XDrawArc(3X) XDrawArc(3X)
NAME
XDrawArc, XDrawArcs - draw arcs and manipulate the XArc
structure
SYNOPSIS
XDrawArc (display, d, gc, x, y, width, height, angle1,
angle2)
Display* display;
Drawable d;
GC gc;
int x,y;
unsigned int width, height;
int angle1, angle2;
XDrawArcs (display, d, gc, arcs, narcs)
Display* display;
Drawable d;
GC gc;
XArc* arcs;
int narcs
DESCRIPTION
XDrawArc draws a single circular or elliptical arc, and
XDrawArcs draws multiple circular or elliptical arcs. Each
arc is specified by a rectangle and two angles. The center
of the circle or ellipse is the center of the rectangle, and
the major and minor axes are specified by width and height.
Positive angles indicate counterclockwise motion, and nega-
tive angles indicate clockwise motion. If the magnitude of
angle2 is greater than 360 degrees, XDrawArc or XDrawArcs
truncates it to 360 degrees.
For an arc specified as
[ x, y, width, height, angle1, angle2], the origin of the
width height
major and minor axes is at [x+ , y+ ], and the in-
2 2
finitely thin path describing the entire circle or ellipse
height
intersects the horizontal axis at [x, y+ ] and
height 2
[x+width, y+ ] and intersects the vertical axis at
width 2 width
[x+ , y] and [x+ , y+height]. These coordinates can
2 2
be fractional and so are not truncated to discrete coordi-
nates. The path should be defined by the ideal mathematical
path. For a wide line with line width lw, the bounding out-
lines for filling are given by the two infinitely thin paths
consisting of all points whose perpendicular distance from
the path of the circle/ellipse is equal to lw/2 (which may
be a fractional value). The cap style and join style are
applied the same as for a line corresponding to the tangent
of the circle/ellipse at the endpoint.
For an arc specified as
[ x, y, width, height, angle1, angle2], the angles must be
specified in the effectively skewed coordinate system of the
November, 1990 1
XDrawArc(3X) XDrawArc(3X)
ellipse (for a circle, the angles and coordinate systems are
identical). The relationship between these angles and an-
gles expressed in the normal coordinate system of the screen
(as measured with a protractor) is as follows:
( _width)
skewed-angle = atan tan(normal-angle)* +adjust
( height)
The skewed-angle and normal-angle are expressed in radians
(rather than in degrees scaled by 64) in the range [0, 2π]
π π
and where atan returns a value in the range [- , ] and ad-
2 2
just is:
π
0 for normal-angle in the range [0, ]
π 2
3π
π for normal-angle in the range [ , ]
2
3π 2
2πfor normal-angle in the range [ , 2π]
2
For any given arc, XDrawArc and XDrawArcs do not draw a pix-
el more than once. If two arcs join correctly and if the
line-width is greater than 0 and the arcs intersect,
XDrawArc and XDrawArcs do not draw a pixel more than once.
Otherwise, the intersecting pixels of intersecting arcs are
drawn multiple times. Specifying an arc with one endpoint
and a clockwise extent draws the same pixels as specifying
the other endpoint and an equivalent counterclockwise ex-
tent, except as it affects joins.
If the last point in one arc coincides with the first point
in the following arc, the two arcs will join correctly. If
the first point in the first arc coincides with the last
point in the last arc, the two arcs will join correctly. By
specifying one axis to be 0, a horizontal or vertical line
can be drawn. Angles are computed based solely on the coor-
dinate system and ignore the aspect ratio.
Both functions use these graphic context components: func-
tion, plane-mask, line-width, line-style, cap-style, join-
style, fill-style, subwindow-mode, clip-x-origin, clip-y-
origin, and clip-mask. They also use these mode-dependent
graphic contents components: foreground, background, tile,
stipple, tile-stipple-x-origin, tile-stipple-y-origin,
dash-offset, and dash list.
Arguments
These functions accept the following arguments:
angle1 Specifies the start of the arc relative to the three
o'clock position from the center, in units of de-
grees * 64.
angle2 Specifies the path and extent of the arc relative to
the start of the arc, in units of degrees * 64.
2 November, 1990
XDrawArc(3X) XDrawArc(3X)
arcs Specifies a pointer to an array of arcs.
d Specifies the drawable.
display Specifies the connection to the X server.
gc Specifies the graphic context.
narcs Specifies the number of arcs in the array.
width, height
Specify the width and height.
x, y Specify the x and y coordinates.
Structures
The XArc structure contains:
typedef struct {
short x, y;
unsigned short width, height;
short angle1, angle2; /* Degrees * 64 */
} XArc;
All x and y members are signed integers. The width and
height members are 16-bit unsigned integers. You should be
careful not to generate coordinates and sizes out of the
16-bit ranges, because the protocol only has 16-bit fields
for these values.
ERRORS
BadDrawable
A value for a Drawable argument does not name a de-
fined window or pixel map.
BadGC A value for a GC argument does not name a defined
graphic context.
BadMatch
An InputOnly window is used as a drawable; or, an
argument or pair of arguments has the correct type
and range but fails to match in some other required
way.
XDrawArc and XDrawArcs can generate BadDrawable, BadGC, and
BadMatch errors.
SEE ALSO
XCreateGC(3X), XDrawLine(3X), XDrawPoint(3X),
XDrawRectangle(3X)
Xlib - C Language Interface
November, 1990 3