This python script automatically counts pixels of certain colours from all photos in a folder and calculates leaf area and leaf damage. I used it for my research and hope it will be helpful for others too.
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Take photos of leaves and place a calibration object of known size in the image (e.g. a black 1 x 1 cm square, see example photos)
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Install the python library "opencv"
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Paint your leaf photos using EXACTLY the following colours (R, G, B):
Calibration objects: Magenta (255, 000, 255)
Healthy leaf area: Blue (000, 000, 255)
Damaged area: Red (255, 000, 000)
(The "fuzzy select"/magic wand tool of GIMP is super helpful for selecting regions of similar colour.)
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Save images in PNG file format
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Download the script "folderleaf.py"
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Edit the script's "settings" section and set the size of your calibration object
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Run the script in the folder containing the photos
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It will create a CSV file with the results called "leaf_damage.csv"
Thanks to LucxMangaJet for creating the first version!