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# iactsim: Imaging Atmospheric Cherenkov Telescope Simulation

**GPU-accelerated instrument response simulation for IACTs.**

[**Documentation**](https://iact-sim-49dba6.gitlab.io/) | [**Source Code**](https://gitlab.com/davide.mollica/iactsim) | [**Report Bug**](https://gitlab.com/davide.mollica/iactsim/-/issues/new?type=ISSUE&initialCreationContext=list-route)

## Overview

iactsim is a high-performance Python package designed for simulating the response 
of Imaging Atmospheric Cherenkov Telescopes (IACTs). By exploiting the parallel
computational power of GPUs (via CUDA and CuPy), it accelerates computationally 
intensive tasks such as ray-tracing and SiPM response simulation.

## Package structure

The framework is organized into four main modules:

* **optics**: handles optical system, non-sequential ray-tracing and photon sources. 
* **electronics**: manages pixel signal generation, trigger logic and sampling logic.
                   Currently only SiPM signal generation is supported, including prompt cross-talk,
                   after-pulse and micro-cell recovery time
* **visualization**: provides tools for 3D geometry/ray-tracing rendering (via VTK)
                     and helper matplotlib functions for data plotting
* **iactxx**: a general-purpose C++ extension module, currently used for high-performance parallel reading, decompression and conversion of files written with the IACT extension of CORSIKA7

## **Key Features**

* **GPU acceleration:** fast ray-tracing and camera response simulation using NVIDIA HPC SDK and CuPy. 
* **Optics:**  
  * **non-sequential ray-tracing** algorithm supporting multiple reflections (e.g., protective windows, multi-layer coatings, SiPM reflectance);
  * currently supports **spherical, aspherical, flat and cylindrical surfaces**; 
  * flexible geometry modeling: supports **surface segmentation** where each segment is an independent surface, allowing for unique profiles (e.g., curvature variations) and customizable alignment errors (tilts/shifts);
  * run-time configuration of each surface.
* **Electronics:**  
  * detailed **SiPM response simulation** (prompt cross-talk, after-pulse and microcell recovery time);
  * run-time configuration of pixel properties on a per-pixel basis;
  * trigger and sampling logic customization.
* **Visualization:**  
  * built-in plotting capabilities for the detailed analysis of ray-tracing results and camera electronic signals;
  * 3D interactive visualization of optical systems and photon paths using VTK.
* **Python-based:** seamless integration with the scientific Python stack (NumPy, SciPy, Numba, Matplotlib, Jupyter, etc.).

## Table of contents

* [Installation](#installation)
* [Runtime configuration](#runtime-configuration)
* [Usage](#usage)
  * [Building an optical system](#building-an-optical-system)
  * [Mirror segmentation](#mirror-segmentation)
  * [3D visualization](#3d-visualization)
* [For developers](#for-developers)

## Installation

### Prerequisites

* **Python:** \>= 3.11  
* **Compiler:** gcc or nvcc 
* **CMake:** \>= 3.15  
* **Runtime:** NVIDIA Drivers, CUDA, CuPy

### Environment setup

We strongly recommend using a virtual environment (e.g., mamba or conda):
```bash
mamba create -n simenv python=3.13  
mamba activate simenv
```
### Install via PyPI

If you have the prerequisites installed, you can install the latest release directly:
```bash
pip install iactsim -v
```
#### Optional: using NVHPC compilers

Building with gcc is the standard and requires no special configuration. However, 
if you specifically wish to use the nvcc, you must configure the environment **before** installation.

Please refer to the **NVIDIA HPC SDK** section in [Runtime Configuration](#runtime-configuration) 
for detailed instructions on setting up the environment.

### Custom compiler flags

You can customize the build options by passing arguments to CMake via pip. 
The following flags are available:

* **CMAKE\_CXX\_COMPILER=\<compiler\>**: specify the C++ compiler executable (e.g., nvcc, g++).  
* **USE\_ZLIBNG=\<ON|OFF\>**: enable/disable zlib-ng support. Default is ON.  
  For the C++ part, by default [zlib-ng](https://github.com/zlib-ng/zlib-ng) (*zlib data compression library for the next generation systems*)
  will be used (CMake will clone the repo automatically). **This is strongly recommended if you plan to read compressed CORSIKA files.**  
  If you have to use the standard system [zlib](https://zlib.net/), you can set this to OFF.

