BATSim

BATSim is designed to enable the application of non-affine shear transformations to GalSim images. It works by trasnforming the stamp pixel grid, distorting the location of the pixel centers. When a GalSim image is sampled onto this distorted grid, the resulting image will be sheared due to the transformation of the pixel grid.

Non-affine transforms can simualate complex shear effects such as intrinsic alignment, flexion, and optical field distrotion maps. Custom transform functions can also be passed to Stamp objects.

Building and installing BATSim from source (Conda + GalSim C++)

The package is not currently available on PyPI and only installable on Linux while in early development. We suggest using conda/mamba to build the package and install its dependencies. This is because the primary dependency, Galsim, does not ship a pre-built C++ library via pip.

The below instructions will work with pure conda, but we recommend using mamba to make the installation of dependencies much quicker.

First, make sure to clone and switch to the repository root:

git clone https://github.com/CMacM/BATSim.git
cd BATSim

Regular Install (not editable)

1. Create build environment and activate

mamba create -n batsim -c conda-forge -c defaults python=3.10 conda-build boa
mamba activate batsim

2. Build the package

From the repository root:

# mambabuild is sometimes only recognised as a conda command
conda mambabuild --override-channels -c conda-forge -c defaults conda/recipe

This will:

• Create an isolated environment to build and run batsim
• Install all required dependencies
• Compile the BATSim C++ extension and link it to Galsim's C++ API
• Produce a conda package in:

$CONDA_PREFIX/conda-bld/linux-64/

3. Install the built package

Install the locally built package into the new environment:

mamba install -c local batsim

If this fails, install directly from the build artifact:

mamba install $CONDA_PREFIX/conda-bld/linux-64/batsim-*.tar.bz2

Development Installation (Editable)

For development you can install BATSim in editable mode so that Python changes take effect immediately without reinstalling.

1. Create a development environment and activate

Note: You may encounter issues if some of these packages are already installed locally and have different builds. To fix, remove them and ensure they are installed through conda-forge.

mamba create -n batsim-dev -c conda-forge -c defaults \
    python=3.10 \
    galsim \
    eigen \
    pybind11 \
    numpy \
    fitsio \
    astropy \
    matplotlib
mamba activate batsim-dev

2. Install BATSim in editable mode

pip install -e .

After following either of the above installation routes, with the conda environment active, verify the installation:

python -c "import batsim; import batsim._gsinterface; print('BATSim installed successfully')"

Pip only installation

We currently do not provide a pip only installation route as Galsim requires the shared C++ library to be built and linked manually when installed via pip. If you wish to build via pip only, you will need to install all dependencies and build and link the Galsim C++ headers. You can find details on how to do this [here][https://galsim-developers.github.io/GalSim/_build/html/install_pip.html]. You may then need to update your LD_LIBRARY_PATH, LIBRARY_PATH, and CPLUS_INCLUDE_PATH to point to build and include folders for the GalSim C++ shared library.

We plan to add a pure pip installation route in future.

(repo icon created using images by Frepik and brgfx on Freepik)