Curves
A regular curve is a set of highlighted pixels on a raster display (or dots on
a printed page) that define the perimeter (or part of the perimeter) of a
conic section. An irregular curve is a set of pixels that define a curve that does
not fit the perimeter of a conic section. In Windows, the ending point is
excluded from a curve just as it is excluded from a line.
When an application calls one of the Windows curve-drawing functions, GDI
breaks the curve into a number of extremely small, discrete line segments. After
determining the endpoints (starting point and ending point) for each of these
line segments, GDI determines which pixels (or dots) define each line by applying
its DDA.
An application can draw an ellipse or part of an ellipse by calling the
Arc function. This function draws the curve within the perimeter of an invisible
rectangle called a bounding rectangle. The size of the ellipse is specified by
two invisible radials extending from the center of the rectangle to the sides
of the rectangle. The following illustration shows an arc (part of an ellipse)
drawn by using the
Arc function.
When calling the
Arc function, an application specifies the coordinates of the bounding rectangle
and radials. The preceding illustration shows the rectangle and radials with
dashed lines while the actual arc was drawn using a solid line.
When drawing the arc of another object, the application can call the
SetArcDirection and
GetArcDirection functions to control the direction (clockwise or counterclockwise) in which
the object is drawn. The default direction for drawing arcs and other objects is
counterclockwise.
In addition to drawing ellipses or parts of ellipses, Windows-based
applications can draw irregular curves called Bézier curves. A
Bézier curve is an irregular curve whose curvature is defined by four control points (
p1,
p2,
p3, and
p4). The control points
p1 and
p4 define the starting and ending points of the curve, and the control points
p2 and
p3 define the shape of the curve by marking points where the curve reverses
orientation.
An application can draw irregular curves by calling the
PolyBezier function, supplying the appropriate control points.
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