[detectionview] fix bug, when no detection is in the current selection

fixtracks/widgets/detectionview.py

@@ -142,9 +142,10 @@ class DetectionView(QWidget):
         scores = self._data.selectedData("confidence")
 
         image_rect = self._pixmapitem.boundingRect() if self._pixmapitem is not None else QRectF(0,0,0,0)
-
+        num_detections = len(frames)
         for i, (id, f, t, l, s) in enumerate(zip(ids, frames, tracks, userlabeled, scores)):
             c = Tracks.fromValue(t).toColor()
+            c.setAlpha(int(i * 255 / num_detections))
             if keypoint >= 0:
                 x = coordinates[i, keypoint, 0]
                 y = coordinates[i, keypoint, 1]