add BytesIO support to all export functions (STL, STEP, etc)

[jdegenstein]

Currently export_brep supports BytesIO export, consider adding to other export functions. It may make upcoming wasm in-browser usage easier.

ref: https://github.com/yeicor/OCP.wasm/issues/4

[wedward]

BytesIO support for gltf would be a big help for me as well

[goertzenator]

+1 for gltf in particular as it is the preferred format for threejs. stl would also be useful to me.

[jdegenstein]

OK I have done further research and testing, and here is the current state of this proposal (has not been put into build123d yet, just local work). I am particularly disappointed that we don't have an OCCT-native solution for GLTF. It appears that OCCT simply does not support std::ostream for many of the exporter file formats.

file format	ability to export to BytesIO
BREP	✅ already implemented
SVG	✅ tested and working
DXF	✅ tested and working
STEP	✅ tested and working
STL	❌ not possible with OCCT
GLTF/GLB	❌ not possible with OCCT
STL	✅ tested and working using lib3mf / Mesher
3MF	✅ tested and working using lib3mf / Mesher

[gumyr]

After consulting with ChatGPT on this the following seems like it might be an acceptable solution the the STL and GLTF/GLB case:

import io
import os
import tempfile
import sys
from OCP.StlAPI import StlAPI_Writer

def write_stl_to_memory(shape, binary=True) -> io.BytesIO:
    """Write shape to STL entirely in memory (best-effort cross-platform)."""
    writer = StlAPI_Writer()
    writer.ASCIIMode(not binary)

    # --- Linux fast path ---
    if hasattr(os, "memfd_create"):
        fd = os.memfd_create("stl_buffer")
        tmp_path = f"/proc/self/fd/{fd}"
        writer.Write(shape, tmp_path)
        os.lseek(fd, 0, os.SEEK_SET)
        data = os.read(fd, 1 << 24)  # read up to 16 MB; adjust as needed
        os.close(fd)
        return io.BytesIO(data)

    # --- macOS / Windows fallback ---
    with tempfile.SpooledTemporaryFile(max_size=64 * 1024 * 1024) as tmp:
        tmp_name = tmp.name if tmp.name else "tmp.stl"
        writer.Write(shape, tmp_name)
        tmp.seek(0)
        data = tmp.read()
    return io.BytesIO(data)

[jdegenstein]

OK as of the above merged PR, here is the current status:

file format	ability to export to BytesIO
BREP	✅ already released
SVG	✅ merged in PR #1141
DXF	✅ merged in PR #1141
STEP	✅ merged in PR #1141
STL	❌ not possible with OCCT
GLTF/GLB	❌ not possible with OCCT
STL	✅ merged in PR #1141 using lib3mf / Mesher.write_stream
3MF	✅ merged in PR #1141 using lib3mf / Mesher.write_stream

As such, I have decided to keep this issue open (with an updated title). The major missing piece is GLTF/GLB, per the linked issue at OCCT it sounds like they have added this (and STL, maybe others) to their roadmap.