gimp-image-annotator or GIÀ, a lightweight GIMP plug-in to alllow for computer vision-assisted image annotation using the powerful GIMP selection toolbox.
Follow the guide here: https://en.wikibooks.org/wiki/GIMP/Installing_Plugins to find how to install GIMP plug-ins on your system, save the file image-annotator.py in GIMP's plug-in folder.
Updated for GIMP v3! In this version of GIMP, all plug-ins must be placed in a directory of their own with the same name as the plug-in in order to be parsed by the program. Therefore, the image-anntotaor.py file must be placed inside a folder named image-annotator inside the plug-in directory. On Linux, the plug-in may need to be made executable with the command chmod a+x /path/to/image-annotator.py in order to be seen by GIMP.
Once installed, navigate to Annotation then GIMP Image Annotator (GIÀ), add the labels you want, select one, use GIMP's selection tools (e.g. The Fuzzy Select tool - a guide can be found here: https://docs.gimp.org/en/gimp-tools-selection.html) to select an area (use Quick Mask or Shift+Q to quickly see the mask you have created). Make sure antialisaing and feathering is off, you cannot turn it off for rectangle select however it isn't used. Once you have your desired selected area, press Save mask. Repeat until all objects are annotated.
gimp-image-annotator saves a binary mask of each annotation, with class of mask stored in the _annotations.json file. The _annotations.json file is structured as followed
[{
"label": "label",
"id": "0",
"filename": "image.png"
}]The masks can be inputted using most image processing software. For example in opencv-python it would be:
import cv2
mask = cv2.imread(PATH, cv2.IMREAD_GRAYSCALE)