Difference between image_id and id in annotation key of coco annotation format

[AbuBakrCh]

I am making annotation file for my own dataset. I am unable to understand what can be id in annotation key for my dataset?

[MonkeyWithAComputer]

I have the same question, have you figured it out?
annotation{
"id" : int, <---------------------what is this?
"image_id" : int,
"category_id" : int,
"segmentation" : RLE or [polygon],
"area" : float,
"bbox" : [x,y,width,height],
"iscrowd" : 0 or 1,
}

[saiabinesh]

Having the same doubt? Is that an annotation ID? Meaning does each annotation have it's own ID?

[cpoptic]

I emailed info@cocodatset.org this exact question, but got no reply.

I found an article on creating your own COCO-style dataset and it appears the "id" is to uniquely identify each annotation. The "image_id", makes sense, but a unique id for each annotations seems overkill.
Basically you could just loop through every annotation in your dataset, assign an "id" counter to that annotation, and increment it for the next iteration.

http://www.immersivelimit.com/tutorials/create-coco-annotations-from-scratch

[Ostyk]

Yup, it truly was is an overkill to assign a global unique ID to each annotation but that's the way it has to be according to the API.
It helped me after quite a bit of time trying to figure why I had so few annotations..

in the pythonAPI it's here in line 95 of coco.py

if 'annotations' in self.dataset:
            for ann in self.dataset['annotations']:
                imgToAnns[ann['image_id']].append(ann)
                anns[ann['id']] = ann

[cao13jf]

Finally, I figure out the way to create a dataset according to the COCO format. All instance labels should be unique across all images. If you have a list of annotation matrix, e.g, [A_1, A_2, ..., A_n], the anns can be constructed by following these pseudo codes:

save_id = 1
for A in [A_1, A_2, ..., A_n]:
    for label in label_list_of_A:
        label_mask = A==save_id
        #  use pycocotools to get the annotations from label_mask
        #  ......
        save_id += 1