Fix video writing (#3379)

ultralytics/yolo/engine/predictor.py

@@ -38,7 +38,7 @@ from ultralytics.nn.autobackend import AutoBackend
 from ultralytics.yolo.cfg import get_cfg
 from ultralytics.yolo.data import load_inference_source
 from ultralytics.yolo.data.augment import LetterBox, classify_transforms
-from ultralytics.yolo.utils import DEFAULT_CFG, LOGGER, SETTINGS, callbacks, colorstr, ops
+from ultralytics.yolo.utils import DEFAULT_CFG, LOGGER, MACOS, SETTINGS, WINDOWS, callbacks, colorstr, ops
 from ultralytics.yolo.utils.checks import check_imgsz, check_imshow
 from ultralytics.yolo.utils.files import increment_path
 from ultralytics.yolo.utils.torch_utils import select_device, smart_inference_mode
@@ -341,8 +341,10 @@ class BasePredictor:
                     h = int(vid_cap.get(cv2.CAP_PROP_FRAME_HEIGHT))
                 else:  # stream
                     fps, w, h = 30, im0.shape[1], im0.shape[0]
-                save_path = str(Path(save_path).with_suffix('.mp4'))  # force *.mp4 suffix on results videos
+                suffix = '.mp4' if MACOS else '.avi' if WINDOWS else '.avi'
-                self.vid_writer[idx] = cv2.VideoWriter(save_path, cv2.VideoWriter_fourcc(*'mp4v'), fps, (w, h))
+                fourcc = 'avc1' if MACOS else 'WMV2' if WINDOWS else 'MJPG'
+                save_path = str(Path(save_path).with_suffix(suffix))
+                self.vid_writer[idx] = cv2.VideoWriter(save_path, cv2.VideoWriter_fourcc(*fourcc), fps, (w, h))
             self.vid_writer[idx].write(im0)
 
     def run_callbacks(self, event: str):