refactor(main.py): implement safer frame positioning and handling for video segments to improve robustness

main.py

@@ -540,13 +540,18 @@ class MediaGrader:
                 print("Error: Video has insufficient frames for multi-segment mode")
                 return
             
+            # Use a more conservative approach for segment positioning
+            # Instead of using total_frames directly, use a safer range
+            safe_frame_count = max(1, int(self.total_frames * 0.9))  # Use 90% of reported frames as safety margin
+            print(f"Using safe frame count: {safe_frame_count} (90% of reported {self.total_frames})")
+                
             for i in range(self.segment_count):
                 if self.segment_count <= 1:
                     position_ratio = 0
                 else:
                     position_ratio = i / (self.segment_count - 1)
-                start_frame = int(position_ratio * (self.total_frames - 1))
-                start_frame = max(0, min(start_frame, self.total_frames - 1))  # Clamp to valid range
+                start_frame = int(position_ratio * (safe_frame_count - 1))
+                start_frame = max(0, min(start_frame, safe_frame_count - 1))  # Clamp to safe range
                 self.segment_positions.append(start_frame)
             print(f"Segment positions: {self.segment_positions}")
             
@@ -618,7 +623,7 @@ class MediaGrader:
             if not cap.isOpened():
                 return None, None
             
-            # Seek to start frame and read initial frame
+            # Try seeking to the requested frame
             cap.set(cv2.CAP_PROP_POS_FRAMES, start_frame)
             ret, frame = cap.read()
             
@@ -627,6 +632,17 @@ class MediaGrader:
                 cap.set(cv2.CAP_PROP_POS_FRAMES, start_frame)
                 return cap, frame.copy()
             else:
+                # Frame doesn't exist - try seeking to a safer position
+                # Try progressively earlier frames
+                for fallback_frame in [start_frame // 2, start_frame // 4, 0]:
+                    cap.set(cv2.CAP_PROP_POS_FRAMES, fallback_frame)
+                    ret, frame = cap.read()
+                    if ret and frame is not None:
+                        print(f"Segment {segment_index}: fell back from frame {start_frame} to {fallback_frame}")
+                        cap.set(cv2.CAP_PROP_POS_FRAMES, fallback_frame)
+                        return cap, frame.copy()
+                
+                # If all fallbacks failed, give up
                 cap.release()
                 return None, None