/* * linear - perform a simple two point 2D linear interpolation * * given: *	x0, y0		first known point on the line *	x1, y1		second knonw point on the line *	x		a given point to interpolate on * * returns: *	y such that (x,y) is on the line defined by (x0,y0) and (x1,y1) * * NOTE: The line cannot be vertical.  So x0 != y0. */define linear(x0, y0, x1, y1, x){    /* firewall */    if (!isnum(x0) || ! isnum(y0) || !isnum(x1) || ! isnum(y1) || !isnum(x)) {	quit "non-numeric argument passed to linear";    }    if (x0 == x1) {	quit "linear given a line with an infinite slope";    }    /* return y = y0 + (delta_Y/delta_X) * (x - x0) */    return y0 + (((y1-y0)/(x1-x0)) * (x - x0));}