python - 通过面法线值阈值选择 STL 的面

Question

我想用 Python 编写一个脚本，它可以根据人脸法线值条件在 STL 中生成人脸组。例如，提供的是 Stl 的快照，不同的颜色表示包含满足我给定的人脸法线阈值的三角形面的人脸组。在python中有没有简单的方法来做到这一点？ 人脸组 STL

Answer 1

我确定有一个 python 库来加载 stl 文件，但我一直只是自己编写，因为文件格式非常简单（有关文件格式描述，请参阅Wikipedia 文章）。

这是我读取 stl 文件的代码：

import numpy as np
import struct

def Unique(inputList):
      """ 
      Given an M x N list, this function gets the unique rows by treating all
      M Ntuples as single objects. This function also returns the indexing
      to convert the unique returned list back to the original non-unique list.
      """

      hashTable=dict()

      indexList=[]
      uniqueList=[]

      indx=0
      for ntuple in inputList:
            if not ntuple in hashTable:
                hashTable[ntuple]=indx
                indexList.append(indx)
                uniqueList.append(ntuple)
                indx+=1
            else:
                indexList.append(hashTable.get(ntuple))      

      return uniqueList, indexList


def IsBinarySTL(filename):
    try:
        with open(filename,'r') as f:
              test=f.readline()
    except UnicodeDecodeError:
        return True

    if len(test) < 5:
        return True
    elif test[0:5].lower() == 'solid':
        return False  # ASCII STL
    else:
        return True

def ReadSTL(filename):
    """ Returns numpy arrays for vertices and facet indexing """
    def GetListFromASCII(filename):
        """ Returns vertex listing from ASCII STL file """
        outputList=[]

        with open(filename,'r') as f:
            lines=[line.split() for line in f.readlines()]
        for line in lines:
            if line[0] == 'vertex':
                    outputList.append(tuple([float(x) for x in line[1:]]))
        return outputList

    def GetListFromBinary(filename):
        """ Returns vertex listing from binary STL file """
        outputList=[]
        with open(filename,'rb') as f:
            f.seek(80) # skip header
            nFacets=struct.unpack('I',f.read(4))[0] # number of facets in piece

            for i in range(nFacets):
                  f.seek(12,1) # skip normal
                  outputList.append(struct.unpack('fff',f.read(12))) # append each vertex triple to list (each facet has 3 vertices)
                  outputList.append(struct.unpack('fff',f.read(12))) 
                  outputList.append(struct.unpack('fff',f.read(12)))
                  f.seek(2,1) # skip attribute
        return outputList

    if IsBinarySTL(filename):
        vertexList = GetListFromBinary(filename)
    else:
        vertexList = GetListFromASCII(filename)

    coords, tempindxs = Unique(vertexList)

    indxs = list()
    templist = list()
    for i in range(len(tempindxs)):
        if (i > 0 ) and not (i % 3):
            indxs.append(templist)
            templist = list()
        templist.append(tempindxs[i])
    indxs.append(templist)

    return np.array(coords), np.array(indxs)

这是计算刻面法线的代码（假设右手规则）

def GetNormals(vertices, facets):
    """ Returns normals for each facet of mesh """
    u = vertices[facets[:,1],:] - vertices[facets[:,0],:]
    v = vertices[facets[:,2],:] - vertices[facets[:,0],:]
    normals = np.cross(u,v)
    norms = np.sqrt(np.sum(normals*normals, axis=1))
    return normals/norms[:, np.newaxis]

最后，写出 stl 文件的代码（假设每个方面都有一个属性列表）：

def WriteSTL(filename, vertices, facets, attributes, header):
    """
    Writes vertices and facets to an stl file. Notes:
    1.) header can not be longer than 80 characters
    2.) length of attributes must be equal to length of facets
    3.) attributes must be integers
    """
    nspaces = 80 - len(header)
    header += nspaces*'\0'

    nFacets = np.shape(facets)[0]
    stl = vertices[facets,:].tolist()

    with open(filename,'wb') as f: # binary
        f.write(struct.pack('80s', header.encode('utf-8'))) # header
        f.write(struct.pack('I',nFacets)) # number of facets
        for i in range(nFacets):
            f.write(struct.pack('fff',0,0,0)) # normals set to 0
            for j in range(3):
                f.write(struct.pack('fff',stl[i][j][0], stl[i][j][1], stl[i][j][2])) # 3 vertices per facet 
            f.write(struct.pack("H", attributes[i])) # 2-byte attribute

将所有这些放在一起，您可以执行以下操作：

if __name__ == "__main__":
    filename = "bunny.stl"

    vertices, facets = ReadSTL(filename)  # parse stl file
    normals = GetNormals(vertices, facets)  # compute normals

    # Get some value related to normals
    attributes = []
    for i in range(np.shape(normals)[0]):
        attributes.append(int(255*np.sum(normals[i])**2))

    # Write new stl file
    WriteSTL("output.stl", vertices, facets, attributes, "stlheader")

此代码片段读取一个 stl 文件，计算法线，然后根据每个法线的平方和分配一个属性值（注意该属性必须是整数）。

此脚本的输入和输出如下所示：