThe results are device dependent,
but the desired effect is that a single pixel is drawn.CapRoundthinThe results are the same as for
CapButt /thin.CapProjectingthinThe results are the same as for
CapButt /thin.CapButtwideNothing is drawn.CapRoundwideThe closed path is a circle, centered at the endpoint, and
with the diameter equal to the line-width.CapProjectingwideThe 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,
,
,
,
,
and
);
for line requests
with line-style
LineSolid
(for example,
,
,
,
);
and for the even dashes for line requests with line-style
LineOnOffDash
or
LineDoubleDash,
the following apply:
FillSolidForegroundFillTiledTileFillOpaqueStippledA tile with the same width and height as stipple,
but with background everywhere stipple has a zero
and with foreground everywhere stipple has a oneFillStippledForeground masked by stipple
When drawing lines with line-style
LineDoubleDash,
the odd dashes are controlled by the fill-style in the following manner:
FillSolidBackgroundFillTiledSame as for even dashesFillOpaqueStippledSame as for even dashesFillStippledBackground 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
.
Specifying a
value of N is equivalent to specifying the two-element list [N, N] in
.
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
or
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
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
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
and
requests (and any similar requests defined by extensions).
To create a new GC that is usable on a given screen with a
depth of drawable, use
.
Graphics contextinitializingXCreateGCGC XCreateGCDisplay *displayDrawable dunsignedlong valuemaskXGCValues *valuesdisplay
Specifies the connection to the X server.
d
Specifies the drawable.
valuemask
Specifies which components in the GC are to be (Vm.
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.
The
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.
can generate
BadAlloc,
BadDrawable,
BadFont,
BadMatch,
BadPixmap,
and
BadValue
errors.
To copy components from a source GC to a destination GC, use
.
XCopyGCXCopyGCDisplay *displayGCsrc, destunsignedlong valuemaskdisplay
Specifies the connection to the X server.
src
Specifies the components of the source GC.
valuemask
Specifies which components in the GC are to be (Vm.
This argument is the bitwise inclusive OR of zero or more of the valid
GC component mask bits.
dest
Specifies the destination GC.
The
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
.
can generate
BadAlloc,
BadGC,
and
BadMatch
errors.
To change the components in a given GC, use
.
XChangeGCXChangeGCDisplay *displayGC gcunsignedlong valuemaskXGCValues *valuesdisplay
Specifies the connection to the X server.
gc
Specifies the GC.
valuemask
Specifies which components in the GC are to be (Vm.
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.
The
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
.
Changing the clip-mask overrides any previous
request on the context.
Changing the dash-offset or dash-list
overrides any previous
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.
can generate
BadAlloc,
BadFont,
BadGC,
BadMatch,
BadPixmap,
and
BadValue
errors.
To obtain components of a given GC, use
.
XGetGCValuesStatus XGetGCValuesDisplay *displayGC gcunsignedlong valuemaskXGCValues *values_returndisplay
Specifies the connection to the X server.
gc
Specifies the GC.
valuemask
Specifies which components in the GC are to be (Vm.
This argument is the bitwise inclusive OR of zero or more of the valid
GC component mask bits.
values_return
Returns the GC values in the specified
XGCValues
structure.
The
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,
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.
To free a given GC, use
.
XFreeGCXFreeGCDisplay *displayGC gcdisplay
Specifies the connection to the X server.
gc
Specifies the GC.
The
function destroys the specified GC as well as all the associated storage.
can generate a
BadGC
error.
To obtain the
GContext
resource ID for a given GC, use
.
XGContextFromGCGContext XGContextFromGCGC gcgc
Specifies the GC (Gc.
Xlib usually defers sending changes to the components of a GC to the server
until a graphics function is actually called with that GC.
This permits batching of component changes into a single server request.
In some circumstances, however, it may be necessary for the client
to explicitly force sending the changes to the server.
An example might be when a protocol extension uses the GC indirectly,
in such a way that the extension interface cannot know what GC will be used.
To force sending GC component changes, use
.
XFlushGCvoid XFlushGCDisplay *displayGC gcdisplay
Specifies the connection to the X server.
gc
Specifies the GC.
Using Graphics Context Convenience Routines
This section discusses how to set the:
Foreground, background, plane mask, or function components
Line attributes and dashes components
Fill style and fill rule components
Fill tile and stipple components
Font component
Clip region component
Arc mode, subwindow mode, and graphics exposure components
Setting the Foreground, Background, Function, or Plane Mask
To set the foreground, background, plane mask, and function components
for a given GC, use
.
