在 CityEngine 中使用 Python

​ CityEngine的同义词几乎就是“规则建模”，规则指的是CGA脚本。说到脚本，很多用户又会联想到 Python， 衍生出来的问题就是，“CityEngine中是用python脚本建模吗？” 答案是No。

​ CityEngine中确实也引入了python，只不过使用python的姿势不是建模，而是自动化。通过python脚本，可以增强CityEngine的建模功能，自动化驱动脚本建模工作流，提高建模效率。我们既可以在控制台交互式执行python命令，也可以在python editor 中书写python执行文件，甚至可以将一些自定义功能扩展到CityEngine中。如下是CityEngine官方教程中的几个示例代码：

Example 1 选择部分对象

'''
Created on 2018-4-9
@author: kikita
'''
from scripting import *

# get a CityEngine instance
ce = CE()

def selectByAttribute(attr, value):
    objects = ce.getObjectsFrom(ce.scene)
    selection = []
    for o in objects:
        attrvalue = ce.getAttribute(o, attr)
        if attrvalue  ==  value:
            selection.append(o)     
    ce.setSelection(selection)

if __name__ == '__main__':
    selectByAttribute("connectionStart","JUNCTION")

Example 2 修改对象属性

'''
Created on 2018-4-9
@author: kikita
'''
from scripting import *

# get a CityEngine instance
ce = CE()

''' increment the street width parameter of all selected street segments'''
# @noUIupdate
def incrementStreetWidths(increment):
    selectedSegments = ce.getObjectsFrom(ce.selection, ce.isGraphSegment)
    for segment in selectedSegments:
        oldWidth = ce.getAttribute(segment, "/ce/street/streetWidth")
        newWidth = oldWidth+increment
        ce.setAttribute(segment, "/ce/street/streetWidth", newWidth)

if __name__ == '__main__':
    incrementStreetWidths(10)

Example 3 动态模拟建筑物生长

'''
Created on 2018-4-12
@author: kikita
'''
from scripting import *

# get a CityEngine instance
ce = CE()

def growBuilding():
    for i in range(1,14):
        height = 20+i
        doStep(i,height,1)
    for i in range(15,35):
        height = 34
        width = i-14
        doStep(i,height,width)

def doStep(i,height,width):    
    object = ce.getObjectsFrom(ce.scene, ce.withName("'Lot1'"))
    ce.setAttributeSource(object, "height", "OBJECT")
    ce.setAttributeSource(object, "width", "OBJECT")
    ce.setAttribute(object, "height", height)
    ce.setAttribute(object, "width", width)  
    Generate(object)
    
def Generate(object):
    ce.generateModels(object)

if __name__ == '__main__':
   growBuilding()

Example 4 陈列资源库

'''
Created on 2018-4-12
@author: kikita
'''
from scripting import *

# get a CityEngine instance
ce = CE()

def writeCGAlib():
    # open the cga file to be written
    cga = "/*Asset Library Loader : Generated by asset_lib.py*/\n version \"2014.0\"\n\n"   
    # write start rule
    cga += "Lot -->  Geometries Textures"  
    # write rule showing geometries
    cga += "\n\nGeometries --> "   
    # get all .obj files from asset directory, and call their loader
    for obj in ce.getObjectsFrom("/", ce.isFile, ce.withName("/Tutorial_10*/assets/*.obj")):    
        # and write 
        cga += "\n\t t(2,0,0)  Geometry(\""+obj+"\")"
        print obj
    # write rule showing jpg textures
    cga += "\n\nTextures --> \n\ts(1,0,0) set(scope.tz,0) set(scope.ty,3) i(\"facades/xy-plane.obj\")"
    # get all .jpg files from asset directory, and call their loader
    for jpg in ce.getObjectsFrom("/", ce.isFile, ce.withName("/Tutorial_10*/assets/*.jpg")):
        cga += "\n\tt(2,0,0)  Texture(\""+jpg+"\")"
    #write geometry loader rule
    cga += "\n\n Geometry(asset) --> s(1,0,0) i(asset) set(scope.ty,0) set(scope.tz,0)"
    #write texture loader rule
    cga += "\n\n Texture(asset) --> set(material.colormap, asset)"    
    cgafile = ce.toFSPath("rules/asset_lib2.cga")
    CGA = open(cgafile, "w")
    CGA.write(cga)
    CGA.close()
    print "written file "+cgafile
        
def assignAndGenerateLib():
    object = ce.getObjectsFrom(ce.scene, ce.withName("'Lot2'"))
    ce.refreshWorkspace()
    ce.setRuleFile(object, "asset_lib2.cga")
    ce.setStartRule(object, "Lot")
    ce.generateModels(object)
    
if __name__ == '__main__':
    writeCGAlib()
    assignAndGenerateLib() 

Example 5 扩展CityEngine功能

相关的python功能实现后，确保在CityEngine的python editor中可以正常运行。然后，在CityEngine的工作空间根目录创建 scripting.py 文件，内容如下：

import sys

sys.path.append({PATH_TO_YOUR_SCRIPTS_DIRECTORY})
# e.g. sys.path.append("C:\user\CityEngine\MyProject\scripts")

# import customized python module
import myEfficiencyModule

重启CE之后，可以在python编辑器或控制台中直接使用此模块的功能。

>>> myEfficiencyModule.selectByAttribute("connectionEnd", "JUNCTION")