OCC: interface to open-cascade

Preamble

OCC performs CAD operations and surface meshing using open-cascade.

To use the module with the Converter.array interface:

import OCC

To use the module with the CGNS/Python interface:

import OCC.PyTree as OCC

List of functions

– CAD to surface mesh

OCC.PyTree.meshAll	Get a first TRI meshed tree linked to CAD.
OCC.PyTree._projectOnFaces	Project t on CAD.
OCC.PyTree._meshDeviation

– CAD to surface mesh (legacy functions)

OCC.convertCAD2Arrays	Convert a CAD (IGES or STEP) file to arrays.
OCC.PyTree.convertCAD2PyTree	Convert a CAD (IGES or STEP) file to pyTree.

– CAD manipulation

OCC.readCAD	Read CAD file and return a CAD hook.
OCC.writeCAD	Write CAD file.
OCC.getNbEdges	Return the number of edges in CAD hook.
OCC.getNbFaces	Return the number of faces in CAD hook.
OCC.getFaceArea	Return the area of given faces.
OCC._translate	Translate all or given faces.
OCC._rotate	Rotate all or given faces.
OCC._scale	Scale all or given faces.
OCC._splitFaces	Split all faces to be less than area.
OCC._mergeFaces	Merge some faces.
OCC._sewing	Sew some faces (suppress redundant edges).
OCC._removeFaces	Remove given faces.
OCC._trimFaces	Trim a set of faces with another set of faces.

Contents

CAD to surface mesh

OCC.PyTree.meshAll(hook, hmin=-1, hmax=-1., hausd=-1.)

Mesh a CAD with triangles. If hmin=hmax, mesh with a regular h. If hmin, hmax and hausd are set, mesh with isotropic triangles with local size adapted to curvature. hausd is the max chordal error of mesh to CAD. This function returns a tree with two bases (EDGES and FACES). EDGES contains discretized edges with a link to the global edge number in CAD. FACES contains discretized faces with a link to the global face number in CAD.

Parameters:

• hook (CAD hook) – CAD hook

• hmin (float) – minimum step size on output mesh.

• hmax (float) – maximum step size on output mesh.

• hausd (float) – maximum chordal error.

Return type:

mesh

Example of use:

• Mesh a CAD (pyTree):

# - meshAll (pyTree) -
import OCC.PyTree as OCC
import Converter.PyTree as C

hook = OCC.readCAD("cube.step", "fmt_step")
# constant h
m = OCC.meshAll(hook, hmin=1., hmax=1.)
# varying h between hmin and hmax with deflection hausd
m = OCC.meshAll(hook, hmin=1., hmax=5., hausd=1.)
# get components in final mesh
cs = OCC.getComponents(m)
# check if components are watertight
for c in cs:
    ret = OCC.isWatertight(c)
    print("Component watertight: ", ret)
C.convertPyTree2File(m, 'out.cgns')

OCC.PyTree._projectOnFaces(hook, t, faceList=None)

Project mesh on given CAD faces (in place).

Parameters:

• hook (CAD hook) – CAD hook

• t (zone, list of zones or tree) – mesh to project

• faceList (list of face index (starts 1)) – list of faces number to calculate the area

Example of use:

• Project a mesh on CAD faces (pyTree):

# - projectOnFaces (PyTree) -
import OCC.PyTree as OCC
import Converter.PyTree as C
import Generator.PyTree as G

hook = OCC.readCAD("cube.step", "fmt_step")

# project on all CAD faces
a = G.cart((-27,-17,-33), (1,1,1), (1,10,10))
OCC._projectOnFaces(hook, a, None)
C.convertPyTree2File(a, 'out.cgns')

# project on certain CAD faces
a = G.cart((-27,-17,-33), (1,1,1), (1,10,10))
OCC._projectOnFaces(hook, a, [1,2,3,4,5,6])
C.convertPyTree2File(a, 'out.cgns')

OCC.PyTree.meshDeviation(hook, t)

Mesaure deviation of mesh from CAD. Project the center of mesh triangles on CAD and return the projection distance in a field.

Parameters:

• hook (CAD hook) – CAD hook

• t (zone, list of zones or tree) – mesh

Example of use:

• Measure deviation of mesh from CAD (pyTree):

# - meshDeviation (pyTree) -
import OCC.PyTree as OCC
import Converter.PyTree as C

hook = OCC.readCAD("cube.step", "fmt_step")
m = OCC.meshAll(hook, hmin=1., hmax=1.)

OCC._meshDeviation(hook, m)

C.convertPyTree2File(m, 'out.cgns')

CAD to surface mesh (legacy functions)

OCC.convertCAD2Arrays(fileName, format='fmt_iges', h=0., chordal_err=0., growth_ratio=0., algo=1)

Read a CAD and return arrays.

