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Simple viewer for STEP files on UNIX

date: 2021-11-04
reading time: 2 min.
category: howto
tags: STEP

The STEP format is probably the most used accurate 3D geometry exchange format. (It can contain a lot more than just geometry!)

STL files (most often used in 3D printers) are based on triangles, which are basically only an approximation of the real geometry.

A lot of available STEP-file viewer are windows-only. I’ve been looking for a simple STEP-file viewer for UNIX-like operating systems, so that I can see the parts that I’m making using cadquery (see also: cadquery documentation).

I’m not a fan of IDE’s like cq-editor or IDE’s in general. In particular, cq-editor pulls in the whole of spyder (a Python IDE) just to get syntax highlighting in its built-in editor.

Most programs working with accurate 3D data (including cadquery) use the OpenCASCADE library. This comes with its own draw test harness, named DRAWEXE on UNIX systems. We can use that to show STEP files.

The DRAWEXE program can be controlled using tcl commands.

The contents of a minimal shell-script to view a STEP file are shown below.

export CASROOT="/usr/local/OpenCAS"
export CSF_OCCTResourcePath="${CASROOT}/resources"
export DRAWHOME="${CSF_OCCTResourcePath}/DrawResources"
export CSF_DrawPluginDefaults="${CSF_OCCTResourcePath}/DrawResources"
export DRAWDEFAULT="${CSF_OCCTResourcePath}/DrawResources/DrawDefault"
CMDS="pload ALL;vinit;stepread \"${1}\" a *;"
CMDS="${CMDS}vsetdispmode 1;vdisplay a_1;vchangeselected a_1;"
CMDS="${CMDS}vsetmaterial aluminium;vaxo;vfit"

The exported variables seem to be needed by DRAWEXE, based on other scripts in the distribution. Note that you might have to change them, especially CASROOT, depending on where OpenCASCADE is installed.

The commands used in CMDS are documented in draw test harness. The first line in CMDS loads all libraries, initializes the viewer window and reads the STEP file which is the first and only argument provided to the scipt. The second line sets the display mode to shaded, displays the model and selects it. The latter makes the lines/edges visible. The third line sets the visual properties of the model to that of aluminium, switches to axonometric view (orientation +X-Y+Z) and fits the model to the window.

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