| #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.
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.
The return value is the previous poly mode if the escape is successful. Otherwise, the return value is zero.
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.
Polygon, Polyline