OpenCV equivalent of np.where()

Question

When using gocv package it is possible, for example, to perform template matching of a pattern within an image. The package also provide the MinMaxLoc function to retrieve locations of minimums and maximums within the matrix.

However, in below python example, the writer uses numpy.Where to threshold the matrix and get locations of multiple maximums. The python zip function is used to glue values together so they are like a slice [][2]int, the inner slice being xs and ys of the matches found.

The syntax loc[::-1] reverses the array.

The star operator in zip(*loc..) is being used to unpack the slices given to zip.

https://docs.opencv.org/master/d4/dc6/tutorial_py_template_matching.html

import cv2 as cv
import numpy as np
from matplotlib import pyplot as plt

img_rgb = cv.imread('mario.png')
img_gray = cv.cvtColor(img_rgb, cv.COLOR_BGR2GRAY)
template = cv.imread('mario_coin.png',0)
w, h = template.shape[::-1]
res = cv.matchTemplate(img_gray,template,cv.TM_CCOEFF_NORMED)
threshold = 0.8
loc = np.where( res >= threshold)

for pt in zip(*loc[::-1]):
    cv.rectangle(img_rgb, pt, (pt[0] + w, pt[1] + h), (0,0,255), 2)
cv.imwrite('res.png',img_rgb)

How do I implement the same np.where algorithm in Go to get the multiple locations after the threshold is applied?

Answer 1

OpenCV has a built-in (semi-)equivalent function to np.where(), which is findNonZero(). As implied by the name, it finds the non-zero elements in an image, which is what np.where() does when called with a single argument, as the numpy docs state.

And this is available in the golang bindings as well. From the gocv docs on FindNonZero:

func FindNonZero(src Mat, idx *Mat)

FindNonZero returns the list of locations of non-zero pixels.

For further details, please see: https://docs.opencv.org/master/d2/de8/group__core__array.html#gaed7df59a3539b4cc0fe5c9c8d7586190

Note: np.where() returns indexes in array order, that is, (row, col) or (i, j) which is opposite to typical image indexing (x, y). That is why loc is reversed in Python. When using findNonZero() you won't need to do that, since OpenCV always uses (x, y) for points.