XDrawArc(3) XLIB FUNCTIONS XDrawArc(3)
NAME
XDrawArc, XDrawArcs, XArc - draw arcs and arc structure
SYNTAX
int XDrawArc(Display *display, Drawable d, GC gc, int x, int y, unsigned
int width, unsigned int height, int angle1, int angle2);
int XDrawArcs(Display *display, Drawable d, GC gc, XArc *arcs, int
narcs);
ARGUMENTS
angle1 Specifies the start of the arc relative to the three-o'clock
position from the center, in units of degrees * 64.
angle2 Specifies the path and extent of the arc relative to the start
of the arc, in units of degrees * 64.
arcs Specifies an array of arcs.
d Specifies the drawable.
display Specifies the connection to the X server.
gc Specifies the GC.
narcs Specifies the number of arcs in the array.
width
height Specify the width and height, which are the major and minor
axes of the arc.
x
y Specify the x and y coordinates, which are relative to the
origin of the drawable and specify the upper-left corner of
the bounding rectangle.
DESCRIPTION
XDrawArc draws a single circular or elliptical arc, and XDrawArcs draws
multiple circular or elliptical arcs. Each arc is specified by a rec-
tangle and two angles. The center of the circle or ellipse is the cen-
ter of the rectangle, and the major and minor axes are specified by the
width and height. Positive angles indicate counterclockwise motion, and
negative 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 major and minor axes is at [x+wi2th,y+hei2ht], and the infinitely
thin path describing the entire circle or ellipse intersects the hori-
zontal axis at [x,y+hei2ht] and [x+width,y+hei2ht] and intersects the
vertical axis at [x+wi2th,y] and [x+wi2th,y+height]. These coordinates
can be fractional and so are not truncated to discrete coordinates. The
path should be defined by the ideal mathematical path. For a wide line
with line-width lw, the bounding outlines 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 el-
lipse (for a circle, the angles and coordinate systems are identical).
The relationship between these angles and angles expressed in the normal
coordinate system of the screen (as measured with a protractor) is as
follows:
skewed-angle=atan⎝tan(normal-angle)*height⎠+adjust
The skewed-angle and normal-angle are expressed in radians (rather than
in degrees scaled by 64) in the range [0,2π] and where atan returns a
value in the range [−2,2] and adjust is:
l l. 0 for normal-angle in the range [0,2] π for normal-angle in
the range [2,32] 2π for normal-angle in the range [32,2π]
For any given arc, XDrawArc and XDrawArcs do not draw a pixel more than
once. If two arcs join correctly and if the line-width is greater than
zero 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 extent, except as it affects joins.
If the last point in one arc coincides with the first point in the fol-
lowing 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 zero, a horizontal or
vertical line can be drawn. Angles are computed based solely on the co-
ordinate system and ignore the aspect ratio.
Both functions use these GC components: function, 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 GC
mode-dependent components: foreground, background, tile, stipple, tile-
stipple-x-origin, tile-stipple-y-origin, dash-offset, and dash-list.
XDrawArc and XDrawArcs can generate BadDrawable, BadGC, and BadMatch er-
rors.
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 co-
ordinates and sizes out of the 16-bit ranges, because the protocol only
has 16-bit fields for these values.
DIAGNOSTICS
BadDrawable
A value for a Drawable argument does not name a defined Window
or Pixmap.
BadGC A value for a GContext argument does not name a defined GCon-
text.
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 re-
quest.
SEE ALSO
XDrawLine(3), XDrawPoint(3), XDrawRectangle(3)
Xlib - C Language X Interface
X Version 11 libX11 1.8.12 XDrawArc(3)
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