Parameters:

• fileName (string) – CAD file name

• format (string) – file format (‘fmt_iges’ or ‘fmt_step’)

• h (float) – step size on output mesh. If 0., automatic setting [algo=1,2].

• chordal_error (float) – max error between CAD and mesh. Result in curvature adaptation. If 0., automatic setting.

• growth_ratio (float) – max growth ratio between adjacent triangles [algo=1,2].

• algo (int) – algo=0: mesh with only respect to curvature, algo=1 or algo=2: mesh with regular triangles.

Return type:

a list of TRI arrays

Example of use:

• Read a CAD (array):

# - convertCAD2Arrays (arrays) -
import Converter as C
import OCC

# IGES avec T3Mesher
A = OCC.convertCAD2Arrays('hammer.iges', format='fmt_iges',
                          h=0., chordal_err=0., growth_ratio=0.8, algo=1)
C.convertArrays2File(A, 'hammer1.plt')

# IGES avec OCC
A = OCC.convertCAD2Arrays('hammer.iges', format='fmt_iges',
                          chordal_err=1, algo=0)
C.convertArrays2File(A, 'hammer2.plt')

# STEP avec T3Mesher
A = OCC.convertCAD2Arrays('as1-oc-214.stp', format='fmt_step',
                          h=0., chordal_err=0., growth_ratio=0.8, algo=1)
C.convertArrays2File(A, 'as1.plt')

# STEP avec OCC
A = OCC.convertCAD2Arrays('as1-oc-214.stp', format='fmt_step',
                          chordal_err=1, algo=0)
C.convertArrays2File(A, 'as2.plt')

---

OCC.PyTree.convertCAD2PyTree(fileName, format='fmt_iges', h=0., chordal_err=0., growth_ratio=0., algo=1)

Read a CAD and return a zone.

Parameters:

• fileName (string) – CAD file name

• format (string) – file format (‘fmt_iges’ or ‘fmt_step’)

• h (float) – step size on output mesh. If 0., automatic setting [algo=1,2].

• chordal_error (float) – max error between CAD and mesh. Result in curvature adaptation. If 0., automatic setting.

• growth_ratio (float) – max growth ratio between adjacent triangles [algo=1,2].

• algo (int) – algo=0: mesh with only respect to curvature, algo=1 or algo=2: mesh with regular triangles.

Return type:

CGNS pyTree

Example of use:

• Read a CAD (pyTree):

# - convertIGES2PyTree (PyTree) -
import OCC.PyTree as OCC
import Converter.PyTree as C

t = OCC.convertCAD2PyTree('hammer.iges', h=0., chordal_err=0., algo=1)
C.convertPyTree2File(t, 'out.cgns')

CAD manipulation

OCC.readCAD(fileName, format='fmt_step')

Read a CAD file and return a CAD hook.

Parameters:

• fileName (string) – CAD file name

• format (string) – file format (‘fmt_iges’ or ‘fmt_step’)

Return type:

CAD hook

Example of use:

• Read a CAD:

# - readCAD (pyTree) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cube.step", "fmt_step")

---

OCC.writeCAD(hook, fileName, format='fmt_step')

Write a CAD hook to a file.

Parameters:

• hook (CAD hook) – CAD hook

• fileName (string) – CAD file name

• format (string) – file format (‘fmt_iges’ or ‘fmt_step’)

Example of use:

• Write a CAD:

# - writeCAD (pyTree) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cube.step", "fmt_step")
OCC.writeCAD(hook, "out.step", "fmt_step")

---

OCC.getNbEdges(hook)

Return the total number of edges in a CAD hook.

Parameters:

hook (CAD hook) – CAD hook

Return type:

int

Example of use:

• Get the number of edges:

# - getNbEdges (pyTree) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cube.step", "fmt_step")
print(OCC.getNbEdges(hook))
#> 12

---

OCC.getNbFaces(hook)

Return the number of faces in a CAD hook.

Parameters:

hook (CAD hook) – CAD hook

Return type:

int

Example of use:

• Get the number of faces:

# - getNbFaces (pyTree) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cube.step", "fmt_step")
print(OCC.getNbFaces(hook))
#> 6

---

OCC.getFaceArea(hook, faceList=[])

Return the area of given faces.

Parameters:

• hook (CAD hook) – CAD hook

• faceList (list of face index (starts 1)) – list of faces number to calculate the area

Return type:

float

Example of use:

• Get face area:

# - getFaceArea (pyTree) -
import OCC.PyTree as OCC

h = OCC.readCAD('cube.step', 'fmt_step')

# Area of all model
print(OCC.getFaceArea(h))

# Area of seome faces
print(OCC.getFaceArea(h, [1,2]))

---

OCC._translate(hook, vector, listOfFaces=None)

Translate a CAD hook by a given vector.

