How to calculate the mean IU score in image segmentation?

Question

How to compute the mean IU (mean Intersection over Union) score as in this paper?

Long, Jonathan, Evan Shelhamer, and Trevor Darrell. "Fully Convolutional Networks for Semantic Segmentation."

Answer 1

For each class Intersection over Union (IU) score is:

true positive / (true positive + false positive + false negative)

The mean IU is simply the average over all classes.

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Regarding the notation in the paper:

• n_cl : the number of classes
• t_i : the total number of pixels in class i

• n_ij : the number of pixels of class i predicted to belong to class j. So for class i:

  • n_ii : the number of correctly classified pixels (true positives)
  • n_ij : the number of pixels wrongly classified (false positives)
  • n_ji : the number of pixels wrongly not classifed (false negatives)

You can find the matlab code to compute this directly in the Pascak DevKit here

Answer 2

 from sklearn.metrics import confusion_matrix  
 import numpy as np

 def compute_iou(y_pred, y_true):
     # ytrue, ypred is a flatten vector
     y_pred = y_pred.flatten()
     y_true = y_true.flatten()
     current = confusion_matrix(y_true, y_pred, labels=[0, 1])
     # compute mean iou
     intersection = np.diag(current)
     ground_truth_set = current.sum(axis=1)
     predicted_set = current.sum(axis=0)
     union = ground_truth_set + predicted_set - intersection
     IoU = intersection / union.astype(np.float32)
     return np.mean(IoU)

Answer 3

This should help

def computeIoU(y_pred_batch, y_true_batch):
    return np.mean(np.asarray([pixelAccuracy(y_pred_batch[i], y_true_batch[i]) for i in range(len(y_true_batch))])) 

def pixelAccuracy(y_pred, y_true):
    y_pred = np.argmax(np.reshape(y_pred,[N_CLASSES_PASCAL,img_rows,img_cols]),axis=0)
    y_true = np.argmax(np.reshape(y_true,[N_CLASSES_PASCAL,img_rows,img_cols]),axis=0)
    y_pred = y_pred * (y_true>0)

    return 1.0 * np.sum((y_pred==y_true)*(y_true>0)) /  np.sum(y_true>0)

Answer 4

The jaccard_similarity_score (per How to find IoU from segmentation masks?) can be used to get the same results as @Alex-zhai's code above:

import numpy as np
from sklearn.metrics import jaccard_score

y_true = np.array([[0, 1, 1],
                   [1, 1, 0]])
y_pred = np.array([[1, 1, 1],
                   [1, 0, 0]])

labels = [0, 1]
jaccards = []
for label in labels:
    jaccard = jaccard_score(y_pred.flatten(),y_true.flatten(), pos_label=label)
    jaccards.append(jaccard)
print(f'avg={np.mean(jaccards)}')