使用VTK与Python实现机械臂三维模型可视化

　　三维可视化系统的建立依赖于三维图形平台， 如 OpenGL、VTK、OGRE、OSG等， 传统的方法多采用OpenGL进行底层编程，即对其特有的函数进行定量操作， 需要开发人员熟悉相关函数， 从而造成了开发难度大、 周期长等问题。VTK、 ORGE、OSG等平台使用封装更好的函数简化了开发过程。下面将使用Python与VTK进行机器人上位机监控界面的快速原型开发。

　　完整的上位机程序需要有三维显示模块、机器人信息监测模块（位置/角度/速度/电量/温度/错误信息...）、通信模块（串口/USB/WIFI/蓝牙...）、控制模块等功能模块。三维显示模块主要用于实时显示机器人的姿态（或位置）信息。比如机器人上肢手臂抬起，程序界面中的虚拟机器人也会同时进行同样的动作。三维显示模块也可以用于对机器人进行控制，实现良好的人机交互。比如在三维图像界面中可以点击拾取机器人某一关节，拖拽部件(肢体)控制真实的机器人完成同样的运动。Aldebaran Robotics的图形化编程软件Choregraphe可以完成上述的一些功能对NAO机器人进行控制。

　　对于简单的模型可以自己编写函数进行创建，但这种方法做出来的模型过于简单不够逼真。因此可以先在SolidWorks、Blender、3DMax、Maya、Rhino等三维设计软件中建立好模型，然后导出为通用的三维文件格式，再使用VTK将其读入并进行渲染。

　　在SolidWorks等三维设计软件中设计好机器人的大臂（upperarm）和小臂（forearm），然后创建装配体如下图所示。在将装配体导出为STL文件前需要注意几点：

　　1. 当从外界读入STL类型的模型时，其会按照它内部的坐标位置进行显示，因此它的位置和大小是确定的。为了以后的定位以及移动、旋转等操作的方便，需要先在SolidWorks中创建一个坐标系。如下图所示，坐标系建立在大臂关节中心点。

　　2. 如果将装配体整体输出为一个STL文件，则导入VTK后无法控制零部件进行相对运动。因此，需要将装配体各可动部件分别导出。

　　在SolidWorks的另存为STL对话框中，点开输出选项卡，如下图所示。注意之前提到的几点：如果勾选“在单一文件中保存装配体的所有零部件”则会将整个装配体导出为一个STL文件，否则就是分别命名的两个STL文件；输出坐标系下拉列表中选择之前创建的坐标系1，并勾选“不要转换STL输出数据到正的坐标空间”。

　　下面的Python代码简单实现了一个2自由度机械臂的三维仿真，可以拖动滑块或按键盘上的方向键控制肩关节或肘关节运动。当然程序还存在一些问题有待完善...

#!/usr/bin/env python

 import vtk
import math
from vtk.util.colors import *

filenames = ["upperarm.stl","forearm.stl"]

dt = 1.0        # degree step in rotation
angle = [0, 0]  # shoulder and elbow joint angle

renWin = vtk.vtkRenderWindow()
assembly = vtk.vtkAssembly()
slider_shoulder = vtk.vtkSliderRepresentation2D()
slider_elbow = vtk.vtkSliderRepresentation2D()
actor  = list() # the list of links

# Customize vtkInteractorStyleTrackballCamera
class MyInteractor(vtk.vtkInteractorStyleTrackballCamera):
    def __init__(self,parent=None):
        self.AddObserver("CharEvent",self.OnCharEvent)
        self.AddObserver("KeyPressEvent",self.OnKeyPressEvent)

    # Override the default key operations which currently handle trackball or joystick styles is provided
    # OnChar is triggered when an ASCII key is pressed. Some basic key presses are handled here
    def OnCharEvent(self,obj,event):
        pass

    def OnKeyPressEvent(self,obj,event):
        global angle
        # Get the compound key strokes for the event
        key = self.GetInteractor().GetKeySym()
        # Output the key that was pressed
        #print "Pressed: " , key

        # Handle an arrow key
        if(key == "Left"):
            actor[1].RotateY(-dt)

        if(key == "Right"):
            actor[1].RotateY(dt)

        if(key == "Up"):
            assembly.RotateY(-dt)
            angle[0] += dt
            if angle[0] >= 360.0:
                angle[0] -= 360.0
            slider_shoulder.SetValue(angle[0])

        if(key == "Down"):
            assembly.RotateY(dt)
            angle[0] -= dt
            if angle[0] < 0.0:
                angle[0] += 360.0
            slider_shoulder.SetValue(angle[0])

