WriteVrml works only after XDisplay - Forum Open Cascade Technology

andreasplesch_159166
Fri, 05/01/2020 - 22:41

Forums:

I am trying to export VRML with Draw using WriteVrml. It works but only after I use XDisplay to render the document on the view. Without, the VRML is generated but only has a header and no geometry content:

Draw[25]> pwd
C:/OpenCASCADE-7.4.0-vc14-64/opencascade-7.4.0/data/step
Draw[28]> ReadStep D screw.step
 File STEP to read : screw.step
Document saved with name D
Draw[31]> WriteVrml D screw.wrl

screw.wrl:

#VRML V2.0 utf8

WorldInfo {
  info [
    "Generated by Open CASCADE Technology 7.4"
  ]
}

After rendering with XDisplay it works:

Draw[26]> vclear
Draw[27]> vinit View1
Driver1/Viewer1/View1
Draw[28]> ReadStep D screw.step
 File STEP to read : screw.step
Document saved with name D
Draw[29]> XDisplay -dispMode 1 D
D:the product name
Draw[31]> WriteVrml D screw.wrl

How can I avoid the rendering ?

andreasplesch_159166
Mon, 05/04/2020 - 16:27

Let me know if I can provide anything else to help. WriteStep and WriteIges works as expected, eg. without displaying.

Patrik Mueller
Tue, 05/05/2020 - 20:58

Hi Andreas,

never worked with Draw, but could you try to do

incmesh

after reading and before writing?

I assume

WriteVrml

need tesselation on the shape before writing. If you do a display this is done by AIS.

Best regards,

Patrik

andreasplesch_159166
Wed, 05/06/2020 - 00:27

Thank you much. Indeed, generating a mesh worked:

Draw[1]> pload ALL
1
Draw[2]> cd data/step
Draw[3]> ReadStep A screw.step
 File STEP to read : screw.step
Document saved with name A
Draw[4]> XGetFreeShapes A shapes
shapes
Draw[5]> incmesh shapes 0.01 -relative
Incremental Mesh, multi-threading OFF
Meshing statuses: NoError
Draw[7]> WriteVrml A screwMeshed.vrml

The incmesh command is not documented in https://www.opencascade.com/doc/occt-7.4.0/overview/html/occt_user_guides__test_harness.html

I only used draw to reproduce a problem with python. I see that incmesh is the tcl command interface to https://www.opencascade.com/doc/occt-7.4.0/refman/html/class_b_rep_mesh___incremental_mesh.html

Are there examples or recommendations on how to use that class ? How does AIS determine good deflection values ?

Draw[12]> incmesh
Builds triangular mesh for the shape
usage: incmesh Shape LinearDeflection [options]
options:
        -a val          angular deflection for edges in deg
                        (default ~28.64 deg = 0.5 rad)

        -ai val         angular deflection inside of faces in deg
                        (default ~57.29 deg = 1 rad)

        -di val         Linear deflection used to tessellate the face interior.
        -min            minimum size parameter limiting size of triangle's
                        edges to prevent sinking into amplification in case
                        of distorted curves and surfaces

        -relative       notifies that relative deflection is used
                        (switched off by default)

        -int_vert_off   disables insertion of internal vertices into mesh
                        (enabled by default)
        -surf_def_off   disables control of deflection of mesh from real
                        surface (enabled by default)
        -parallel       enables parallel execution (switched off by default)
        -adjust_min     enables local adjustment of min size depending on edge size (switched off by default)

andreasplesch_159166
Wed, 05/06/2020 - 02:48

Thanks that worked !

Draw[1]> pload ALL
1
Draw[2]> cd data/step
Draw[3]> ReadStep A screw.step
 File STEP to read : screw.step
Document saved with name A
Draw[4]> XGetFreeShapes A shapes
shapes
Draw[5]> incmesh shapes 0.01 -relative
Incremental Mesh, multi-threading OFF
Meshing statuses: NoError
Draw[7]> WriteVrml A screwMeshed.vrml
Draw[8]> help incmesh
incmesh         : Builds triangular mesh for the shape, run w/o args for help

I am using draw to reproduce an issue I had with python. incmesh uses

https://www.opencascade.com/doc/occt-7.4.0/refman/html/class_b_rep_mesh___incremental_mesh.html#details

Do you of examples or recommendation on how to use BRepMesh_IncrementalMesh ?

Especially with regards to automating deviation parameter selection ? How does AIS choose those for presentation triangulation ?

Draw[12]> incmesh
Builds triangular mesh for the shape
usage: incmesh Shape LinearDeflection [options]
options:
        -a val          angular deflection for edges in deg
                        (default ~28.64 deg = 0.5 rad)

        -ai val         angular deflection inside of faces in deg
                        (default ~57.29 deg = 1 rad)

        -di val         Linear deflection used to tessellate the face interior.
        -min            minimum size parameter limiting size of triangle's
                        edges to prevent sinking into amplification in case
                        of distorted curves and surfaces

        -relative       notifies that relative deflection is used
                        (switched off by default)

        -int_vert_off   disables insertion of internal vertices into mesh
                        (enabled by default)
        -surf_def_off   disables control of deflection of mesh from real
                        surface (enabled by default)
        -parallel       enables parallel execution (switched off by default)
        -adjust_min     enables local adjustment of min size depending on edge size (switched off by default)

Thanks for any feedback or input.

Patrik Mueller
Thu, 05/14/2020 - 11:35

You should ask the OCCT people about the calculation/definition of the default parameters.

andreasplesch_159166
Thu, 05/14/2020 - 22:25

Well, I started to dig through the source but I could not find anything special, just default values, and I think relative to bounding box size.