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#define SET_POLY_MODE 4104 
short Control(lpDevice, SET_POLY_MODE, lpMode, NULL)
LPPDEVICE lpDevice;
LPINT lpMode;
 
The SET_POLY_MODE escape enables a device driver to draw shapes (such as
Bezier curves) that are not supported directly by GDI. This permits
applications that draw complex curves to send the curve description directly
to a device without having to simulate the curve as a polygon with a large
number of points.
 
The SET_POLY_MODE escape sets the poly mode for the device driver. The poly
mode is a state variable indicating how to interpret calls to the Polygon
(GDI.63) and Polyline (GDI.37) functions.
 
Parameter  Description
────────────────────────────────────────────────────────────────────────────
 
 
lpDevice   Points to a PDEVICE structure specifying the destination device.
 
lpMode     Points to a 16-bit variable that specifies the desired poly mode.
           The parameter can be one of the following values.
 
           Value                   Meaning
           ─────────────────────────────────────────────────────────────────
           PM_POLYLINE (1)         The points define a conventional polygon
                                   or polyline.
 
           PM_BEZIER (2)           The points define a sequence of 4-point
                                   Bezier spline curves. The first curve
                                   passes through the first four points,
                                   with the first and fourth points as end
                                   points, and the second and third points
                                   as control points. Each subsequent curve
                                   in the sequence has the end point of the
                                   previous curve as its start point, the
                                   next two points as control points, and
                                   the third as its end point.
 
                                   The last curve in the sequence is
                                   permitted to have fewer than four points.
                                   If the curve has only one point, it is
                                   considered a point. If it has two points,
                                   it is a line segment. If it has three
                                   points, it is a parabola defined by
                                   drawing a Bezier curve with the end
                                   points equal to the first and third
                                   points and the two control points equal
                                   to the second point.
 
           PM_POLYLINESEGMENT (3)  The points specify a list of coordinate
                                   pairs. Line segments are drawn connecting
                                   each successive pair of points.
 
           The device driver need not support all the possible modes. It is
           expected to return zero if it does not support the specified
           mode.
 
Return Value
 
The return value is the previous poly mode if the escape is successful.
Otherwise, the return value is zero.
 
Comments
 
An application should issue the SET_POLY_MODE escape before it draws a
complex curve. It should then call Polyline or Polygon with the desired
control points defining the curve. After drawing the curve, the application
should reset the driver to its previous state by reissuing the SET_POLY_MODE
escape.
 
Calls to the Polyline function are drawn using the currently selected pen.
 
Calls to the Polygon function are drawn using the currently selected pen and
brush. If the start and end points are not equal, a line is drawn from the
start point to the end point before filling the polygon (or curve).
 
Calls to the Polygon function using PM_POLYLINESEGMENT mode are treated
exactly the same as calls to Polyline.
 
A Bezier curve is defined by four points. The curve is generated by
connecting the first and second, second and third, and third and fourth
points. The midpoints of these consecutive line segments are then connected.
Then the midpoints of the lines connecting the midpoints are connected.
 
The line segments drawn in this way converge to a curve defined by the
following parametric equations, expressed as a function of an independent
variable t.
 
X(t) = (1-t)**3x1 + 3(1-t)**2tx2 + 3(1-t)t**2x3 + t**3x4
 
Y(t) = (1-t)**3y1 + 3(1-t)**2ty2 + 3(1-t)t**2y3 + t**3y4
 
The points (x1,y1), (x2,y2), (x3,y3), and (x4,y4) are the control points
defining the curve. The independent variable t varies from 0 to 1.
 
A second-degree Bezier curve may be expressed as a third-degree Bezier using
the following parameterization:
 
Cx1 = 1/3X1 + 2/3X2    Cy1 = 1/3Y1 + 2/3Y2
Cx2 = 2/3X2 + 1/3X3    Cy2 = 2/3Y2 + 1/3Y3
 
(Cx1, Cy1) and (Cx2, Cy2) are third-degree control points of the
second-degree Bezier specified by the points (x1, y1), (x2, y2), and (x3,
y3).
 
Applications are expected to check the return value from this escape to
determine whether or not the driver supports the specified poly mode.
 
See Also
 
Polygon, Polyline, PDEVICE
 
 
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