

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;
For an arc specified as [ x, y, width , height, angle1, angle2 ], the origin of the major and minor axes is at [x + width/2, y + height/2], and the infinitely thin path describing the entire circle or ellipse intersects the horizontal axis at [x, y + height/2] and [x + width, y + height/2] and intersects the vertical axis at [x + width/2, y] and [x + width/2, 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 linewidth 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 capstyle and joinstyle 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 ellipse (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:
skewedangle = atan [tan(normalangle)*width/height] + adjustThe skewedangle and normalangle are expressed in radians (rather than in degrees scaled by 64) in the range [0, 2*pi] and where atan returns a value in the range [pi/2, pi/2] and adjust is:
0 for normalangle in the range [0, pi/2] pi for normalangle in the range [pi/2, 3*pi/2] 2 pi for normalangle in the range [3*pi/2, 2*pi]For any given arc, XDrawArc and XDrawArcs do not draw a pixel more than once. If two arcs join correctly and if the linewidth 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 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 zero, a horizontal or vertical line can be drawn. Angles are computed based solely on the coordinate system and ignore the aspect ratio.
Both functions use these GC components: function, planemask, linewidth, linestyle, capstyle, joinstyle, fillstyle, subwindowmode, clipxorigin, clipyorigin, and clipmask. They also use these GC modedependent components: foreground, background, tile, stipple, tilestipplexorigin, tilestippleyorigin, dashoffset, and dashlist.
XDrawArc and XDrawArcs can generate ``BadDrawable'', ``BadGC'', and ``BadMatch'' errors.
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 16bit unsigned integers.
You should be careful not to generate coordinates and sizes
out of the 16bit ranges, because the protocol only has 16bit
fields for these values.