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It works in C++ :

    Mat img = imread("g:/lib/opencv/samples/data/lena.jpg");
    imwrite("test.webp", img);
    fstream fs;
    fs.open("test.webp", ios::binary + ios::in);
    fs.seekg(0, fs.end);
    int length = fs.tellg();
    fs.seekg(0, fs.beg);
    std::vector<uchar> buffer(length);
    fs.read((char*)&buffer[0], length);
    fs.close();
    vector<uchar> tmp;
    Mat img2= imdecode(buffer, IMREAD_UNCHANGED);
    imshow("e",img2 );
    waitKey();

In python

import numpy as np
import cv2 as cv
_bytes = open('test.webp', 'rb').read()
buf   = np.frombuffer(_bytes, np.uint8)
img   = cv.imdecode(buf, cv.IMREAD_UNCHANGED)

error is imdecode_(''): can't read data: OpenCV(4.1.0-pre) G:\Lib\opencv\modules\imgcodecs\src\grfmt_webp.cpp:164: error: (-215:Assertion failed) data.rows == 1 in function 'cv::WebPDecoder::readData'

I think python binding is wrong... post an issue

It works in C++ :

    Mat img = imread("g:/lib/opencv/samples/data/lena.jpg");
    imwrite("test.webp", img);
    fstream fs;
    fs.open("test.webp", ios::binary + ios::in);
    fs.seekg(0, fs.end);
    int length = fs.tellg();
    fs.seekg(0, fs.beg);
    std::vector<uchar> buffer(length);
    fs.read((char*)&buffer[0], length);
    fs.close();
    vector<uchar> tmp;
    Mat img2= imdecode(buffer, IMREAD_UNCHANGED);
    imshow("e",img2 );
    waitKey();

In python

import numpy as np
import cv2 as cv
_bytes = open('test.webp', 'rb').read()
buf   = np.frombuffer(_bytes, np.uint8)
img   = cv.imdecode(buf, cv.IMREAD_UNCHANGED)

error is imdecode_(''): can't read data: OpenCV(4.1.0-pre) G:\Lib\opencv\modules\imgcodecs\src\grfmt_webp.cpp:164: error: (-215:Assertion failed) data.rows == 1 in function 'cv::WebPDecoder::readData'

I think python binding is wrong... np.uint8 is convert in char and it should be uchar post an issue