XCreateGC, XCopyGC, XChangeGC, XGetGCValues, XFreeGC, XGContextFromGC, XGCValues
- create or free graphics contexts and graphics context structure
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);
- 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.
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.
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).
- 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.
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