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OpenCL Interferometry Library (liboi)


The OpenCL Interferometry Library (liboi) is a C / C++ library that aims to provide software developers with access to routines that are commonly used in interferometry. The software heavily relies on the heterogeneous computing environment targeted by the Open Compute Language (OpenCL) to target a wide range of traditional and multi-core CPUs; servers, hand-held/embedded devices, specialized hardware, and Graphical Processing Units (GPUs).


The library currently provides:

  • OpenGL / OpenCL Interop (copy OpenGL image to OpenCL buffers)
  • Image to Fourier transform via. a discrete Fourier Transform
  • Fourier transform to interferometric data (visibility squared, bispectra)
  • Image data to chi, chi squared, and log(likelihood).

Installing prerequisites

Apple / OS X

To use liboi on OS X, the following are required:

  • OpenCL 1.1 support (OS X 10.7 or higher)
  • gcc 4.7.4 or later †
  • cmake 2.8 or higher
  • cfitsio and ccfits

The OS can be installed/upgraded through the Appstore. Several of the additional required libraries can be installed through MacPorts:

sudo port install gcc47
sudo port install cmake
sudo port install cfitsio
sudo port install git

Please note that Apple may have installed a fake version of gcc located at /opt/local/bin/gcc that is a wrapper for clang. Besure to specify that you want to use the macport-installed compiler using

export CC=/opt/local/bin/gcc-mp-4.7
export CXX=/opt/local/bin/g++-mp-4.7

before running and ./configure or cmake commands!

Next you will need to download and install ccfits. You can do this using commands similar to the following

tar xvzf CCfits-2.4.tar.gz
cd CCfits
./configure --prefix=/opt/local --with-cfitsio=/opt/local
sudo make install

After this follow the installation instructions below.

† When we last attempted to compile liboi on a Mac the machine had Xcode 4.6 (which included apple clang 4.2 which is based on llvm-clang 3.2svn) that did not have full C++11 support. We do not have access to an Apple system for development, thus we cannot try compiling on a more recent system. If you wish to help get liboi running on a Mac please contact us!


To use liboi on a Debian / Ubuntu system, the following are should be installed:

  • gcc and g++ v4.6.3 (or later)
  • cmake v2.8
  • cfitsio, ccfits
  • An OpenGL library (optional, enables OpenCL-OpenGL interoperability)
  • An OpenCL 1.1 compliant device and library

Most of these packages can be easily installed through apt-get. First the compiler, cmake, cfitsio, and ccfits:

sudo apt-get install build-essential g++ cmake libccfits0 libccfits-dev git

To enable OpenCL-OpenGL interoperability you should also install an OpenGL library. This should install the prerequisites:

sudo apt-get install libglu1-mesa libglu1-mesa-dev

Lastly install the OpenCL Installable Client Driver (ICD) loader:

sudo apt-get install opencl-headers ocl-icd-libopencl1


On systems that do not have NVIDIA unified virtual memory (e.g. Ubuntu 13.10 and earlier) you only need the display drivers (e.g. one of nvidia-current, nvidia-304, or nvidia-331), OpenCL headers, and an OpenCL ICD loader:

sudo apt-get install nvidia-319 opencl-headers nvidia-opencl-dev

On later Ubuntu systems you will need to add the UVM package. You may also need to install special modprobe rules for the NVIDIA UVM drivers. These can be found in the following packages:

sudo apt-get install nvidia-331 nvidia-331-uvm nvidia-modprobe nvidia-opencl-icd-331

If you prefer, you can install the drivers from NVIDIA instead.

If you receive an error involving clCreateFromGLTexture during the linking stage, edit the src/CMakeLists.txt and replace the OpenCL version testing section with the following to force OpenCL 1.1 compatability mode:

#    add_definitions(-DDETECTED_OPENCL_1_0)
#    add_definitions(-DDETECTED_OPENCL_1_2)    
#    add_definitions(-DDETECTED_OPENCL_2_0)
#    add_definitions(-DDETECTED_OPENCL_UNKNOWN_VERSION)     


sudo apt-get install fglrx opencl-headers

or if you so choose you can install the AMD graphic drivers, and then install the [OpenCL SDK] (

Intel CPUs and GPUs

For Intel CPUs you need to install the Intel OpenCL SDK

Unlike on Windows and Apple systems, Intel does not provide support for OpenCL on their GPUs with their display drivers. Instead an open source project called Beignet is filling the gap.

We have successfully compiled liboi and verified that it functions with Beignet, however doing so was not a straightforward process. See the document in this directory for further details.

Checkout / getting a copy of LibOI source code

After installing the aforementioned prerequisites, you simply need to checkout a copy of LibOI

git clone
cd liboi
git submodule update --init

If you wish to have the bleeding-edge development version of liboi (which often includes the latest features and bugfixes) checkout the development version:

git checkout develop

otherwise you can stay on the default master branch.

Building instructions

After you have obtained a copy of the source and initialized the submodules, simply

cd build
cmake ..

If you have installed a library in a non-standard location, please see the [Overriding library locations][] section below. If you have any errors in the compilation steps, please contact us. (If you are on a Apple machine be sure to set the export lines mentioned above!)

