OpenEXR Spectral Image

This repository contains the supporting code to read and write OpenEXR spectral images.

Our article An OpenEXR Layout for Spectral Images published in the Journal of Computer Graphics Techniques provides a description of the layout.

You may also find sample spectral images useful: ZIP Archive.

To load OpenEXR spectral image, you can use Spectral Viewer.

Content

• lib contains the main C++ code.
• app contains sample C++ applications using the provided C++ library.
• python contains a Python example to load spectral OpenEXR files using OpenImageIO.

Compilation

To compile this code, you need a C++11 compliant compiler, the OpenEXR library installed on your system and CMake.

mkdir build
cd build
cmake ..
make

This will compile an example program.

Sample programs

You can find in app several sample programs using spectral OpenEXR.

Macbeth

Macbeth executable (macbeth) generates a Macbeth colour chart using the spectral data from: http://www.babelcolor.com/colorchecker-2.htm. It will generate a file Macbeth.exr from the execution place.

./bin/macbeth

Fluo EXR

Fluo EXR executable (fluo-exr) generates a simple checker-board made of two fluorescent patches: 3M fluorescent yellow Post-It (R) sticker and 3M fluorescent pink Post-It (R) sticker. Data courtesy of Labsphere Inc. It will generate a file BiSpectral.exr

./bin/fluo-exr

Export spectrum

Export spectrum executable (export-spectrum) extracts from the given pixel location the stored spectrum in an ASCII file. Each columns correspond to a polarisation component if present in the image. S_0, S_1, S_2, S_3 for emissive images and T for reflective images. First column is the wavelength in manometers. Each column is separated by a space.

It takes as arguments:

• A spectral EXR
• The x coordinate of the pixel to extract
• The y coordinate of the pixel to extract
• The output file

./bin/export-spectrum Macbeth.exr 15 15 Macbeth.txt

You can plot the spectrum using gnuplot

plot "Macbeth.txt" u 1:2 w line

Export reradiation

Export reradiation executable (export-reradiation) extracts from the give pixel location the stored reradiation matrix in a ASCII file.

It takes as arguments:

• A bi-spectral EXR
• The x coordinate of the pixel to extract
• The y coordinate of the pixel to extract
• The output file

./bin/export-spectrum BiSpectral.exr 15 15 reradiation.txt

You can plot the reradiation matrix using gnuplot

plot "reradiation.txt" matrix w image

Merge EXR

Merge EXR executable (merge-exr) creates an emissive spectral EXR from a folder containing collection of monochromatic EXRs. You can use the Cornell box data as input http://www.graphics.cornell.edu/online/box/data.html.

It takes as arguments:

• Folder path containing the images
• Starting wavelength (in manometers)
• Increment of wavelength between images (in manometers)
• Output file
• Optional additional arguments:
  • Camera response in CSV format (comma separated)
  • Lens transmission in CSV format (comma separated)
  • Each filter corresponding to each channel transmissions in CSV format (comma separated)

To convert the Matlab matrices of the Cornell data in CSV format, we use the following GNU Octave code:

csvwrite("F1.csv", [wavelen F1])
csvwrite("F2.csv", [wavelen F2])
csvwrite("F3.csv", [wavelen F3])
csvwrite("F4.csv", [wavelen F4])
csvwrite("F5.csv", [wavelen F5])
csvwrite("F6.csv", [wavelen F6])
csvwrite("F7.csv", [wavelen F7])

load lens.mat
csvwrite("lens.csv", [wavelen lens])

load camera.mat
csvwrite("camera.csv", [wavelen_cam.' response.'])

Then, we execute the program as follows:

./bin/merge-exr \
    data/cornell 400 50 output/CornellBox.exr \
    data/cornell/camera.csv \
    data/cornell/lens.csv \
    data/cornell/F1.csv \
    data/cornell/F2.csv \
    data/cornell/F3.csv \
    data/cornell/F4.csv \
    data/cornell/F5.csv \
    data/cornell/F6.csv \
    data/cornell/F7.csv

Spectrum to EXR

Spectrum to EXR executable (spectrum-to-exr) creates a 1x1px spectral OpenEXR from a given spectrum.

It takes as arguments:

• A spectrum in CSV format (comma separated)
• Type of spectrum (either reflective or emissive)
• Output file

For example:

./bin/spectrum-to-exr data/D65.csv emissive output/D65.exr

Split channels

Split channels executable (split-channels) splits a spectral EXR is separate single monochromatic EXR files.

It takes as arguments:

• A spectral EXR file
• An output folder

For example:

./bin/split-channels Macbeth.exr output