mAP IoU Precision Sweep

Overview

The standard COCO-style mAP for instance segmentation averages precision across IoU thresholds [0.5, 0.55, ..., 0.95]. This implementation uses pycocotools mask_util.iou for fast RLE-based IoU computation, then sweeps thresholds to compute TP/FP/FN counts and average precision. Essential for offline evaluation of Mask R-CNN, Detectron2, or MMDetection outputs.

Quick Start

import numpy as np
from pycocotools import mask as mask_util

def precision_at(threshold, iou_matrix):
    matches = iou_matrix > threshold
    tp = np.sum(np.sum(matches, axis=1) == 1)
    fp = np.sum(np.sum(matches, axis=0) == 0)
    fn = np.sum(np.sum(matches, axis=1) == 0)
    return tp, fp, fn

def compute_map(pred_masks, gt_masks):
    enc_preds = [mask_util.encode(np.asfortranarray(m)) for m in pred_masks]
    enc_gts = [mask_util.encode(np.asfortranarray(m)) for m in gt_masks]
    ious = mask_util.iou(enc_preds, enc_gts, [0] * len(enc_gts))
    precisions = []
    for t in np.arange(0.5, 1.0, 0.05):
        tp, fp, fn = precision_at(t, ious)
        precisions.append(tp / (tp + fp + fn) if (tp + fp + fn) > 0 else 0)
    return np.mean(precisions)

Workflow

1. Encode predicted and ground-truth binary masks as RLE using mask_util.encode
2. Compute the full IoU matrix with mask_util.iou (fast C implementation)
3. For each threshold in [0.5, 0.55, ..., 0.95], count TP/FP/FN
4. Compute precision per threshold, then average across all 10 thresholds

Key Decisions

• RLE encoding: np.asfortranarray is required — pycocotools expects Fortran-order arrays
• One-to-one matching: each pred matches at most one GT (sum axis check == 1)
• Empty predictions: return 0 if no predictions; missing this causes division errors
• Integration: wrap in a Detectron2 DatasetEvaluator for periodic validation during training

References

• Positive score with Detectron 2/3 - Training
• Sartorius Segmentation - Detectron2 [Training]