Overriding library locations

If you have installed a library in a non-standard location, it may be necessary to override the library installation location. The following environmental variables are checked by CMake when building:

CFITSIO_ROOT_DIR    - path to directory above cfitstio.h and libcfitsio.*
CCFITS_ROOT_DIR     - path to directory above CCfits/ (the folder)
OPENCL_ROOT_DIR     - path to directory containing OpenCL
                      that is OpenCL/cl.hpp (Apple) or CL/cl.hpp (everyone else)

These can be set by typing export VARIABLE=/path/to/directory before calling cmake in the building instructions above. CMake should indicate that the directory you specified is used, rather than the default search path on your computer.

Testing and benchmarking

After compiling liboi it is useful to test that liboi is functioning correcly on your hardware. For this purpose we have packaged the liboi_tests and liboi_benchmark program in the liboi/bin directory.

The liboi_tests program executes a series of unit tests and compares the result of liboi's calculations with analytic results. By default liboi_tests runs on the first OpenCL-compatabile GPU it can find. You can also have it run on an OpenCL-supporting CPU by specifying the -cpu option. See liboi_tests -h for more information.

When you execute liboi_testsyou should see something like this:

liboi/bin$ ./liboi_tests 
[==========] Running 34 tests from 12 test cases.
[----------] Global test environment set-up.
[----------] 14 tests from ChiTest
[ RUN      ] ChiTest.CPU_Chi_ZERO
[       OK ] ChiTest.CPU_Chi_ZERO (3 ms)
[----------] Global test environment tear-down
[==========] 34 tests from 12 test cases ran. (5987 ms total)
[  PASSED  ] 34 tests.

In the ideal situation all tests will pass, but frequently a few tests will fail. In partcular, the following tests often fail for the following reasons:

  • CRoutine_Sum_NVidia.CL_Sum_CPU_CHECK executes a parallel sum. Due to some optimizations specific to NVidia GPUs, this test seems to always fail on non-NVidia hardware. Thus if you have an Intel or AMD GPU, don't worry if this test fails.
  • CRoutine_DFT.CL_UniformDisk this test compares the real (indexed as .s[0]) and imaginary (indexed .s[1]) components of an analytical uniform disk Fourier transform with a discrete Fourier transform of an image. The analytic and DFT answers are required to match at a 3% or better level. Out of the 10 UV points tested, it is common for a few to fail, particularly on older hardware.
  • CRoutine_DFT.CL_DFT_LARGE_UNEVEN_N_UV_POINTS compares an analytical and DFT result when there are a large uneven number of UV points (10221 in total). This test often fails on older hardware where shared memory limitations occur.

The liboi_benchmark program accesses how fast your hardware can execute the following sequence:

  1. Copy a 128x128 image from RAM to OpenCL device memory
  2. Compute the DFT on a reference data set
  3. Compute a chi-squared
  4. Sum the chi-squared
  5. Copy the summed chi-squared back to the host.

By default this will execute on a GPU. You can, specify the -cpu option to run the benchmark program on your OpenCL-compatible CPU. You can also change the image size, image scale, and number of iterations used in the benchmark. See liboi_benchmark -h for this and other options.

liboi_benchmark will print some information about your hardware, print metadata about the data file being used, and then benchmark liboi. When you run liboi_benchmark you should see something like the following:

liboi/bin$ ./liboi_benchmark 
Running Benchmark with: 

Device information: 
Device Name: Tahiti


Data set information for: 
N Vis: 0
N V2 : 525
N T3 : 700
N UV : 529
Average JD: 2456253.90049
Building kernels.
Starting benchmark.
Iteration 0 Chi2: 1993.02527
Iteration 100 Chi2: 1993.02527
Iteration 200 Chi2: 1993.02527
Iteration 300 Chi2: 1993.02527
Iteration 400 Chi2: 1993.02527
Iteration 500 Chi2: 1993.02527
Iteration 600 Chi2: 1993.02527
Iteration 700 Chi2: 1993.02527
Iteration 800 Chi2: 1993.02527
Iteration 900 Chi2: 1993.02527
Benchmark Test completed!
1000 iterations in 6.33000 seconds. Throughput 157.97788 iterations/sec.

The important aspects to note are that (1) the chi-squared is constant as a function of iteration number, (2) the chi-squared is near the reference number above, and (2) your throughput. The performance of liboi is limited by the DFT which is linearly dependent on the product of the number of UV points and number of pixels. In terms of what you expect, here are some representative test values from liboi_benchmark on various hardware:

OpenCL device Iterations/sec Other information
NVIDIA GTX 970m 322 Using optirun / bumblebee
NVIDIA GeForce GTX 570 260
Intel i7-3520M (GPU) 210 Apple driver, HD Graphics 4000
AMD Radeon R9 280x 155
Intel i7-4770K (GPU) 135 Beignet, HD Graphics 4600
NVIDIA GeForce 8600m GT 60
NVIDIA GeForce 8400 GS 50
i7-4770K (CPU) 5 Running on 4 physical cores.

All tests were performed on various Linux distributions using manufacturer drivers unless otherwise noted. Any additional benchmarks would be appreciated.

Licensing and Acknowledgements

LibOI is free software, distributed under the [GNU Lesser General Public License (Version 3)](<

If you use this software as part of a scientific publication, please cite as:

Kloppenborg, B.; Baron, F. (2012), "LibOI: The OpenCL Interferometry Library" (Version X). Available from

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