square_crop.py

#!/usr/bin/env python3
"""
square_crop.py

Crop each COCO bounding box to the smallest square that contains it.

"""
import argparse
import json
from pathlib import Path
import os

from PIL import Image, ImageOps
from tqdm import tqdm

def load_coco(json_path):
    with open(json_path, "r") as f:
        coco = json.load(f)
    id2fname = {img["id"]: img["filename"] for img in coco["images"]}
    return id2fname, coco["annotations"]


def square_from_bbox(x1, y1, x2, y2, img_w, img_h):
    """
    Compute (left, top, side) of the smallest square fully containing the bbox.
    The square is centred on the bbox; if it overflows the image, it is shifted
    (but not resized) so it lies inside the image.  Returns the final crop box
    (left, top, side).
    """
    side = max(x2-x1, y2-y1)
    cx, cy = (x1+x2) / 2.0, (y1+y2) / 2.0
    left = int(round(cx - side / 2.0))
    top = int(round(cy - side / 2.0))

    # Shift the square so it fits inside the image
    left = max(0, min(left, img_w - side))
    top = max(0, min(top, img_h - side))
    return left, top, int(side)


def crop_annotation(img_path, ann, out_dir, pad_color=0):
    with Image.open(img_path) as img:
        img_w, img_h = img.size
        x1, y1, x2, y2 = ann["bbox"]
        left, top, side = square_from_bbox(x1, y1, x2, y2, img_w, img_h)

        # Perform crop (may be smaller than 'side' at edges)
        crop = img.crop((left, top, left + side, top + side))

        print(f"final image {crop.size} from original image {img.size}")

        # If we lost pixels at the edge, pad back to full square
        if crop.size != (side, side):
            delta_w = side - crop.size[0]
            delta_h = side - crop.size[1]
            padding = (0, 0, delta_w, delta_h)  # (left, top, right, bottom)
            crop = ImageOps.expand(crop, padding, fill=pad_color)

        # resize smaller so it takes less time to load
        crop = crop.resize((640,640))
        
        # Build output filename: <stem>_ann<id>.ext
        stem = Path(img_path).stem
        suffix = Path(img_path).suffix
        out_name = f"{stem}_ann{ann['id']}{suffix}"
        crop.save(os.path.join(out_dir, out_name))


def run_square_crop(input_dir, coco_json_path, cropped_dir):
    id2fname, annotations = load_coco(coco_json_path)

    # Group annotations by image for efficient loading
    im2anns = {}
    for ann in annotations:
        im2anns.setdefault(ann["image_id"], []).append(ann)

    for img_id, anns in tqdm(im2anns.items(), desc="Processing images"):
        img_path = os.path.join(input_dir, id2fname[img_id])

        for ann in anns:
            # try:
            crop_annotation(img_path, ann, cropped_dir)
            # except Exception as e:
            #     print(f"Error on {img_path}.")

    return [os.path.join(cropped_dir, fname) for fname in sorted(os.listdir(cropped_dir))]