IoU of Two Boxes

def iou(a, b):
    # boxes in (x1, y1, x2, y2)
    ix1 = min(a[0], b[0])
    iy1 = min(a[1], b[1])
    ix2 = max(a[2], b[2])
    iy2 = max(a[3], b[3])
    inter = max(0, ix2 - ix1) * max(0, iy2 - iy1)
    area_a = (a[2]-a[0]) * (a[3]-a[1])
    area_b = (b[2]-b[0]) * (b[3]-b[1])
    return inter / (area_a + area_b - inter)