def strokes_to_lines(strokes):
    """Convert stroke-3 format to polyline format."""
    x = 0
    y = 0
    lines = []
    line = []
    for i in range(len(strokes)):
        if strokes[i, 2] == 1:
            x += float(strokes[i, 0])
            y += float(strokes[i, 1])
            line.append([x, y])
            lines.append(line)
            line = []
        else:
            x += float(strokes[i, 0])
            y += float(strokes[i, 1])
            line.append([x, y])
    return lines