IoU (Intersection over Union)

How well does a prediction fit the ground truth?

IoU is used to estimate how well a predicted mask or bounding box matches the ground truth data.
IoU also known as Jaccard index or Jaccard similarity coefficient.
Interpretation / calculation
The IoU is calculated by dividing the overlap between the prediction and ground truth label by the union of these. 
The output is a percentage indicating the overlap between the two labels. 
Code implementation
Numpy
PyTorch
Numpy
import numpy as np
​
SMOOTH = 1e-6
​
def iou_numpy(outputs: np.array, labels: np.array):
    outputs = outputs.squeeze(1)
    
    intersection = (outputs & labels).sum((1, 2))
    union = (outputs | labels).sum((1, 2))
    
    iou = (intersection + SMOOTH) / (union + SMOOTH)
    
    thresholded = np.ceil(np.clip(20 * (iou - 0.5), 0, 10)) / 10
    
    return thresholded  # Or thresholded.mean()
PyTorch
import torch
​
SMOOTH = 1e-6
​
def iou_pytorch(outputs: torch.Tensor, labels: torch.Tensor):
    # You can comment out this line if you are passing tensors of equal shape
    # But if you are passing output from UNet or something it will most probably
    # be with the BATCH x 1 x H x W shape
    outputs = outputs.squeeze(1)  # BATCH x 1 x H x W => BATCH x H x W
    
    intersection = (outputs & labels).float().sum((1, 2))  # Will be zero if Truth=0 or Prediction=0
    union = (outputs | labels).float().sum((1, 2))         # Will be zzero if both are 0
    
    iou = (intersection + SMOOTH) / (union + SMOOTH)  # We smooth our devision to avoid 0/0
    
    thresholded = torch.clamp(20 * (iou - 0.5), 0, 10).ceil() / 10  # This is equal to comparing with thresolds
    
    return thresholded  # Or thresholded.mean() if you are interested in average across the batch
Related model families 
​Object Detectors 
​Instance Segmentors​
​Semantic Segmentors​

Last updated 3 months ago