Splitting OpenEXR into different exposure images - image-processing

Question

I'm trying to use this dataset to do Exposure Meging (Fusion) in Python. Each image in the dataset has an OpenEXR file that can be downloaded (i don't have much experience with this file format).
I want to extract different samples (jpg or png) from the OpenEXR file with different exposures .
I managed to do that in Darktable :
Open the OpenEXR file (image)
Change the Exposure
Save as jpg
redo for each exposure value (-3EV, -2EV, -1EV, 0EV, 1EV, 2EV, 3EV).
The problem : I have 100 images and i want to automate this process. any idea on how to do that ?
Thank you in advance

Answer 1

Since each increment of EV ("Exposure Value") corresponds to doubling the exposure, and EXR files are in linear light (not gamma-encoded), you would expect that you can double the pixel values in an EXR file to add 1EV and halve them to do -1EV...
So, I downloaded the Luxo EXR file from here. Then I went into Photoshop and clicked:
Image -> Mode -> 8-bits/channel
and selected Method = Exposure and Gamma and set exposure=+1 and saved the resulting file as a JPEG with +1 in its name. I repeated that for EV-3, EV-2, EV+0, EV+1, EV+2, EV+3.
I then looked at the resulting files with ImageMagick using commands like the following in the Terminal to examine the mean value of the combined RGB image:
magick identify -verbose image-EV+2.jpg
I then went about producing those same mean values, and found that the following works:
# To increase 1 EV
magick input.exr -evaluate multiply 2 result.jpg
# To increase 2 EV
magick input.exr -evaluate multiply 4 result.jpg
# To increase 3 EV
magick input.exr -evaluate multiply 8 result.jpg
And so on...
So, I wrote a bash script to do that as follows, which you could save in your HOME directory as adjust.sh:
#!/bin/bash
# Default file, if none specified
file=${1:-/Users/mark/Desktop/LuxoDoubleChecker.exr}
# Default EV of +1, if none specified
EV=${2:-1}
# Strip extension
base="${file%.*}"
# Apply given EV to file and save with new name
new="${base}EV${EV}.jpg"
echo "Applying EV $EV to $file, saving as $new"
magick "$file" -evaluate multiply $(bc -l <<< "2^$EV") "$new"
Then, just necessary once, make it executable:
chmod +x $HOME/adjust.sh
And then you run it like this to add +3EV to SomeImage.exr:
~/adjust.sh SomeImage.exr 3
Sample Output
Applying EV 3 to SomeImage.exr, saving as SomeImageEV3.jpg
Alternatively, if you save this script as allEVs.sh, it will load the specified image just once and generate all 7 exposures in one go without re-reading the input EXR file 7 times:
#!/bin/bash
# Default file, if none specified
file=${1:-/Users/mark/Desktop/LuxoDoubleChecker.exr}
# Strip extension to get base without extension
base="${file%.*}"
magick "$file" \
\( +clone -evaluate multiply 0.125 -write "${base}EV-3.jpg" +delete \) \
\( +clone -evaluate multiply 0.25 -write "${base}EV-2.jpg" +delete \) \
\( +clone -evaluate multiply 0.5 -write "${base}EV-1.jpg" +delete \) \
\( +clone -evaluate multiply 1 -write "${base}EV-0.jpg" +delete \) \
\( +clone -evaluate multiply 2 -write "${base}EV+1.jpg" +delete \) \
\( +clone -evaluate multiply 4 -write "${base}EV+2.jpg" +delete \) \
-evaluate multiply 8 "${base}EV+3.jpg"
Please check carefully that this works correctly for you before basing a lifetime's analysis on it...
Keywords: Image processing, HDR, High Dynamic Range, EXR, EV, Exposure Value, f-stop, stop, stops, exposure, increase, decrease, tone map, ImageMagick.