cairo:arc (cr x y radius angle1 angle2)

cr -- a cairo:context-t instance
x -- a number for the x position of the center of the arc
y -- a number for the y position of the center of the arc
radius -- a number for the radius of the arc
angle1 -- a number for the start angle, in radians
angle2 -- a number for the end angle, in radians

Adds a circular arc of the given radius to the current path. The arc is centered at (x,y), begins at angle1 and proceeds in the direction of increasing angles to end at angle2. If angle2 is less than angle1 it will be progressively increased by 2*PI until it is greater than angle1.

If there is a current point, an initial line segment will be added to the path to connect the current point to the beginning of the arc. If this initial line is undesired, it can be avoided by calling the cairo:new-sub-path function before calling the cairo:arc function.

Angles are measured in radians. An angle of 0 is in the direction of the positive X axis (in user space). An angle of PI/2 radians (90 degrees) is in the direction of the positive Y axis (in user space). Angles increase in the direction from the positive X axis toward the positive y axis. So with the default transformation matrix, angles increase in a clockwise direction.

This function gives the arc in the direction of increasing angles. See the cairo:arc-negative function to get the arc in the direction of decreasing angles. The arc is circular in user space. To achieve an elliptical arc, you can scale the current coordinate transformation matrix (CTM) by different amounts in the x and y directions. For example, to draw an ellipse in the box given by x, y, width, height:

(cairo:save cr)
(cairo:translate cr (+ x (/ width 2)) (+ y (/ height 2)))
(cairo:scale cr (/ width 2) (/ height 2))
(cairo:arc cr 0 0 1 0 (* 2 pi))
(cairo:restore cr)    

The numbers for the arguments are coerced to double floats before being passed to the foreign C function.