# Read Webp from Memory Error

**URL:** <https://forum.opencv.org/t/read-webp-from-memory-error/2962>\
**Category:** C++\
**Created:** [April 24, 2021, 1:05pm UTC](https://forum.opencv.org/t/read-webp-from-memory-error/2962 "2021-04-24T13:05:25Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![marcown](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/marcown/32/555_2.png) [@marcown](https://forum.opencv.org/u/marcown)\
**Post date:** [April 24, 2021, 1:05pm UTC](https://forum.opencv.org/t/read-webp-from-memory-error/2962/1 "2021-04-24T13:05:25Z")

</div>

Hi, I have function like this:

```cpp
int GetCLAHEImage(char * image, int image_size, double clip_limit, int grid_size, int mode, char** image_out_data, int *image_out_length) {
	std::vector<char> image_data(image, image+image_size);
	auto matInfo = cv::Mat(image_data);
	cv::Mat img = cv::imdecode(cv::Mat(image_data), 1);
	applyClahe(img, clip_limit, grid_size, mode);
	std::vector<uchar> buffer;
	cv::imencode(".png", img, buffer);
	*image_out_data = new char[buffer.size()];
	*image_out_length = buffer.size();
	memcpy(*image_out_data, &buffer[0], buffer.size());
	return DE_NO_ERROR;
}

```

When I run it with a encoded png everything is fine, but running it with a encoded webp (image variable) I get the following error at the line of imdecode:

imdecode\_(’’): can’t read data: OpenCV(4.5.2) /opencv-4.5.2/modules/imgcodecs/src/grfmt\_webp.cpp:164: error: (-215:Assertion failed) data.type() == CV\_8UC1 in function ‘readData’

terminate called after throwing an instance of ‘cv::Exception’  
what(): OpenCV(4.5.2) /opencv-4.5.2/modules/imgproc/src/color.cpp:182: error: (-215:Assertion failed) !\_src.empty() in function ‘cvtColor’

SIGABRT: abort  
PC=0x7f2498b65ef5 m=4 sigcode=18446744073709551610

I use opencv 4.5.2, c++ and manjaro linux (error also occurs on debian and ubuntu)

The function is part of a library and I call it using cgo

```auto
	fmt.Println("image info: ", http.DetectContentType(img.ImageBytes))
        // --> returns image/webp
	imgBytesOut, err := mylib.GetCLAHEImage(img.ImageBytes, m.ClipLimit, m.TileSize, mylib.CLAHE_LAB)

```

with a go\_wrapper:

```auto
func GetCLAHEImage(image []byte, clip_limit float64, grid_size int, mode int) ([]byte, error) {
	var ret C.int
	var retlen C.int
	bytes := C.CString("")

	ret = C.GetCLAHEImage((*C.char)(unsafe.Pointer(&image[0])), C.int(len(image)), C.double(clip_limit), C.int(grid_size), C.int(mode), &bytes, &retlen)

	result := C.GoBytes(unsafe.Pointer(bytes), retlen)
	C.free(unsafe.Pointer(bytes))

	return result, nil
}

```

What could be wrong here? Is there a bug in opencv?

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<div class="post-metadata">

**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\
**Post date:** [April 24, 2021, 1:21pm UTC](https://forum.opencv.org/t/read-webp-from-memory-error/2962/2 "2021-04-24T13:21:02Z")

</div>

> [@marcown](#):
>
> Is there a bug in opencv?

possible. please post a webp image file for testing.

using some webp file from wikipedia, I could _not_ reproduce this.

please reduce your code to the bare minimum that reproduces the issue.

