[LAB 03 · 空间信息：表示、采样与掩码](/course/notebooks/49a2c030eafa4e1db6fc4f1a08c3810a.html)
# 3. 实例掩码 · 实现代码
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同源场景标签、插值、信息碰撞的轻量解析代码
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```
# 本册独立素材准备：内嵌预览与精确原图坐标，不下载外站，也不依赖其他 notebook 的变量。
from pathlib import Path
import base64, json
from IPython.display import SVG, display
scene_uri = 'https://codingai-lec03.pages.dev/course/knowledge-assets/46ffa1170be676ea38dfc1ee.jpg'
annotation = {'version': 'L03-r2-20260922', 'image': 'tabletop-occlusion.png', 'size': [1536, 1024], 'label': '手工教学标注：可见轮廓，非模型输出或数据集真值', 'objects': [{'id': 'cup-red', 'category': 'cup', 'polygon': [[280, 270], [295, 251], [362, 241], [448, 242], [527, 253], [565, 270], [562, 361], [554, 454], [542, 532], [526, 565], [491, 579], [432, 586], [369, 582], [329, 568], [307, 541], [297, 516], [258, 497], [224, 471], [194, 433], [179, 392], [178, 356], [190, 326], [210, 308], [242, 303], [278, 314]], 'holes': [[[280, 351], [251, 328], [227, 328], [209, 343], [200, 368], [204, 400], [219, 432], [244, 459], [276, 480], [290, 481]]], 'box': [177, 238, 390, 351]}, {'id': 'cup-blue', 'category': 'cup', 'polygon': [[642, 266], [663, 248], [732, 238], [814, 238], [888, 249], [925, 267], [925, 306], [954, 296], [985, 296], [1009, 310], [1022, 334], [1025, 366], [1018, 396], [1010, 418], [934, 435], [850, 450], [746, 466], [659, 482], [650, 414], [644, 341]], 'holes': [[[925, 348], [944, 323], [965, 318], [986, 326], [1000, 347], [1002, 373], [993, 404], [984, 424], [919, 438]]], 'box': [641, 235, 386, 249]}, {'id': 'book', 'category': 'book', 'polygon': [[602, 500], [626, 490], [1051, 412], [1450, 548], [1451, 560], [1438, 576], [1438, 633], [1454, 648], [918, 770], [602, 586], [597, 560], [598, 516]], 'holes': [], 'box': [596, 411, 860, 362]}]}
asset_dir = Path.cwd() / 'lecture03-assets-r2'
asset_dir.mkdir(exist_ok=True)
preview_file = asset_dir / 'tabletop-occlusion-preview.jpg'
if not preview_file.exists():
preview_file.write_bytes(base64.b64decode(scene_uri.split(',',1)[1]))
(asset_dir / 'scene-annotations.json').write_text(json.dumps(annotation,ensure_ascii=False,indent=2))
print('可用材料:', preview_file.name, 'scene-annotations.json')
print('标注坐标系:', annotation['size'], '；图片预览768×512，展示时同比例映射。')
# 修改这一项为 cup-red、cup-blue 或 book，预测红框和局部窗口怎样改变。
selected_id = 'cup-blue'
obj = next(o for o in annotation['objects'] if o['id']==selected_id)
x,y,w,h = obj['box']
print('所选对象:',selected_id,'类别:',obj['category'],'框(x,y,w,h):',[x,y,w,h])
svg = f''
display(SVG(svg))
# 将所选框显示在归一化的局部窗口；不把此教学裁剪称为 RoIAlign。
local_svg = f''
display(SVG(local_svg))
```