Linux BUILD_FILAMENT_FROM_SOURCE lists archives under lib instead of lib/x86_64

Checklist

• Clean upstream checkout at the exact reported revision.
• Built pip-package directly without prebuilding ext_filament.
• Preserved the original Ninja error and compared only the archive-directory correction.

Environment

Ubuntu 26.04.1 LTS, x86_64
Linux 7.0.0-31-generic
Python 3.12.13
CMake 4.2.3; Ninja 1.13.2
GCC 15.2.0; Clang 21.1.8
Open3D 1a9eb990f9a20936c30c428568c602bdef760744
Filament v1.54.0, tag commit c1a3450d9c0a64c329a17bff4f3a7f68a2474214
Clean checkout status before configure: empty

Exact reproduction

cmake -S . -B build-byproducts -G Ninja \
  -DCMAKE_BUILD_TYPE=Release \
  -DBUILD_GUI=ON -DBUILD_FILAMENT_FROM_SOURCE=ON \
  -DBUILD_CUDA_MODULE=OFF -DBUILD_WEBRTC=OFF \
  -DPython3_EXECUTABLE=/path/to/python

set -o pipefail
cmake --build build-byproducts --target pip-package --parallel 24 --verbose 2>&1 \
  | tee build-original.log
status=${PIPESTATUS[0]}
printf '\nExit status: %s\n' "$status" | tee -a build-original.log
exit "$status"

I did not prebuild ext_filament. The generated graph fails immediately and deterministically:

Change Dir: '/tmp/open3d-audit-7555-build'
Run Build Command(s): /usr/bin/ninja -v pip-package
ninja: error: 'filament-binaries/lib/x86_64/libfilameshio.a', needed by 'lib/Release/libOpen3D.so.0.19.0', missing and no known rule to make it

Exit status: 1

Same-build graph evidence

At this revision, 3rdparty/filament/filament_build.cmake leaves Linux lib_dir as lib, so the Ninja custom command declares these byproducts:

filament-binaries/lib/libfilameshio.a
filament-binaries/lib/libfilament.a
...
filament-binaries/lib/libvkshaders.a

The imported Filament targets in 3rdparty/find_dependencies.cmake, and Filament's actual install layout, instead use:

filament-binaries/lib/x86_64/libfilameshio.a
filament-binaries/lib/x86_64/libfilament.a
...
filament-binaries/lib/x86_64/libvkshaders.a

Ninja therefore has no rule for the imported archive path even though the external project is the intended producer.

Isolated archive-directory correction

The control changed only Linux archive-directory selection:

elseif(UNIX)
    if(CMAKE_SYSTEM_PROCESSOR MATCHES "^(aarch64|arm64)$")
        set(lib_dir lib/aarch64)
    else()
        set(lib_dir lib/x86_64)
    endif()
endif()

After regeneration, both the ext_filament phony edge and external-project build edge declare all 15 archives under filament-binaries/lib/x86_64/. The archives were then present in that directory, including:

b8b23a5152e0ae2c70feeacdfdbcc35795fdca608090647e25db46708f7640d6  lib/x86_64/libfilameshio.a
938d9cfbd4ad8ef5cab54a5485adb01e00141ab777a63c23eeb1f3801b65466e  lib/x86_64/libfilament.a
8900084f7901f938be163b48d1dbb44507d1e2fd4f88eeb57ba5ef981e378cdf  lib/x86_64/libvkshaders.a

The direct pip-package build then completed with exit status 0. Produced wheel:

0f02385e6f0bcc90f2ae54bfd02cc9b942a9c9ffa35918f254494b59383a9167  open3d_cpu-0.19.0+1a9eb99-cp312-cp312-manylinux_2_43_x86_64.whl

Separately recorded host prerequisites

These were required only after the graph defect was corrected and are not part of the candidate fix:

• Filament v1.54.0 with Clang 21 needs <cstddef> in libs/utils/include/utils/memalign.h plus compatibility warning flags.
• This Ubuntu/GCC 15 host needs a temporary -include cstdint workaround for unrelated Assimp/tinyusdz missing includes.
• WITH_STUBGEN=OFF was used because pybind11_stubgen is not installed.

The original missing-rule failure occurs before any of those compilation prerequisites matter.

Result

Confirmed on Linux x86_64. The declared-byproduct directory is lib/, while both Open3D imports and Filament installation use lib/x86_64/. Selecting the architecture-specific directory for the external project's BUILD_BYPRODUCTS fixes the Ninja graph and the full target succeeds. This report does not claim an untested architecture.