> <https://github.com/opencv/opencv/blob/fcaeeac931922cd2cab4382419b4af77f0c4b0dd/modules/imgcodecs/src/grfmt_webp.cpp#L164>

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<div class="post-metadata">

**Author:** ![marcown](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/marcown/32/555_2.png) [@marcown](https://forum.opencv.org/u/marcown)\
**Post date:** [April 24, 2021, 1:41pm UTC](https://forum.opencv.org/t/read-webp-from-memory-error/2962/3 "2021-04-24T13:41:39Z")

</div>

Hey,

here is a small test code:

```cpp
	std::cout<<"before testimg"<<std::endl;
	auto testimg = cv::imread("./1.webp");
	std::vector<uchar> test_buffer;
	cv::imencode(".webp", testimg, test_buffer);

	char *buf = new char[test_buffer.size()];
	memcpy(buf, test_buffer.data(), test_buffer.size());

	std::vector<char> image_data_test(buf, buf+test_buffer.size());
	cv::Mat img_test = cv::imdecode(cv::Mat(image_data_test), 1);

	std::cout<<"after testimg"<<std::endl;

```

and here is the webp file:

[https://www.gstatic.com/webp/gallery/1.webp](https://www.gstatic.com/webp/gallery/1.webp)

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<div class="post-metadata">

**Author:** ![marcown](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/marcown/32/555_2.png) [@marcown](https://forum.opencv.org/u/marcown)\
**Post date:** [April 24, 2021, 2:40pm UTC](https://forum.opencv.org/t/read-webp-from-memory-error/2962/4 "2021-04-24T14:40:04Z")

</div>

I got a “fix” or workaround (i am not sure^^)

instead of reading the char \* using std:vector I do this:

cv::Mat rawData\_test = cv::Mat( 1, test\_buffer.size(), CV\_8UC1, buf );

so the example changes to

```cpp
	std::cout<<"before testimg"<<std::endl;
	auto testimg = cv::imread("./1.webp");
	std::vector<uchar> test_buffer;
	cv::imencode(".webp", testimg, test_buffer);

	char *buf = new char[test_buffer.size()];
	memcpy(buf, test_buffer.data(), test_buffer.size());

	// std::vector<char> image_data_test(buf, buf+test_buffer.size());
    cv::Mat rawData_test = cv::Mat( 1, test_buffer.size(), CV_8UC1, buf );
	cv::Mat img_test = cv::imdecode(cv::Mat(rawData_test), 1);

	std::cout<<"after testimg"<<std::endl;

```

this works

the problem seems to be that

cv::Mat(std::vector image\_data\_test(buf, buf+test\_buffer.size())) is of type CV\_8S and not CV\_8UC1, but when I tried to convert it the resulting Mat was empty

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<div class="post-metadata">

**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\
**Post date:** [April 24, 2021, 2:53pm UTC](https://forum.opencv.org/t/read-webp-from-memory-error/2962/5 "2021-04-24T14:53:18Z")

</div>

> [@marcown](#):
>
> ```auto
> std::vector<char> image_data_test(buf, buf+test_buffer.size());
> cv::Mat img_test = cv::imdecode(cv::Mat(image_data_test), 1);
> 
> ```

the type of that temporary Mat instance is CV\_8S because `image_data_test` has type `char`, which is signed.

the API expects `CV_8U` (same as CV\_8UC1), not `CV_8S`

it’s best to use `uint8_t` to be absolutely clear about the type.

it’s also a good idea to remove as many “conversion steps” as possible… if they’re superfluous, their only effect is to give more chance for bugs to happen.

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<div class="post-metadata">

**Author:** ![marcown](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/marcown/32/555_2.png) [@marcown](https://forum.opencv.org/u/marcown)\
**Post date:** [April 24, 2021, 3:32pm UTC](https://forum.opencv.org/t/read-webp-from-memory-error/2962/6 "2021-04-24T15:32:54Z")

</div>

I wonder why it is working with png’s

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<div class="post-metadata">

**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\
**Post date:** [April 24, 2021, 3:37pm UTC](https://forum.opencv.org/t/read-webp-from-memory-error/2962/7 "2021-04-24T15:37:41Z")

</div>

that type check is in the webp-specific code.