// This function assumes a 16-bit drawing surface to be already locked
// locking is introduced in the previous tutorial
// lpitch and lpscreen are global
// lpitch - pitch of the locked surface
// lpscreen - points to the first pixel in the locked surface

void Line (int x1, int y1, int x2, int y2, ushort color)
{
		int		dx,		//deltas
            dy,
            dx2,	//scaled deltas
            dy2,
            ix,		//increase rate on the x axis
            iy,		//increase rate on the y axis
            err,	//the error term
            i;		//looping variable

	int pitch = lpitch;

	// identify the first pixel
	ushort *ptr_vid = lpscreen + x1 + (y1 * (pitch >> 1));

	// difference between starting and ending points
	dx = x2 - x1;
	dy = y2 - y1;

	// calculate direction of the vector and store in ix and iy
	if (dx >= 0)
		ix = 1;

	if (dx < 0)
	{
		ix = -1;
		dx = abs(dx);
	}

	if (dy >= 0)
		iy = (pitch >> 1);

	if (dy < 0)
	{
		iy = -(pitch >> 1);
		dy = abs(dy);
	}

	// scale deltas and store in dx2 and dy2
	dx2 = dx * 2;
	dy2 = dy * 2;

        
	if (dx > dy)	// dx is the major axis
	{
		// initialize the error term
		err = dy2 - dx;

		for (i = 0; i <= dx; i++)
		{
			*ptr_vid = color;
			if (err >= 0)
			{
				err -= dx2;
				ptr_vid += iy;
			}
			err += dy2;
			ptr_vid += ix;
		}
	}
	
	else 		// dy is the major axis
	{
		// initialize the error term
		err = dx2 - dy;

		for (i = 0; i <= dy; i++)
		{
			*ptr_vid = color;
			if (err >= 0)
			{
				err -= dy2;
				ptr_vid += ix;
			}
			err += dx2;
			ptr_vid += iy;
		}
	}
} // end of Line(...