XDrawArc(XS)      X Version 11 (Release 5) 6 January 1993       XDrawArc(XS)


 Name

    XDrawArc - draw arcs and arc structure

 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;

    XDrawArcs(display, d, gc, arcs, narcs)
          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 ori-
              gin 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 rectan-
    gle 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 the 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 ori-
    gin 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 inter-
    sects 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 identi-
    cal).  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:

       0       for normal-angle in the range  [0, pi/2]
       pi      for normal-angle in the range  [pi/2, 3*pi/2]
       2 pi    for normal-angle 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 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
    coordinate 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'' errors.

 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.

 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
                   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.

 See also

    XDrawLine(XS), XDrawPoint(XS), XDrawRectangle(XS)
    Xlib - C Language X Interface