Opencv template matching

Object Counting with OpenCV Template Matching

Count how many times a template image appears in a scene image using cv2.matchTemplate.

Core Pattern

import cv2
import numpy as np

def count_objects(scene_path, template_path, threshold=0.7):
    scene = cv2.imread(scene_path, cv2.IMREAD_GRAYSCALE)
    template = cv2.imread(template_path, cv2.IMREAD_GRAYSCALE)

    # Try multiple scales if needed
    best_count = 0
    h, w = template.shape

    result = cv2.matchTemplate(scene, template, cv2.TM_CCOEFF_NORMED)
    locations = np.where(result >= threshold)

    # Non-maximum suppression to avoid counting the same object multiple times
    points = list(zip(*locations[::-1]))  # (x, y) pairs
    count = nms_count(points, w, h)
    return count

def nms_count(points, w, h):
    """Count unique detections using simple grid-based NMS."""
    if not points:
        return 0
    used = set()
    count = 0
    for (x, y) in points:
        key = (x // (w // 2), y // (h // 2))
        if key not in used:
            used.add(key)
            count += 1
    return count

Parameters

• threshold — confidence threshold (0.0–1.0). Start with 0.7; lower if missing detections, raise if false positives appear.
• cv2.TM_CCOEFF_NORMED — normalized cross-correlation; robust to lighting differences.

Multi-Scale Matching (when template size differs from scene)

scales = [0.5, 0.75, 1.0, 1.25, 1.5]
for scale in scales:
    resized = cv2.resize(template, None, fx=scale, fy=scale)
    if resized.shape[0] > scene.shape[0] or resized.shape[1] > scene.shape[1]:
        continue
    result = cv2.matchTemplate(scene, resized, cv2.TM_CCOEFF_NORMED)
    ...

Writing Results to CSV

import csv

rows = []
for frame_path in sorted_frames:
    coins = count_objects(frame_path, '/root/coin.png')
    enemies = count_objects(frame_path, '/root/enemy.png')
    turtles = count_objects(frame_path, '/root/turtle.png')
    rows.append({'frame_id': frame_path, 'coins': coins, 'enemies': enemies, 'turtles': turtles})

with open('/root/counting_results.csv', 'w', newline='') as f:
    writer = csv.DictWriter(f, fieldnames=['frame_id', 'coins', 'enemies', 'turtles'])
    writer.writeheader()
    writer.writerows(rows)

Tuning Tips

• If template and scene are both grayscale already, IMREAD_GRAYSCALE is fine.
• If counts seem off, visualize matches with cv2.rectangle to debug.
• For very small sprites (< 10px), lower NMS suppression window.