#### Example: explicitly use nvcc and disable zlib-ng  
```bash
python -m pip install iactsim -v -C cmake.args="-DCMAKE_CXX_COMPILER=nvcc;-DUSE_ZLIBNG=OFF"
```

### Install from source

1. Clone the repository:
   ```bash
   git clone https://gitlab.com/davide.mollica/iactsim.git
   cd iactsim
   ```

2. Editable install:  
   First, install the build dependencies required for the editable install:
   ```bash
   pip install scikit-build-core pybind11 "setuptools_scm[toml]>=8.0" cmake ninja
   ```
   Then, install the package in editable mode. This configuration disables build isolation, ensuring that **only modified C++ files will be recompiled**:
   ```bash  
   python -m pip install --no-build-isolation -e .
   ```

## Runtime configuration

### Install CuPy (required)

iactsim relies on CuPy for GPU offloading. You must install the cupy package that matches your specific CUDA version.
```bash
pip install cupy-cuda<XXX>
```
Replace \<XXX\> with your CUDA version (e.g., cupy-cuda12x for CUDA 12).  
For detailed instructions, refer to the [CuPy documentation](https://docs.cupy.dev/en/stable/install.html).


### Configure CUDA environment

To ensure the custom kernels compile successfully, NVCC and CUDA_PATH must be set correctly. We provide helper scripts to automate this for conda/mamba environments.

#### Case 1: NVIDIA HPC SDK

If you are using the NVIDIA HPC SDK to provide CUDA libraries,
you **must** [configure the enviroment](https://docs.nvidia.com/hpc-sdk//hpc-sdk-install-guide/index.html#end-user-environment-settings) so that CuPy can locate the necessary libraries
(you can download HPC SDK from the [NVIDIA website](https://developer.nvidia.com/hpc-sdk)).

We suggest to use [Environment Modules](https://modules.readthedocs.io/en/latest/) to handle SDK configuration and then define `NVCC` and `CUDA_PATH` enviromental variables:
```bash
module load nvhpc
export NVCC=$NVHPC_ROOT/compilers/bin/nvcc
export CUDA_PATH=$NVHPC_ROOT/cuda
```
With conda/mamba enviroments you can use the provided configuration script `configure_conda_env_hpcsdk`
```bash
mamba activate simenv
configure_conda_env_hpcsdk simenv
mamba deactivate
```
This adds an activation script and a deactivation script to the `simenv` enviroment that will automatically handle the configuration when it is activated or deactivated.

#### Case 2: CUDA toolkit (conda-forge/nvidia)
If you prefer a self-contained environment without external modules, you can install the CUDA Toolkit via mamba:
```bash
mamba install -c nvidia cuda-toolkit
```
and then configure the environment variables `NVCC` and `CUDA_PATH`. You can use the provided configuration script `configure_conda_env_cudatoolkit`:
```bash
mamba activate simenv
configure_conda_env_cudatoolkit simenv
mamba deactivate
```
## Usage

### Building an optical system
The following code provides an example of how to build a simple optical system with a spherical mirror and a flat focal surface.

```python
import matplotlib.pyplot as plt
import iactsim
from iactsim.optics import (
    ApertureShape,
    SurfaceType,
    OpticalSystem,
)

# Use iactsim matplotlib style
plt.style.use('iactsim.iactsim')

# Spherical mirror
mirror_curvature_radius = 20000
plate_scale = mirror_curvature_radius/57.296/2.
mirror = iactsim.optics.SphericalSurface(
    half_aperture=10000., 
    curvature=1./mirror_curvature_radius,
    position=(0,0,0),
    # reflective in the pointing direction
    surface_type=SurfaceType.REFLECTIVE_FRONT,
    name = 'Mirror'
)

# Flat focal surface (5deg hexagon)
focal_plane = iactsim.optics.FlatSurface(
    half_aperture = 5*plate_scale, 
    position = (0,0,0.5*mirror_curvature_radius),
    aperture_shape = ApertureShape.HEXAGONAL,
    # sensitive surface opposite to the pointing direction
    surface_type=SurfaceType.SENSITIVE_BACK,
    name = 'Focal Plane'
)

# Optical system
os = OpticalSystem(
    surfaces=[focal_plane, mirror],
    name='TEST-OS'
)

# Telescope position
pos = (0,0,0)

# Telescope pointing (alt,az)
poi = (0.,0.)

# IACT
telescope = iactsim.IACT(
    os,
    position=pos,
    pointing=poi
)

# Copy optical system data to the device
telescope.cuda_init()

# Photon source initialized on-axis 
source = iactsim.optics.sources.Source(telescope)
source.positions.random = False # To make spot diagrams

# Plot spot diagram at different off-axis angles
n_plots = 5
fig, axes = plt.subplots(1,n_plots,figsize=(3.5*n_plots,3.5))

source.directions.altitude = telescope.altitude - 2.
for ax in axes:
    # Adjust photon position to match the mirror position
    source.set_target('Mirror')
    
    # Generate photons
    ps, vs, wls, ts = source.generate(10000)
    
    # Perform ray-tracing
    telescope.trace_photons(ps, vs, wls, ts)
    
    # Plot spot diagram
    iactsim.visualization.scatter(ps, s=0.2, ax=ax, color='black', alpha=0.5, scale=plate_scale)
    ax.set_xlabel('X (deg)')
    ax.set_ylabel('Y (deg)')
    ax.grid(ls='--')

    # Move the source
    source.directions.altitude += 1. # degree

plt.tight_layout()
plt.show()
```

