作业帮式错题标注：OCR 定位错误红框 + 解题思路。

- PaddleOCR 行级坐标 + AI 识别错答区域，生成标注图

- 解法拆分为「解题思路」与「详细解答」

- 详情页标注图/原图切换，列表显示标注缩略图

backend/app/services/annotation.py

@@ -0,0 +1,109 @@
+import json
+import re
+from pathlib import Path
+
+from PIL import Image, ImageDraw, ImageFont
+
+from app.core.config import settings
+
+
+def _parse_llm_json(text: str) -> dict | None:
+    text = text.strip()
+    match = re.search(r"\{[\s\S]*\}", text)
+    if not match:
+        return None
+    try:
+        return json.loads(match.group())
+    except json.JSONDecodeError:
+        return None
+
+
+def heuristic_wrong_line_ids(lines: list[dict]) -> list[int]:
+    wrong: list[int] = []
+    for i, line in enumerate(lines):
+        t = line.get("text", "")
+        if any(c in t for c in ("×", "✗", "❌", "错")):
+            wrong.append(i)
+            continue
+        if re.search(r"[×xX]\s*$", t.strip()):
+            wrong.append(i)
+    if wrong:
+        return wrong
+    # 单题照片：标注最后几行作答区域
+    if len(lines) == 1:
+        return [0]
+    if len(lines) <= 4:
+        return list(range(max(0, len(lines) - 2), len(lines)))
+    return list(range(len(lines) - 2, len(lines)))
+
+
+def parse_wrong_line_ids(llm_response: str, lines: list[dict]) -> list[int]:
+    data = _parse_llm_json(llm_response)
+    if data and isinstance(data.get("wrong_line_ids"), list):
+        ids = [int(x) for x in data["wrong_line_ids"] if isinstance(x, (int, float, str))]
+        ids = [i for i in ids if 0 <= i < len(lines)]
+        if ids:
+            return ids
+    return heuristic_wrong_line_ids(lines)
+
+
+def regions_from_lines(lines: list[dict], wrong_ids: list[int]) -> list[dict]:
+    regions = []
+    for i in wrong_ids:
+        if i >= len(lines):
+            continue
+        line = lines[i]
+        bbox = line.get("bbox") or [0, 0, 0, 0]
+        regions.append(
+            {
+                "line_id": i,
+                "text": line.get("text", ""),
+                "bbox": bbox,
+                "type": "wrong",
+                "label": "错",
+            }
+        )
+    return regions
+
+
+def draw_annotated_image(
+    src_path: str,
+    lines: list[dict],
+    wrong_ids: list[int],
+    dest_rel_path: str,
+) -> str:
+    img = Image.open(src_path).convert("RGBA")
+    overlay = Image.new("RGBA", img.size, (255, 255, 255, 0))
+    draw = ImageDraw.Draw(overlay)
+
+    try:
+        font = ImageFont.truetype("DejaVuSans-Bold.ttf", max(14, img.size[0] // 40))
+    except OSError:
+        font = ImageFont.load_default()
+
+    for i in wrong_ids:
+        if i >= len(lines):
+            continue
+        bbox = lines[i].get("bbox") or [0, 0, 0, 0]
+        x1, y1, x2, y2 = bbox
+        pad = 6
+        box = [x1 - pad, y1 - pad, x2 + pad, y2 + pad]
+        draw.rounded_rectangle(box, radius=4, fill=(255, 59, 48, 55), outline=(255, 59, 48, 220), width=3)
+        draw.text((x1, max(0, y1 - 18)), "×", fill=(255, 59, 48, 255), font=font)
+
+    combined = Image.alpha_composite(img, overlay).convert("RGB")
+    full_path = Path(settings.UPLOAD_DIR) / dest_rel_path
+    full_path.parent.mkdir(parents=True, exist_ok=True)
+    combined.save(full_path, format="JPEG", quality=92)
+    return dest_rel_path
+
+
+def split_solution_sections(text: str) -> tuple[str | None, str]:
+    if "## 解题思路" not in text:
+        return None, text
+    parts = re.split(r"\n##\s*", text, maxsplit=1)
+    if len(parts) < 2:
+        return None, text
+    approach = parts[0].replace("## 解题思路", "").strip()
+    rest = "## " + parts[1]
+    return approach or None, rest.strip()

backend/app/services/llm.py

@@ -1,5 +1,3 @@
-import enum
-
 import httpx
 from sqlalchemy.orm import Session
 
@@ -36,7 +34,7 @@ OCR 原文：
 {ocr_text}
 """
 
-SOLUTION_PROMPT = """你是一位耐心的{stage}{subject}老师。请为以下题目给出详细解法。
+SOLUTION_PROMPT = """你是一位耐心的{stage}{subject}老师。请像「作业帮」一样，先讲清楚解题思路，再给出完整解答。
 
 【学段要求 — 严禁超纲】
 {curriculum}
@@ -44,14 +42,31 @@ SOLUTION_PROMPT = """你是一位耐心的{stage}{subject}老师。请为以下
 题目：
 {question_text}
 
-请按以下结构输出：
-1. 考点分析（{stage}范围内）
-2. 解题步骤（逐步推导，每步说明依据）
-3. 易错点提醒
-4. 若必须使用超纲方法才能解，请改用{stage}可理解的方法重新解答，不得输出超纲解法。
+请严格按以下 Markdown 结构输出：
+
+## 解题思路
+（2-5 句话：这题考什么、从哪里入手、关键一步是什么，让学生先懂「怎么想」）
+
+## 详细解答
+（分步骤完整推导，每步说明依据）
+
+## 易错点
+（指出常见错误及正确做法）
+
+严禁使用超纲方法；若原题超纲，请给出{stage}课内可理解的解法。
 """
 