        # Ask each renderer owned by this RenderWindow to render its image and synchronize this process
        renWin.Render()
        return

def LoadSTL(filename):
    reader = vtk.vtkSTLReader()
    reader.SetFileName(filename)
    mapper = vtk.vtkPolyDataMapper() # maps polygonal data to graphics primitives
    mapper.SetInputConnection(reader.GetOutputPort())
    actor = vtk.vtkLODActor()
    actor.SetMapper(mapper)
    return actor   # represents an entity in a rendered scene

def CreateCoordinates():
    # create coordinate axes in the render window
    axes = vtk.vtkAxesActor()
    axes.SetTotalLength(100, 100, 100)  # Set the total length of the axes in 3 dimensions 

    # Set the type of the shaft to a cylinder:0, line:1, or user defined geometry.
    axes.SetShaftType(0) 

    axes.SetCylinderRadius(0.02)
    axes.GetXAxisCaptionActor2D().SetWidth(0.03)
    axes.GetYAxisCaptionActor2D().SetWidth(0.03)
    axes.GetZAxisCaptionActor2D().SetWidth(0.03)
    #axes.SetAxisLabels(0)  # Enable:1/disable:0 drawing the axis labels
    #transform = vtk.vtkTransform()
    #transform.Translate(0.0, 0.0, 0.0)
    #axes.SetUserTransform(transform)
    #axes.GetXAxisCaptionActor2D().GetCaptionTextProperty().SetColor(1,0,0)
    #axes.GetXAxisCaptionActor2D().GetCaptionTextProperty().BoldOff() # disable text bolding
    return axes

def ShoulderSliderCallback(obj,event):
    sliderRepres = obj.GetRepresentation()
    pos = sliderRepres.GetValue()
    assembly.SetOrientation(0,-pos,0)

    renWin.Render()

def ElbowSliderCallback(obj,event):
    sliderRepres = obj.GetRepresentation()
    pos = sliderRepres.GetValue()
    actor[1].SetOrientation(0,-pos,0)

    renWin.Render()

def ConfigSlider(sliderRep, TitleText, Yaxes):
    sliderRep.SetMinimumValue(0.0)
    sliderRep.SetMaximumValue(360.0)
    sliderRep.SetValue(0.0) # Specify the current value for the widget
    sliderRep.SetTitleText(TitleText) # Specify the label text for this widget

    sliderRep.GetSliderProperty().SetColor(1,0,0) # Change the color of the knob that slides
    sliderRep.GetSelectedProperty().SetColor(0,0,1) # Change the color of the knob when the mouse is held on it
    sliderRep.GetTubeProperty().SetColor(1,1,0) # Change the color of the bar
    sliderRep.GetCapProperty().SetColor(0,1,1) # Change the color of the ends of the bar
    #sliderRep.GetTitleProperty().SetColor(1,0,0)  # Change the color of the text displaying the value

    # Position the first end point of the slider
    sliderRep.GetPoint1Coordinate().SetCoordinateSystemToDisplay()
    sliderRep.GetPoint1Coordinate().SetValue(50, Yaxes) 

    # Position the second end point of the slider
    sliderRep.GetPoint2Coordinate().SetCoordinateSystemToDisplay()
    sliderRep.GetPoint2Coordinate().SetValue(400, Yaxes) 

    sliderRep.SetSliderLength(0.02) # Specify the length of the slider shape.The slider length by default is 0.05
    sliderRep.SetSliderWidth(0.02) # Set the width of the slider in the directions orthogonal to the slider axis
    sliderRep.SetTubeWidth(0.005)
    sliderRep.SetEndCapWidth(0.03)

    sliderRep.ShowSliderLabelOn() # display the slider text label
    sliderRep.SetLabelFormat("%.1f")

    sliderWidget = vtk.vtkSliderWidget()
    sliderWidget.SetRepresentation(sliderRep)
    sliderWidget.SetAnimationModeToAnimate()

    return sliderWidget

def CreateGround():
    # create plane source
    plane = vtk.vtkPlaneSource()
    plane.SetXResolution(50)
    plane.SetYResolution(50)
    plane.SetCenter(0,0,0)
    plane.SetNormal(0,0,1)