Parameters:

• hook (CAD hook) – CAD hook

• vector (tuple of floats) – translation vector (dx, dy, dz)

• listOfFaces (list of face indices (starts 1)) – if None, translate all else translate only given faces

Example of use:

• Translate a CAD:

# - translate (pyTre) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cube.step", "fmt_step")
OCC._translate(hook, (1,0,0))
OCC.writeCAD(hook, 'out.step', 'fmt_step')

---

OCC._rotate(hook, Xc, axis, angle, listOfFaces=None)

Rotate a CAD hook around a given axis by a given angle.

Parameters:

• hook (CAD hook) – CAD hook

• Xc (tuple of floats) – rotation center (x, y, z)

• axis (tuple of floats) – rotation axis

• angle (float) – rotation angle in degrees

• listOfFaces (list of face indices (starts 1)) – if None, rotate all else rotate only given faces

Example of use:

• Rotate a CAD:

# - rotate (pyTree) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cube.step", "fmt_step")
OCC._rotate(hook, (0,0,0), (0,0,1), 30.)
OCC.writeCAD(hook, 'out.step', 'fmt_step')

---

OCC._scale(hook, factor, X, listOfFaces=None)

Scale a CAD hook by a given factor.

Parameters:

• hook (CAD hook) – CAD hook

• factor (float) – scale factor

• X (tuple of floats) – invariant point (x, y, z)

• listOfFaces (list of face indices (starts 1)) – if None, scale all else scale only given faces

Example of use:

• Rotate a CAD:

# - scale (pyTre) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cube.step", "fmt_step")
OCC._scale(hook, 0.5, (0,0,0))
OCC.writeCAD(hook, 'out.step', 'fmt_step')

---

OCC._splitFaces(hook, area)

Split faces in a CAD hook.

Parameters:

• hook (CAD hook) – CAD hook

• area (float) – split each face if area greater than this value

Example of use:

• Split faces:

# - splitFaces (pyTree) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cube.step", "fmt_step")
OCC._splitFaces(hook, 20.)
OCC.writeCAD(hook, "out.step", "fmt_step")

---

OCC._mergeFaces(hook, faceList=None)

Merge faces in a CAD hook.

Parameters:

• hook (CAD hook) – CAD hook

• faceList (list of face indices (starts 1)) – if None, merge all faces else list of faces indices to merge

Example of use:

• Merge faces:

# - mergeFaces (array) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cube.step", "fmt_step")
OCC._splitFaces(hook, 200.)
# merge given faces
OCC._mergeFaces(hook, [1,2,3])
# merge all faces
OCC._mergeFaces(hook)
OCC.writeCAD(hook, "out.step", "fmt_step")

---

OCC._sewing(hook, faceList=None, tol=1.e-6)

Sew faces. Supress redundant edges.

Parameters:

• hook (CAD hook) – CAD hook

• faceList (list of face indices (starts 1)) – if None, merge all faces else list of faces indices to merge

• tol (float) – tolerance for sewing

Example of use:

• Sew faces:

# - sewing (pyTree) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cube.step", "fmt_step")
OCC._sewing(hook, tol=1.e-6, faceList=[1,2])
OCC._sewing(hook, tol=1.)
OCC.writeCAD(hook, 'out.step', 'fmt_step')

---

OCC._removeFaces(hook, faceList)

Remove given list of faces from CAD.

Parameters:

• hook (CAD hook) – CAD hook

• faceList (list of face indices (starts 1)) – list of faces

Example of use:

• Remove faces:

# - removeFaces (pyTree) -
import OCC

hook = OCC.readCAD("cubeNoTrim.step", "fmt_step")

ret = OCC.getFaceNameInOCAF(hook)
cube1 = ret[3]
OCC._removeFaces(hook, cube1)

OCC.writeCAD(hook, 'out.step', 'fmt_step')

---

OCC._trimFaces(hook, faces1, faces2)

Trim set of faces1 with set of faces2.

Parameters:

• hook (CAD hook) – CAD hook

• faces1 (list of face indices (starts 1)) – first set of faces

• faces2 (list of face indices (starts 1)) – second set of faces

Example of use:

• Trim faces:

# - trimFaces (pyTree) -
import OCC.PyTree as OCC

hook = OCC.readCAD("cubeNoTrim.step", "fmt_step")

ret = OCC.getFaceNameInOCAF(hook)
cube1 = ret[3]
cube2 = ret[5]
#OCC._translate(hook, (+300.,0,0), cube2)
OCC._trimFaces(hook, cube1, cube2)
OCC._sewing(hook, tol=1.e-2)

OCC.writeCAD(hook, 'out.step', 'fmt_step')