OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor,

asked 2017-09-29 03:18:19 -0600

papabiceps
39 ●1 ●3 ●9

I'm trying to run a code that does template matching. I'm getting assertion failed error on cvtColor function.

Here is the error.

OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor, file /home/travis/miniconda/conda-
bld/conda_1485299292920/work/opencv-3.2.0/modules/imgproc/src/color.cpp, line 9748
Traceback (most recent call last):
File "opencv_basics.py", line 60, in <module>
img_gray = cv2.cvtColor(img_rgb,cv2.COLOR_BGR2GRAY)
cv2.error: /home/travis/miniconda/conda-bld/conda_1485299292920/work/opencv-
3.2.0/modules/imgproc/src/color.cpp:9748: error: (-215) scn == 3 || scn == 4 in function cvtColor

Here is the code I'm trying to run.

import cv2
import numpy as np

img_rgb = cv2.imread('template.jpg')
img_gray = cv2.cvtColor(img_rgb,cv2.COLOR_BGR2GRAY)

template = cv2.imread('opencv-template-for-matching.jpg',0)
w, h = template.shape[::-1]

res = cv2.matchTemplate(img_gray,template,cv2.TM_CCOEFF_NORMED)
threshold = 0.8
loc = np.where( res >= threshold)

for pt in zip(*loc[::-1]):
    cv2.rectangle(img_rgb, pt, (pt[0] + w, pt[1] + h), (0,255,255), 2)

cv2.imshow('Detected',img_rgb)

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answered 2017-09-29 03:36:08 -0600

berak
32993 ●7 ●81 ●312

this is probably the most common rookie error there is ...

you're loading an image, but never check the outcome.

since the error came from cvtColor, obviously img_rgb was never loaded (wrong path)

lesson here: whenever you load anything, check !

img_rgb = cv2.imread('template.jpg')
if img_rgb == None: 
    raise Exception("could not load image !")

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So I found out that the image is not getting loaded. What am I supposed to do? I checked the path, the image is there, but the it not getting loaded.

papabiceps ( 2017-09-29 03:40:47 -0600 )edit

How can I be so..... It was in the wrong path.

papabiceps ( 2017-09-29 03:46:06 -0600 )edit

try an absolute path, or, if it is relative, it is relative to where you start your program

(this is not an opencv problem at all, but a basic resource handling one)

berak ( 2017-09-29 03:51:11 -0600 )edit

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answered 2017-09-29 07:30:27 -0600

supra56

943 ●9 ●6

There is nothing wrong with filename. Try this:

import numpy as np
import cv2

image = cv2.imread('photo.jpg')
template = cv2.imread('template.jpg')

# resize images
image = cv2.resize(image, (0,0), fx=0.5, fy=0.5) 
template = cv2.resize(template, (0,0), fx=0.5, fy=0.5) 

# Convert to grayscale
imageGray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
templateGray = cv2.cvtColor(template, cv2.COLOR_BGR2GRAY)

# Find template
result = cv2.matchTemplate(imageGray,templateGray, cv2.TM_CCOEFF)
min_val, max_val, min_loc, max_loc = cv2.minMaxLoc(result)
top_left = max_loc
h,w = templateGray.shape
bottom_right = (top_left[0] + w, top_left[1] + h)
cv2.rectangle(image,top_left, bottom_right,(0,0,255),4)

# Show result
cv2.imshow("Template", template)
cv2.imshow("Result", image)

cv2.moveWindow("Template", 10, 50);
cv2.moveWindow("Result", 150, 50);

cv2.waitKey(0)

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Btw, I'm using raspberry pi 3, picamera version 2, Debian Strecth version 9, python 3.5 and OpenCV 3.3.0.

supra56 ( 2017-09-29 07:47:31 -0600 )edit

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OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor,

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