XSetStateXSetStateDisplay *displayGC gcunsignedlongforeground, backgroundint functionunsignedlong plane_maskdisplay
Specifies the connection to the X server.
gc
Specifies the GC.
foreground
Specifies the foreground you want to set for the specified GC.
background
Specifies the background you want to set for the specified GC.
function
Specifies the function you want to set for the specified GC.
plane_mask
Specifies the plane mask.
can generate
BadAlloc,
BadGC,
and
BadValue
errors.
To set the foreground of a given GC, use
.
XSetForegroundXSetForegroundDisplay *displayGC gcunsignedlong foregrounddisplay
Specifies the connection to the X server.
gc
Specifies the GC.
foreground
Specifies the foreground you want to set for the specified GC.
can generate
BadAlloc
and
BadGC
errors.
To set the background of a given GC, use
.
XSetBackgroundXSetBackgroundDisplay *displayGC gcunsignedlong backgrounddisplay
Specifies the connection to the X server.
gc
Specifies the GC.
background
Specifies the background you want to set for the specified GC.
can generate
BadAlloc
and
BadGC
errors.
To set the display function in a given GC, use
.
XSetFunctionXSetFunctionDisplay *displayGC gcint functiondisplay
Specifies the connection to the X server.
gc
Specifies the GC.
function
Specifies the function you want to set for the specified GC.
can generate
BadAlloc,
BadGC,
and
BadValue
errors.
To set the plane mask of a given GC, use
.
XSetPlaneMaskXSetPlaneMaskDisplay *displayGC gcunsignedlong plane_maskdisplay
Specifies the connection to the X server.
gc
Specifies the GC.
plane_mask
Specifies the plane mask.
can generate
BadAlloc
and
BadGC
errors.
Setting the Line Attributes and Dashes
To set the line drawing components of a given GC, use
.
XSetLineAttributesXSetLineAttributesDisplay *displayGC gcunsignedint line_widthint line_styleint cap_styleint join_styledisplay
Specifies the connection to the X server.
gc
Specifies the GC.
line_width
Specifies the line-width you want to set for the specified GC.
line_style
Specifies the line-style you want to set for the specified GC.
You can pass
LineSolid,
LineOnOffDash,
or
LineDoubleDash.
cap_style
Specifies the line-style and cap-style you want to set for the specified GC.
You can pass
CapNotLast,
CapButt,
CapRound,
or
CapProjecting.
join_style
Specifies the line join-style you want to set for the specified GC.
You can pass
JoinMiter,
JoinRound,
or
JoinBevel.
can generate
BadAlloc,
BadGC,
and
BadValue
errors.
To set the dash-offset and dash-list for dashed line styles of a given GC, use
.
XSetDashesXSetDashesDisplay *displayGC gcint dash_offsetchar dash_list[]int ndisplay
Specifies the connection to the X server.
gc
Specifies the GC.
dash_offset
Specifies the phase of the pattern for the dashed line-style you want to set
for the specified GC.
dash_list
Specifies the dash-list for the dashed line-style
you want to set for the specified GC.
n
Specifies the number of elements in dash_list.
The
function sets the dash-offset and dash-list attributes for dashed line styles
in the specified GC.
There must be at least one element in the specified dash_list,
or a
BadValue
error results.
The initial and alternating elements (second, fourth, and so on)
of the dash_list are the even dashes, and
the others are the odd dashes.
Each element specifies a dash length in pixels.
All of the elements must be nonzero,
or a
BadValue
error results.
Specifying an odd-length list is equivalent to specifying the same list
concatenated with itself to produce an even-length list.
The dash-offset defines the phase of the pattern,
specifying how many pixels into the dash-list the pattern
should actually begin in any single graphics request.
Dashing is continuous through path elements combined with a join-style
but is reset to the dash-offset between each sequence of joined lines.
The unit of measure for dashes is the same for the ordinary coordinate system.
Ideally, a dash length is measured along the slope of the line, but implementations
are only required to match this ideal for horizontal and vertical lines.
Failing the ideal semantics, it is suggested that the length be measured along the
major axis of the line.
The major axis is defined as the x axis for lines drawn at an angle of between
−45 and +45 degrees or between 135 and 225 degrees from the x axis.
For all other lines, the major axis is the y axis.
can generate
BadAlloc,
BadGC,
and
BadValue
errors.
Setting the Fill Style and Fill Rule
To set the fill-style of a given GC, use
.
XSetFillStyleXSetFillStyleDisplay *displayGC gcint fill_styledisplay
Specifies the connection to the X server.
gc
Specifies the GC.
fill_style
Specifies the fill-style you want to set for the specified GC.
You can pass
FillSolid,
FillTiled,
FillStippled,
or
FillOpaqueStippled.
can generate
BadAlloc,
BadGC,
and
BadValue
errors.
