GEOS SDK TechDocs|
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2.4 Ellipses | 
2.6 Three-Point Arcs GrDrawArc(), GrFillArc()
An arc is a partial ellipse. An arc is defined in terms of its base ellipse, the angle at which to start drawing the arc, and the angle at which to stop drawing. Angles are counted in degrees counter-clockwise with 0° corresponding to the positive x axis (i.e., "3 o'clock").
GrDrawArc()
draws the outline of an elliptical arc, a curved line. It does so using the GState's current line attributes.
GrFillArc()
fills the arc. There are two ways to fill an arc: you can fill in the wedge described by the arc, or you can fill just the region between the arc and its chord; set the style with an
ArcCloseType
value.
GEOS SDK TechDocs|
| 2.4 Ellipses | 2.6 Three-Point Arcs