COCO Dataset Export filename mismatch if frame_step != 1

[justinkay]

When using Export as a dataset -> MS COCO, if a task has a frame_step not equal to 1, image filenames and annotations do not match up. In the exported file instances_default.json:

• "images": "id" is incremented by 1, matching filenames (e.g. 0.jpg has id=0, 1.jpg has id=1, etc)
• "annotations": "image_id" for each annotation is incremented by frame_step (e.g. for a task with frame_step=5, annotations for 0.jpg have image_id=0, but annotations for 1.jpg have image_id=5)

I notice in cvat/apps/annotation/annotation.py, the _export_... methods all follow this logic. E.g.

def _export_labeled_shape(self, shape):
        return Annotation.LabeledShape(
            type=shape["type"],
            label=self._get_label_name(shape["label_id"]),
            ## here: ##
            frame=self._db_task.start_frame + shape["frame"] * self._frame_step,
            ##########
            points=shape["points"],
            occluded=shape["occluded"],
            z_order=shape.get("z_order", 0),
            group=shape.get("group", 0),
            attributes=self._export_attributes(shape["attributes"]),
        )

I'm not familiar enough with the codebase to know how essential this logic is. If it used elsewhere, the easiest method may be to change the exported filenames to increment by frame_step as well (though, it would not match how CVAT stores the files internally)..? E.g. 0.jpg, frame_step.jpg, 2*frame_step.jpg....

Attached is an exported dataset of an example task with frame_step=5

task_yft_test_coco.zip