To set the fill-rule of a given GC, use
.
XSetFillRuleXSetFillRuleDisplay *displayGC gcint fill_ruledisplay
Specifies the connection to the X server.
gc
Specifies the GC.
fill_rule
Specifies the fill-rule you want to set for the specified GC.
You can pass
EvenOddRule
or
WindingRule.
can generate
BadAlloc,
BadGC,
and
BadValue
errors.
Setting the Fill Tile and Stipple
Some displays have hardware support for tiling or
stippling with patterns of specific sizes.
Tiling and stippling operations that restrict themselves to those specific
sizes run much faster than such operations with arbitrary size patterns.
Xlib provides functions that you can use to determine the best size,
tile, or stipple for the display
as well as to set the tile or stipple shape and the tile or stipple origin.
To obtain the best size of a tile, stipple, or cursor, use
.
XQueryBestSizeStatus XQueryBestSizeDisplay *displayint classDrawable which_screenunsignedintwidth, heightunsignedint*width_return, *height_returndisplay
Specifies the connection to the X server.
class
Specifies the class that you are interested in.
You can pass
TileShape,
CursorShape,
or
StippleShape.
which_screen
Specifies any drawable on the screen.
widthheight
Specify the width and height.
width_returnheight_return
Return the width and height of the object best supported
by the display hardware.
The
function returns the best or closest size to the specified size.
For
CursorShape,
this is the largest size that can be fully displayed on the screen specified by
which_screen.
For
TileShape,
this is the size that can be tiled fastest.
For
StippleShape,
this is the size that can be stippled fastest.
For
CursorShape,
the drawable indicates the desired screen.
For
TileShape
and
StippleShape,
the drawable indicates the screen and possibly the window class and depth.
An
InputOnly
window cannot be used as the drawable for
TileShape
or
StippleShape,
or a
BadMatch
error results.
can generate
BadDrawable,
BadMatch,
and
BadValue
errors.
To obtain the best fill tile shape, use
.
XQueryBestTileStatus XQueryBestTileDisplay *displayDrawable which_screenunsignedintwidth, heightunsignedint*width_return, *height_returndisplay
Specifies the connection to the X server.
which_screen
Specifies any drawable on the screen.
widthheight
Specify the width and height.
width_returnheight_return
Return the width and height of the object best supported
by the display hardware.
The
function returns the best or closest size, that is, the size that can be
tiled fastest on the screen specified by which_screen.
The drawable indicates the screen and possibly the window class and depth.
If an
InputOnly
window is used as the drawable, a
BadMatch
error results.
can generate
BadDrawable
and
BadMatch
errors.
To obtain the best stipple shape, use
.
XQueryBestStippleStatus XQueryBestStippleDisplay *displayDrawable which_screenunsignedintwidth, heightunsignedint*width_return, *height_returndisplay
Specifies the connection to the X server.
which_screen
Specifies any drawable on the screen.
widthheight
Specify the width and height.
width_returnheight_return
Return the width and height of the object best supported
by the display hardware.
The
function returns the best or closest size, that is, the size that can be
stippled fastest on the screen specified by which_screen.
The drawable indicates the screen and possibly the window class and depth.
If an
InputOnly
window is used as the drawable, a
BadMatch
error results.
can generate
BadDrawable
and
BadMatch
errors.
To set the fill tile of a given GC, use
.
XSetTileXSetTileDisplay *displayGC gcPixmap tiledisplay
Specifies the connection to the X server.
gc
Specifies the GC.
tile
Specifies the fill tile you want to set for the specified GC.
The tile and GC must have the same depth,
or a
BadMatch
error results.
can generate
BadAlloc,
BadGC,
BadMatch,
and
BadPixmap
errors.
To set the stipple of a given GC, use
.
XSetStippleXSetStippleDisplay *displayGC gcPixmap stippledisplay
Specifies the connection to the X server.
gc
Specifies the GC.
stipple
Specifies the stipple you want to set for the specified GC.
The stipple must have a depth of one,
or a
BadMatch
error results.
can generate
BadAlloc,
BadGC,
BadMatch,
and
BadPixmap
errors.
To set the tile or stipple origin of a given GC, use
.
XSetTSOriginXSetTSOriginDisplay *displayGC gcintts_x_origin, ts_y_origindisplay
Specifies the connection to the X server.
gc
Specifies the GC.
ts_x_origints_y_origin
Specify the x and y coordinates of the tile and stipple origin.
When graphics requests call for tiling or stippling,
the parent's origin will be interpreted relative to whatever destination
drawable is specified in the graphics request.
can generate
BadAlloc
and
BadGC
errors.
Setting the Current Font
To set the current font of a given GC, use
.
