When you use GpiPolySpline to draw a spline, each curve is tangential to the first and last of three connected lines. The following figure shows how to draw a spline.
#include <os2.h> HPS hps; /* Presentation-space handle */ MRESULT EXPENTRY wpClient(HWND hwnd, ULONG msg, MPARAM mp1, MPARAM mp2){ POINTL aptl[3]; /* Structure for control points */ aptl[0].x = 50; /* Loads x-coord. of first control point */ aptl[0].y = 100; /* Loads y-coord. of first control point */ GpiMove(hps, aptl); /* Sets current position */ aptl[0].x = 75; /* Loads x-coord. of second control point */ aptl[0].y = 200; /* Loads y-coord. of second control point */ aptl[1].x = 100; /* Loads x-coord. of third control point */ aptl[1].y = 0; /* Loads y-coord. of third control point */ aptl[2].x = 125; /* Loads x-coord. of fourth control point */ aptl[2].y = 100; /* Loads y-coord. of fourth control point */ GpiPolySpline(hps, 3L, aptl); /* Draws spline */ } /* wpClient */