XDrawArc

Syntax

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;

Arguments

Description

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 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 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:

skewed-angle = atan ( tan ( normal-angle ) * width / height ) + adjust

The skewed-angle and normal-angle 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:

For any given arc, XDrawArc() does 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() does 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.

This function uses 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. It also uses these GC mode-dependent components: foreground, background, tile, stipple, tile-stipple-x-origin, tile-stipple-y-origin, dash-offset, and dash-list.

XDrawArc() can generate BadDrawable , BadGC , and BadMatch errors.

Diagnostics

See also