added cmd line arg for saving only positives

Added a simple command line argument (default False) where if set to true, only the images and videos which contain a positive identification are saved to the output. 

Location in code is not extremely efficient, but is present at that location for readability and minimal intrusiveness.

Signed-off-by: GarbageHaus <90212322+GarbageHaus@users.noreply.github.com>

detect.py

@@ -82,6 +82,7 @@ def run(
     save_conf=False,  # save confidences in --save-txt labels
     save_crop=False,  # save cropped prediction boxes
     nosave=False,  # do not save images/videos
+    savepositives=False, # save only positive detections
     classes=None,  # filter by class: --class 0, or --class 0 2 3
     agnostic_nms=False,  # class-agnostic NMS
     augment=False,  # augmented inference
@@ -233,7 +234,7 @@ def run(
                 cv2.waitKey(1)  # 1 millisecond
 
             # Save results (image with detections)
-            if save_img:
+            if save_img and ( (savepositives and len(det) ) or not savepositives ):
                 if dataset.mode == "image":
                     cv2.imwrite(save_path, im0)
                 else:  # 'video' or 'stream'
@@ -281,6 +282,7 @@ def parse_opt():
     parser.add_argument("--save-conf", action="store_true", help="save confidences in --save-txt labels")
     parser.add_argument("--save-crop", action="store_true", help="save cropped prediction boxes")
     parser.add_argument("--nosave", action="store_true", help="do not save images/videos")
+    parser.add_argument("--savepositives", action="store_true", help="save files only with a positive detection")
     parser.add_argument("--classes", nargs="+", type=int, help="filter by class: --classes 0, or --classes 0 2 3")
     parser.add_argument("--agnostic-nms", action="store_true", help="class-agnostic NMS")
     parser.add_argument("--augment", action="store_true", help="augmented inference")