Custom build steps: PyVista on ARM64 example

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Tesseracts use by default an official Python docker image as the base image. Although this covers many useful cases, some system dependencies sometimes require a custom image and extra build steps.

Via tesseract_config.yaml, it is possible to somewhat flexibly alter the build process to accomodate different needs. As a concrete example, here’s what we had to do internally in order to to build an arm64 Tesseract with PyVista installed as a dependency:

build_config:
  base_image: "debian:trixie"

  target_platform: "linux/arm64"

  extra_packages:
   - python3-vtk9
   - python3-venv

  custom_build_steps:
    - |
      USER root
      # Symlink the VTK Python bindings into the Python site-packages directory
      RUN python_site=$(python -c "import site; print(site.getsitepackages()[0])") && \
          ln -s /usr/lib/python3/dist-packages/vtk* $python_site && \
          ls -l $python_site/vtk* && \
          python -c "import vtk"
      # Must install pyvista with --no-deps to avoid installing vtk (which we copied from the system)
      RUN pip install matplotlib numpy pillow pooch scooby && pip install --no-deps pyvista==0.44.1
      USER tesseractor

Here is what the various configurations set above do to the build process:

  • We started from a base image that had VTK 9.2 installed (debian:trixie), which was specified via base_image.
  • The target_platform is set to linux/arm64, which will build the resulting image for an ARM64 architecture.
  • We installed python3-vtk9 and python3-venv to get the VTK Python bindings and venv by specifying them in extra_packages. You can think of extra_packages as a list of packages which are apt-get installed right on the base image just immediately after some other dependencies (like git, ssh, and build-essential) are installed.
  • We can then run arbitrary commands on the image which is being built via custom_build_steps. This list of commands need to be specified as if they were in a Dockerfile. In particular, we start here by temporarily setting the user to root, as the default user in the Tesseract build process is tesseractor – which does not have root privileges – and then switch back to the tesseractor user at the very end. We then run commands directly on the shell via RUN commands. All these steps specified in custom_build_steps are executed at the very end of the build process, followed only by a last execution of tesseract-runtime check that checks that the runtime can be launched and the user-defined tesseract_api module can be imported.