OpenCV equivalent of np.where() - opencv

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.

Answer 2

For anyone coming across this I hope a full example keeps you from spending days hitting your head against the wall and reading the same google results over and over until something clicks.
package main
import (
"fmt"
"image"
"image/color"
"os"
"gocv.io/x/gocv"
)
func OpenImage(path string) (image.Image, error) {
f, err := os.Open(path)
if err != nil {
return nil, err
}
defer f.Close()
img, _, err := image.Decode(f)
return img, err
}
func main() {
src := gocv.IMRead("haystack.png", gocv.IMReadGrayScale)
tgt := gocv.IMRead("needle.png", gocv.IMReadGrayScale)
if src.Empty() {
fmt.Printf("failed to read image")
os.Exit(1)
}
if tgt.Empty() {
fmt.Printf("failed to read image")
os.Exit(1)
}
// Get image size
tgtImg, _ := tgt.ToImage()
iX, iY := tgtImg.Bounds().Size().X, tgtImg.Bounds().Size().Y
// Perform a match template operation
res := gocv.NewMat()
gocv.MatchTemplate(src, tgt, &res, gocv.TmSqdiffNormed, gocv.NewMat())
// Set a thresh hold. Using the `gocv.TmSqdiffNormed` confidence levels are
// reversed. Meaning the lowest value is actually the greatest confidence.
// So here I perform an Inverse Binary Threshold setting all values
// above 0.16 to 1.
thresh := gocv.NewMat()
gocv.Threshold(res, &thresh, 0.16, 1.0, gocv.ThresholdBinaryInv)
// Filter out all the non-zero values.
gocv.FindNonZero(thresh, &res)
// FindNonZero returns a list or vector of locations in the form of a gocv.Mat when using gocv.
// There may be a better way to do this, but I iterate through each found location getting the int vector in value
// at each row. I have to convert the returned int32 values into ints. Then draw a rectangle around each point.
//
// The result of get res.GetVeciAt(i, 0) is just a slice of x, y integers so each value can be accessed by
// using slice/array syntax.
for i := 0; i < res.Rows(); i++ {
x, y := res.GetVeciAt(i, 0)[0], res.GetVeciAt(i, 0)[1]
xi, yi := int(x), int(y)
gocv.Rectangle(&src, image.Rect(xi, yi, xi+iX, yi+iY), color.RGBA{0, 0, 0, 1}, 2)
}
w := gocv.NewWindow("Test")
w.IMShow(src)
if w.WaitKey(0) > 1 {
os.Exit(0)
}
}