Instance segmentation with fastai using json annotations

Hi all!

I would like to train an instance segmentation model with fastai. Training semantic segmentation models in fastai is really simple, thanks to the common idiom of storing both images and labels (masks) as images. As a matter of fact, if images and labels are stored this way, the SegmentationDataLoaders.from_label_func makes it very simple to read (image, label) pairs.

Unfortunately, my use case is quite different. I have an instance segmentation task, where the labels are stored as .json files, in the VGG VIA format. How can I create an appropriate DataBlock object? In order to fix ideas, here is a very simple dataset, containing two images in the images folder, and the corresponding json file with the annotations in the labels folder:

My goal is to read this dataset with an appropriate DataBlock object. Can you help me?

I also include an annotated example of a VIA project json file, so that you can more easily understand the syntax, if that helps.

  "_via_settings": {                # settings used by the VIA application
    "ui": {
      "annotation_editor_height": 25,
      "annotation_editor_fontsize": 0.8,
      "leftsidebar_width": 18,
      "image_grid": {
        "img_height": 80,
        "rshape_fill": "none",
        "rshape_fill_opacity": 0.3,
        "rshape_stroke": "yellow",
        "rshape_stroke_width": 2,
        "show_region_shape": true,
        "show_image_policy": "all"
      "image": {
        "region_label": "__via_region_id__",
        "region_color": "__via_default_region_color__",
        "region_label_font": "10px Sans",
        "on_image_annotation_editor_placement": "NEAR_REGION"
    "core": {
      "buffer_size": 18,
      "filepath": {},
      "default_filepath": ""
    "project": {
      "name": "via_project_16Feb2021_13h17m"
  "_via_img_metadata": {              # stores information about all images and their associated metadata
    "adutta_swan.jpg-1": {            # each image is indexed using a unique key: FILENAME-FILESIZE
      "filename": "adutta_swan.jpg",  # image filename
      "size": -1,                     # file size in bytes (-1 indicates unknown)
      "regions": [                    # an array of all manually defined regions (only 1 region here)
          "shape_attributes": {       # shape of the first region
            "name": "rect",           # region shape: {rect, polygon, circle, ellipse, point, ...}
            "x": 108,                 # x-coordinate of the top-left point
            "y": 123,                 # y-coordinate of the top-left point
            "width": 283,             # width of rectangle
            "height": 150             # height of rectangle
          "region_attributes": {      # attributes (i.e. metadata) of the first region
            "name": "Swan"            # "name" is a region attribute and it has a value of "Swan"
      "file_attributes": {                # attributes associated with the full image
        "caption": "Swan in lake Geneve", # "caption" is a file attribute and it has a value of "Swan in ..."
    "wikimedia_death_of_socrates.jpg-1": {
      "filename": "wikimedia_death_of_socrates.jpg",
      "size": -1,
      "regions": [],                  # this image has no regions (so far)
      "file_attributes": {            # the "caption" file attribute for this image has a user defined value
        "caption": "The Death of Socrates by David",
  "_via_attributes": {                # attributes that can be attached to image and its regions
    "region": {                       # definition of region attributes (i.e. attributes belonging to an image region)
      "name": {                       # "name" is region attribute which defines the name of the object contained in that region
        "type": "text",               # attribute type can be {text, dropdown, radio, checkbox, ...}
        "description": "Name of the object",
        "default_value": "not_defined"
    "file": {                         # file attributes correspond to the full image (and not image region)
      "caption": {                    # "caption" is a file attribute
        "type": "text",
        "description": "",
        "default_value": ""
  "_via_data_format_version": "2.0.10",
  "_via_image_id_list": [             # this contains the list of image-id present in the "_via_img_metadata" dictionary

Let me know if there’s anything I could do, to help you to help me :slightly_smiling_face: thanks,