-OLYMPIAD_SOLUTION_PROMPT = """你是一位{stage}奥数教练。请为以下奥数题给出详细解题思路与完整解答。
+ERROR_DETECT_PROMPT = """你是{stage}{subject}老师。以下是试卷/作业 OCR 识别结果，每行前有编号。
+请找出「学生答错的部分」：错误答案、被打叉的作答、明显不正确的计算结果等。
+
+{numbered_lines}
+
+只输出 JSON，不要其他文字：
+{{"wrong_line_ids": [行编号整数列表]}}
+若整张图就是一道错题，请标注含有错误答案或作答的行；找不到则标注最后作答行。
+"""
+
+OLYMPIAD_SOLUTION_PROMPT = """你是一位{stage}奥数教练。请像优秀辅导老师一样，先讲解题思路，再完整解答。
 
 【奥数学段要求 — 严禁超纲】
 {curriculum}
@@ -59,11 +74,18 @@ OLYMPIAD_SOLUTION_PROMPT = """你是一位{stage}奥数教练。请为以下奥
 题目：
 {question_text}
 
-请按以下结构输出：
-1. 题型与思路切入点（{stage}奥数常见技巧）
-2. 详细解答步骤
-3. 关键技巧总结（仅限{stage}奥数范围）
-4. 严禁使用超出上述范围的方法；若题目过难，给出{stage}可接受的培优思路。
+请严格按以下 Markdown 结构输出：
+
+## 解题思路
+（点明题型、突破口、{stage}奥数常用技巧）
+
+## 详细解答
+（完整步骤）
+
+## 关键技巧
+（总结，仅限{stage}奥数范围）
+
+严禁超纲；过难题给出{stage}可接受的培优思路。
 """
 
 
@@ -167,3 +189,15 @@ async def generate_solution(
         question_text=question_text,
     )
     return await generate_text(prompt, cfg)
+
+
+async def detect_wrong_line_ids(
+    cfg: AIConfig,
+    subject: str,
+    ocr_lines: list[dict],
+    school_level=None,
+) -> str:
+    stage = school_level_label(school_level)
+    numbered = "\n".join(f"[{i}] {line.get('text', '')}" for i, line in enumerate(ocr_lines))
+    prompt = ERROR_DETECT_PROMPT.format(stage=stage, subject=subject, numbered_lines=numbered)
+    return await generate_text(prompt, cfg)

backend/app/services/migrate.py

@@ -65,3 +65,17 @@ def run_migrations() -> None:
             with engine.begin() as conn:
                 for clause in alters:
                     conn.execute(text(f"ALTER TABLE system_settings {clause}"))
+
+    if "wrong_questions" in tables:
+        wq_columns = {col["name"] for col in inspector.get_columns("wrong_questions")}
+        wq_alters: list[str] = []
+        if "solution_approach" not in wq_columns:
+            wq_alters.append("ADD COLUMN solution_approach TEXT")
+        if "mark_regions_json" not in wq_columns:
+            wq_alters.append("ADD COLUMN mark_regions_json TEXT")
+        if "annotated_image_path" not in wq_columns:
+            wq_alters.append("ADD COLUMN annotated_image_path VARCHAR(512)")
+        if wq_alters:
+            with engine.begin() as conn:
+                for clause in wq_alters:
+                    conn.execute(text(f"ALTER TABLE wrong_questions {clause}"))

backend/app/services/ocr.py

@@ -1,5 +1,7 @@
 from pathlib import Path
 
+from PIL import Image
+
 from app.core.config import settings
 
 _ocr_engine = None
@@ -14,18 +16,52 @@ def get_ocr_engine():
     return _ocr_engine
 
 
-def run_ocr(image_path: str) -> str:
+def _bbox_from_box(box: list) -> list[float]:
+    xs = [float(p[0]) for p in box]
+    ys = [float(p[1]) for p in box]
+    return [min(xs), min(ys), max(xs), max(ys)]
+
+
+def run_ocr_with_regions(image_path: str) -> dict:
+    """Return OCR text plus line-level bounding boxes for annotation."""
     engine = get_ocr_engine()
     result = engine.ocr(image_path, cls=True)
-    if not result or not result[0]:
-        return ""
-    lines = []
-    for line in result[0]:
-        if line and len(line) >= 2:
-            text = line[1][0]
-            if text:
-                lines.append(text)
-    return "\n".join(lines)
+    lines: list[dict] = []
+    if result and result[0]:
+        for item in result[0]:
+            if not item or len(item) < 2:
+                continue
+            box, rec = item[0], item[1]
+            text = rec[0] if rec else ""
+            conf = float(rec[1]) if rec and len(rec) > 1 else 0.0
+            if not text:
+                continue
+            lines.append(
+                {
+                    "text": text,
+                    "confidence": conf,
+                    "box": box,
+                    "bbox": _bbox_from_box(box),
+                }
+            )
+
+    width, height = 0, 0
+    try:
+        with Image.open(image_path) as img:
+            width, height = img.size
+    except OSError:
+        pass
+
+    return {
+        "text": "\n".join(line["text"] for line in lines),
+        "lines": lines,
+        "width": width,
+        "height": height,
+    }
+
+
+def run_ocr(image_path: str) -> str:
+    return run_ocr_with_regions(image_path)["text"]
 
 
 def save_upload_file(user_id: str, question_id: str, filename: str, content: bytes) -> str:
@@ -38,3 +74,8 @@ def save_upload_file(user_id: str, question_id: str, filename: str, content: byt
     full_path = Path(settings.UPLOAD_DIR) / rel_path
     full_path.write_bytes(content)
     return rel_path
+
+
+def annotated_rel_path(original_rel: str) -> str:
+    p = Path(original_rel)
+    return str(p.parent / f"{p.stem}_marked.jpg")