OpenCV TypeError: Expected cv::UMat for argument [Solved]

# OpenCV TypeError: Expected cv::UMat for argument [Solved]

To solve the OpenCV "TypeError: Expected cv::UMat for argument", make sure your image is of the correct form.

The first thing you should try is to pass the cv2.UMat object directly to cv2.cvtColor().

main.py

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import cv2

img = cv2.imread('thumbnail.webp')

img_UMat = cv2.UMat(img)

gray_UMat = cv2.cvtColor(img_UMat, cv2.COLOR_RGB2GRAY)

print(gray_UMat)

pass umat object directly to cv2 cvt color

The first argument the cv2.cvtColor() method takes is the image src.

# Convert the image to a NumPy array

If that doesn't work, try to convert the image to a NumPy array by passing it to numpy.array().

First, make sure you have the numpy module installed.

shell

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pip install numpy

# or with pip3
pip3 install numpy

Now, import the numpy module and convert the image to a NumPy array.

main.py

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import cv2
import numpy as np

img = cv2.imread('thumbnail.webp')

# ✅ Convert image to NumPy array
img = np.array(img)

img_UMat = cv2.UMat(img)

gray_UMat = cv2.cvtColor(img_UMat, cv2.COLOR_RGB2GRAY)

print(gray_UMat)

convert image to numpy array

I only added this line to the code sample.

main.py

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# ✅ Convert an image to a NumPy array
img = np.array(img)

If that doesn't work, try to use the numpy.ascontiguousarray() method instead.

main.py

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import cv2
import numpy as np

img = cv2.imread('thumbnail.webp')

# ✅ using numpy.ascontiguousarray()
img = np.ascontiguousarray(img)

img_UMat = cv2.UMat(img)

gray_UMat = cv2.cvtColor(
    img_UMat,
    cv2.COLOR_RGB2GRAY
)

print(gray_UMat)

using numpy ascontiguousarray method instead

# Try to create a copy of the image

If the error persists, try to reassign the image to a copy of itself by calling img.copy().

main.py

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import cv2

img = cv2.imread('thumbnail.webp')

# ✅ Create a copy of the image
img = img.copy()

img_UMat = cv2.UMat(img)

gray_UMat = cv2.cvtColor(img_UMat, cv2.COLOR_RGB2GRAY)

print(gray_UMat)

create copy of the image

Notice that we reassigned the value of the img variable to the result of calling img.copy().

main.py

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# ✅ Create a copy of the image
img = img.copy()

# Try to pass the `cv2.UMat` object to the `cv2.UMat` class

If the error persists, try to pass the cv2.UMat object to the cv2.UMat class when calling cv2.cvtColor().

main.py

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import cv2

img = cv2.imread('thumbnail.webp')

img_UMat = cv2.UMat(img)

gray_UMat = cv2.cvtColor(
    cv2.UMat(img_UMat),
    cv2.COLOR_RGB2GRAY
)

print(gray_UMat)

pass cv2 umat object to cv2 umat class

The only line I changed is the following.

main.py

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gray_UMat = cv2.cvtColor(
    cv2.UMat(img_UMat),
    cv2.COLOR_RGB2GRAY
)

I passed the already-created cv2.UMat object to the cv2.UMat class when calling cv2.cvtColor().

If you get one of the following OpenCV errors, click on the link and follow the instructions:

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OpenCV TypeError: Expected cv::UMat for argument [Solved]