<img src="https://gitlab.com/davide.mollica/iactsim/-/raw/dev/media/psf.png?ref_type=heads" alt="Alt text" width="90%">

### Mirror segmentation

The following code provides an example of how to segment a surface (`AsphericalSurface`, `SphericalSurface` or `FlatSurface`) 
starting from a mother surface (in this case `mirror`).
Note that each segment is an independent surface and does not need a mother surface, which is used here simply for convenience.

```python
import numpy as np

# List of segments
segments = []

# Segment ID
k = 0

# Segments on a 10X10 grid, 80 total
n = 10
segment_distance = 2*mirror.half_aperture / (n+3)

for i in range(n+3):
    for j in range(n+3):
        offset = [
            -mirror.half_aperture+segment_distance*i,
            -mirror.half_aperture+segment_distance*j
        ]
        r_segment = np.sqrt(offset[0]**2+offset[1]**2)
        
        # Do not create segments outside the original mirror aperture
        if r_segment > mirror.half_aperture-segment_distance*np.sqrt(2):
            continue

        # Ideal segment position
        segment_position = [
            offset[0],
            offset[1],
            mirror.sagitta(r_segment),
        ]
        
        # Create the surface
        segment = iactsim.optics.SphericalSurface(
            curvature=mirror.curvature,
            half_aperture=0.45*segment_distance, 
            position=segment_position,
            surface_type=mirror.type,
            name = f'Segment-{k}',
            aperture_shape=ApertureShape.SQUARE,
            # Big random dispersion for visualization purpose
            tilt_angles=np.random.normal(0,1,3),
            # Gaussian scattering
            scattering_dispersion=0.05
        )
        
        # Specify the segment offset
        # When a segment is created in this way:
        #  - it will be oriented with the same surface normal 
        #    of the mother surface at the specified offset 
        #  - `tilt_angles` attribute will define a deviation from this orientation.
        segment.offset = offset
        
        segments.append(segment)
        k += 1

# Optical system
segmented_os = iactsim.optics.OpticalSystem(
    surfaces=[focal_plane, *segments],
    name='SEGMENTED-TEST-OS'
)

# IACT
segmented_telescope = iactsim.IACT(segmented_os, position=pos, pointing=poi)
segmented_telescope.cuda_init()

# Plot spot diagram at different off-axis angles
n_plots = 5
fig, axes = plt.subplots(1,n_plots,figsize=(3.5*n_plots,3.5))

# Photon source initialized on-axis 
source = iactsim.optics.sources.Source(segmented_telescope)
source.positions.radial_uniformity = False
source.positions.random = False
source.positions.r_max = mirror.half_aperture*1.5

source.directions.altitude -= 2.
for ax in axes:
    # Adjust photon position to match the mirror position
    source.set_target()
    
    # Generate photons
    ps, vs, wls, ts = source.generate(10000)
    
    # Perform ray-tracing
    segmented_telescope.trace_photons(ps, vs, wls, ts)
    
    # Plot spot diagram
    iactsim.visualization.scatter(ps, s=0.2, ax=ax, color='black', alpha=0.5, scale=plate_scale)
    ax.set_xlabel('X (deg)')
    ax.set_ylabel('Y (deg)')
    ax.grid(ls='--')

    # Move the source
    source.directions.altitude += 1. # degree

plt.tight_layout()
plt.show()
```
<img src="https://gitlab.com/davide.mollica/iactsim/-/raw/dev/media/segmented_psf.png?ref_type=heads" alt="Alt text" width="90%">

### 3D visualization

Verify your geometry or visualize ray-tracing paths using the built-in VTK backend.

#### Interactive 3D view of the optical system  
```python
from iactsim.visualization import VTKOpticalSystem
renderer = VTKOpticalSystem(segmented_telescope.optical_system)  
renderer.start_render()
```

<div style="text-align: center;">
    <img src="https://gitlab.com/davide.mollica/iactsim/-/raw/dev/media/vtk_os.png?ref_type=heads" alt="Alt text" width="35%">
</div>


#### Visualize photon paths  
```python
segmented_telescope.visualize_ray_tracing(
    *source.generate(10000),
    map_wavelength_color=False,
    focal_point='FocalPlane',
    show_not_detected=False
)
```

<div style="text-align: center;">
    <img src="https://gitlab.com/davide.mollica/iactsim/-/raw/dev/media/iactsim_visualize_raytracing.png?ref_type=heads" alt="Alt text" width="35%">
</div>

## **For developers**

* **Performance benchmarks:** [Current vs Main Branch](https://iact-sim-49dba6.gitlab.io/comparison_performance_graph.svg)  
* **Contributions:** Contributions are welcome\! Please open an issue before submitting a MR.