arc.md
In rswipl: Embed 'SWI'-'Prolog'

class arc {#class-arc}

An arc object is a graphical object that represents part of an ellipse. An arc is a subclass of class joint and thus may have arrows at either end. Arcs may be used to construct piecharts as well as to display a bounded connection line.

The basic representation of an arc consists of

| ->position    | Position of the center          |
| ->size        | Width and height of the ellipse |
| ->start_angle | Degrees of starting angle       |
| ->size_angle  | Degrees of angle (> 0 and < 0)  |
| ->close       | Describes how the arc is closed |

This basic specification is satisfactory for the specification of pie-charts. The method ->points is useful for creating bounded lines from one point to another. The method ->connect_angle is useful to relate two lines with an arc (like in and/or trees).

arc<-close: {none,pie_slice,chord} Determines how the arc is closed and filled if ->fill does not equal @nil.
- none Only the arc is visible
- chord The arc and a straight line from <-start to <-end are displayed
- pie_slice The arc and two start lines from <-start and <-end to <-position are displayed.
arc<-fill: colour|{foreground,background}* Image used to fill the arc. When @nil, the arc is not filled. The behaviour of this method is dependent of the value of ->close:
- none or pie_slice Creates a pie_slice. If ->close is none no line is painted around the straight edges of the slice.
- chord Fills the area between a straight line connecting <-start and <-end and the arc.
arc<-position: point The ->position describes the center of the ellipse the arc is part of.
arc<-size: size The ->size describes the width and height of the ellipse the arc is part of. Note that (currently) the axes of the ellipse are always horizontal and vertical.
arc<-size_angle: num Describes the angle in degrees of the ellipse segment. Positive values are counter clockwise.
arc<-start_angle: num Describes the starting angle of the arc in degrees. 0 degrees denotes 3 o'clock. Positive values are anti-clockwise.

The position of the starting point may be requested with <-start.

arc->compute Compute the <-area from the principal representation. The <-area is defined to be the bounding box of the arc.
arc->connect_angle: line, line Connect two lines with an arc. The ->position of the arc will be located are the line <-intersection of the two lines. The <-start_angle will make the arc start at the first line. The <-size_angle will connect it to the second line.

This method may be used to paint and/or trees.
arc->end_angle: num @see arc->start_angle
arc->geometry: x=[int], y=[int], width=[int], height=[int] Only the X and Y parameters are used to move the arc.
arc->initialise: radius=[int], start=[num], size=[num] Create an arc from its radius (setting ->size), ->start_angle and ->size_angle. Both angles are in degrees and measured counter-clockwise.

The default radius is determined by arc.radius. The default ->start_angle is 0 and the default ->size_angle is 90.
arc->points: start_x=int, start_y=int, end_x=int, end_y=int, curvature=int Create a bounded connection between two points. The first 4 integers describe x1,y1 and x2, y2. The last value specifies the number of pixels the line is bounded.
arc->radius: int Specifies both ->width and ->height.
arc->start_angle: num @see arc->end_angle

arc<-end: -> point End-point of the arc.
arc<-radius: -> int Equivalent to <-width

Bugs: Not properly defined if <-width is not equal to <-height
arc<-start: -> point Start position of the arc.