Model builder (#29)

Co-authored-by: Ayush Chaurasia <ayush.chuararsia@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

ultralytics/yolo/utils/metrics.py

@@ -4,6 +4,12 @@ Model validation metrics
 """
 
 import numpy as np
+import torch
+
+
+def box_area(box):
+    # box = xyxy(4,n)
+    return (box[2] - box[0]) * (box[3] - box[1])
 
 
 def bbox_ioa(box1, box2, eps=1e-7):
@@ -26,3 +32,24 @@ def bbox_ioa(box1, box2, eps=1e-7):
 
     # Intersection over box2 area
     return inter_area / box2_area
+
+
+def box_iou(box1, box2, eps=1e-7):
+    # https://github.com/pytorch/vision/blob/master/torchvision/ops/boxes.py
+    """
+    Return intersection-over-union (Jaccard index) of boxes.
+    Both sets of boxes are expected to be in (x1, y1, x2, y2) format.
+    Arguments:
+        box1 (Tensor[N, 4])
+        box2 (Tensor[M, 4])
+    Returns:
+        iou (Tensor[N, M]): the NxM matrix containing the pairwise
+            IoU values for every element in boxes1 and boxes2
+    """
+
+    # inter(N,M) = (rb(N,M,2) - lt(N,M,2)).clamp(0).prod(2)
+    (a1, a2), (b1, b2) = box1[:, None].chunk(2, 2), box2.chunk(2, 1)
+    inter = (torch.min(a2, b2) - torch.max(a1, b1)).clamp(0).prod(2)
+
+    # IoU = inter / (area1 + area2 - inter)
+    return inter / (box_area(box1.T)[:, None] + box_area(box2.T) - inter + eps)