    # mapper
    mapper = vtk.vtkPolyDataMapper()
    mapper.SetInputConnection(plane.GetOutputPort())

    # actor
    actor = vtk.vtkActor()
    actor.SetMapper(mapper)
    actor.GetProperty().SetRepresentationToWireframe()
    #actor.GetProperty().SetOpacity(0.4)  # 1.0 is totally opaque and 0.0 is completely transparent
    actor.GetProperty().SetColor(light_grey)

    '''
    # Load in the texture map. A texture is any unsigned char image.
    bmpReader = vtk.vtkBMPReader()
    bmpReader.SetFileName("ground_texture.bmp")
    texture = vtk.vtkTexture()
    texture.SetInputConnection(bmpReader.GetOutputPort())
    texture.InterpolateOn()
    actor.SetTexture(texture)
    '''
    transform = vtk.vtkTransform()
    transform.Scale(2000,2000, 1)
    actor.SetUserTransform(transform)

    return actor

def CreateScene():
    # Create a rendering window and renderer
    ren = vtk.vtkRenderer()
    #renWin = vtk.vtkRenderWindow()
    renWin.AddRenderer(ren)

    # Create a renderwindowinteractor
    iren = vtk.vtkRenderWindowInteractor()
    iren.SetRenderWindow(renWin)
    style = MyInteractor()
    style.SetDefaultRenderer(ren)
    iren.SetInteractorStyle(style)

    for id, file in enumerate(filenames):
        actor.append(LoadSTL(file))
        #actor[id].GetProperty().SetColor(blue)
        r = vtk.vtkMath.Random(.4, 1.0)
        g = vtk.vtkMath.Random(.4, 1.0)
        b = vtk.vtkMath.Random(.4, 1.0)
        actor[id].GetProperty().SetDiffuseColor(r, g, b)
        actor[id].GetProperty().SetDiffuse(.8)
        actor[id].GetProperty().SetSpecular(.5)
        actor[id].GetProperty().SetSpecularColor(1.0,1.0,1.0)
        actor[id].GetProperty().SetSpecularPower(30.0)

        assembly.AddPart(actor[id])

        # Add the actors to the scene
        #ren.AddActor(actor[id])

    # Also set the origin, position and orientation of assembly in space.
    assembly.SetOrigin(0, 0, 0) # This is the point about which all rotations take place
    #assembly.AddPosition(0, 0, 0)
    #assembly.RotateX(45)

    actor[1].SetOrigin(274, 0, 0)  # initial elbow joint position

    ren.AddActor(assembly)

    # Add coordinates
    axes = CreateCoordinates()
    ren.AddActor(axes)

    # Add ground
    ground = CreateGround()
    ren.AddActor(ground)

    # Add slider to control the robot
    sliderWidget_shoulder = ConfigSlider(slider_shoulder,"Shoulder Joint", 80)
    sliderWidget_shoulder.SetInteractor(iren)
    sliderWidget_shoulder.EnabledOn()
    sliderWidget_shoulder.AddObserver("InteractionEvent", ShoulderSliderCallback)

    sliderWidget_elbow = ConfigSlider(slider_elbow,"Elbow Joint", 160)
    sliderWidget_elbow.SetInteractor(iren)
    sliderWidget_elbow.EnabledOn()
    sliderWidget_elbow.AddObserver("InteractionEvent", ElbowSliderCallback)

    # Set background color
    ren.SetBackground(.2, .2, .2)

    # Set window size
    renWin.SetSize(600, 600)

    # Set up the camera to get a particular view of the scene
    camera = vtk.vtkCamera()
    camera.SetFocalPoint(300, 0, 0)
    camera.SetPosition(300, -400, 350)
    camera.ComputeViewPlaneNormal()
    camera.SetViewUp(0, 1, 0)
    camera.Zoom(0.4)
    ren.SetActiveCamera(camera)

    # Enable user interface interactor
    iren.Initialize()
    iren.Start()

if __name__ == "__main__":
    CreateScene()

　　下面是使用MFC搭建的机器人上位机监控平台，可以实现上述的一些基本功能。这个GIF动画使用开源软件ScreenToGif生成，非常好用！

参考 ：

基于OpenGL的六自由度机械臂三维仿真工具的设计

03-VTK基础概念(2)

第04章-VTK基础（4）

Using VTK Interactors

VTK/Examples/Cxx/PolyData/TransformPipeline

VTK/Examples/Cxx/Visualization/DisplayCoordinateAxes

VTK/Examples/Cxx/GeometricObjects/Axes

Using VTK to Visualize Scientific Data (online tutorial)

VTK/Tutorials/Widgets

VTK/Tutorials/InteractorStyleSubclass