XSetFontXSetFontDisplay *displayGC gcFont fontdisplay
Specifies the connection to the X server.
gc
Specifies the GC.
font
Specifies the font.
can generate
BadAlloc,
BadFont,
and
BadGC
errors.
Setting the Clip Region
Xlib provides functions that you can use to set the clip-origin
and the clip-mask or set the clip-mask to a list of rectangles.
To set the clip-origin of a given GC, use
.
XSetClipOriginXSetClipOriginDisplay *displayGC gcintclip_x_origin, clip_y_origindisplay
Specifies the connection to the X server.
gc
Specifies the GC.
clip_x_originclip_y_origin
Specify the x and y coordinates of the clip-mask origin.
The clip-mask origin is interpreted relative to the origin of whatever
destination drawable is specified in the graphics request.
can generate
BadAlloc
and
BadGC
errors.
To set the clip-mask of a given GC to the specified pixmap, use
.
XSetClipMaskXSetClipMaskDisplay *displayGC gcPixmap pixmapdisplay
Specifies the connection to the X server.
gc
Specifies the GC.
pixmap
Specifies the pixmap or
None.
If the clip-mask is set to
None,
the pixels are always drawn (regardless of the clip-origin).
can generate
BadAlloc,
BadGC,
BadMatch,
and
BadPixmap
errors.
To set the clip-mask of a given GC to the specified list of rectangles, use
.
XSetClipRectanglesXSetClipRectanglesDisplay *displayGC gcintclip_x_origin, clip_y_originXRectangle rectangles[]int nint orderingdisplay
Specifies the connection to the X server.
gc
Specifies the GC.
clip_x_originclip_y_origin
Specify the x and y coordinates of the clip-mask origin.
rectangles
Specifies an array of rectangles that define the clip-mask.
n
Specifies the number of rectangles.
ordering
Specifies the ordering relations on the rectangles.
You can pass
Unsorted,
YSorted,
YXSorted,
or
YXBanded.
The
function changes the clip-mask in the specified GC
to the specified list of rectangles and sets the clip origin.
The output is clipped to remain contained within the
rectangles.
The clip-origin is interpreted relative to the origin of
whatever destination drawable is specified in a graphics request.
The rectangle coordinates are interpreted relative to the clip-origin.
The rectangles should be nonintersecting, or the graphics results will be
undefined.
Note that the list of rectangles can be empty,
which effectively disables output.
This is the opposite of passing
None
as the clip-mask in
,
,
and
.
If known by the client, ordering relations on the rectangles can be
specified with the ordering argument.
This may provide faster operation
by the server.
If an incorrect ordering is specified, the X server may generate a
BadMatch
error, but it is not required to do so.
If no error is generated, the graphics
results are undefined.
Unsorted
means the rectangles are in arbitrary order.
YSorted
means that the rectangles are nondecreasing in their Y origin.
YXSorted
additionally constrains
YSorted
order in that all
rectangles with an equal Y origin are nondecreasing in their X
origin.
YXBanded
additionally constrains
YXSorted
by requiring that,
for every possible Y scanline, all rectangles that include that
scanline have an identical Y origins and Y extents.
can generate
BadAlloc,
BadGC,
BadMatch,
and
BadValue
errors.
Xlib provides a set of basic functions for performing
region arithmetic.
For information about these functions,
see section 16.5.
Setting the Arc Mode, Subwindow Mode, and Graphics Exposure
To set the arc mode of a given GC, use
.
XSetArcModeXSetArcModeDisplay *displayGC gcint arc_modedisplay
Specifies the connection to the X server.
gc
Specifies the GC.
arc_mode
Specifies the arc mode.
You can pass
ArcChord
or
ArcPieSlice.
can generate
BadAlloc,
BadGC,
and
BadValue
errors.
To set the subwindow mode of a given GC, use
.
XSetSubwindowModeXSetSubwindowModeDisplay *displayGC gcint subwindow_modedisplay
Specifies the connection to the X server.
gc
Specifies the GC.
subwindow_mode
Specifies the subwindow mode.
You can pass
ClipByChildren
or
IncludeInferiors.
can generate
BadAlloc,
BadGC,
and
BadValue
errors.
To set the graphics-exposures flag of a given GC, use
.
XSetGraphicsExposuresXSetGraphicsExposuresDisplay *displayGC gcBool graphics_exposuresdisplay
Specifies the connection to the X server.
gc
Specifies the GC.
graphics_exposures
Specifies a Boolean value that indicates whether you want
GraphicsExpose
and
NoExpose
events to be reported when calling
and
with this GC.
can generate
BadAlloc,
BadGC,
and
BadValue
errors.