Can I use (( cv::resize )) with matrices of type double?

Problem Breakdown:

The error message:

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opencv/modules/core/src/matrix_wrap.cpp:72: error: (-215:Assertion failed) 0 <= i && i < (int)vv.size() in function 'getMat_'
indicates that an assertion in OpenCV's matrix code is failing. This assertion checks if the index i is within a valid range for the data structure it is trying to access (typically a matrix or vector). The issue stems from the fact that an index i is being accessed that doesn't fall within the valid range of the matrix or vector (0 <= i && i < vv.size()).

Context:

You mentioned that you are working with two arrays (or matrices), a and b, which are both of type double and represent matrices with rows and columns. You're also performing a resizing operation using OpenCV’s cv::resize function. Here's a breakdown of what each part of your code does:

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cv::resize(a, b, cv::Size(newrows, newcols), 0, 0, INTER_CUBIC);
a: This is the input matrix that you want to resize.

b: This is the output matrix where the resized data will be stored.

newrows and newcols: These are the new dimensions for the resized matrix.

INTER_CUBIC: This specifies the interpolation method used for resizing, which is cubic interpolation in this case.

The error suggests that there's an issue with the resizing operation, particularly when trying to access or modify the matrix.

Possible Causes of the Error:

Invalid Matrix Dimensions (empty matrix or invalid index): The matrix a might not be properly initialized or it might be empty, causing the resize operation to fail. This can also happen if newrows or newcols is zero or negative, leading to an invalid dimension for resizing.

Mismatched Data Types: OpenCV matrices typically store their data as CV_8U, CV_32F, CV_64F, etc., but the type might not match the expected format in some parts of the operation. If a and b are defined as double arrays but are not properly converted to OpenCV matrices (i.e., cv::Mat), this can cause issues when performing matrix operations.

Incorrect Matrix Initialization or Resize Parameters: If b (the destination matrix) is not pre-allocated or initialized correctly, OpenCV might not know how to store the resized data.

Issue with vv.size() in the Source Matrix: The assertion error itself suggests that OpenCV is attempting to access an element or index in the matrix (or vector) that is out of bounds. The index i that is being accessed may not be valid, possibly because vv (which is a reference to some internal matrix data structure) has an unexpected size or is empty.

Steps to Resolve the Issue:

Let's go through a few troubleshooting steps to resolve this issue:

1. Check Matrix a Initialization

First, ensure that matrix a is properly initialized and has valid dimensions. An empty matrix or a matrix with invalid dimensions (e.g., rows or columns set to 0) could be the source of the problem.

Example:

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cv::Mat a = cv::Mat::zeros(rows, cols, CV_64F); // Initialize with valid dimensions
Also, ensure that a is not empty at the time of the cv::resize operation:

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if (a.empty()) { std::cerr << "Matrix 'a' is empty!" << std::endl; return -1; }

2. Pre-allocate Matrix b

Ensure that the destination matrix b (the resized matrix) is properly allocated with the new size. Although cv::resize should handle this internally, it's good practice to allocate memory explicitly before using the matrix.

Example:

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cv::Mat b(newrows, newcols, CV_64F); // Pre-allocate the destination matrix

3. Validate Resize Parameters

Make sure the new dimensions (newrows and newcols) are valid. If either of these is zero or negative, OpenCV will fail to resize the matrix properly.

Example:

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if (newrows <= 0 || newcols <= 0) { std::cerr << "Invalid new dimensions!" << std::endl; return -1; }

4. Verify the Interpolation Method

Make sure that the interpolation method you're passing is supported by OpenCV. INTER_CUBIC should be valid for resizing operations, but it's always good to verify that you're using the correct OpenCV constants. You could experiment with other methods like INTER_LINEAR if you're unsure:

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cv::resize(a, b, cv::Size(newrows, newcols), 0, 0, cv::INTER_LINEAR); // Try a different interpolation method

5. Check Matrix Type Compatibility

If a and b are both double arrays, ensure that they are properly wrapped into OpenCV's cv::Mat objects before resizing. OpenCV's cv::resize function works on cv::Mat objects, so you need to ensure that the matrices a and b are of type cv::Mat.

Example:

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cv::Mat a(rows, cols, CV_64F, a_data); // Assuming a_data is a double pointer to the array data cv::Mat b; // OpenCV will allocate the output matrix automatically

6. Use Debugging Statements

Add debugging statements to trace where the issue occurs. For example, before the cv::resize call, print the dimensions of both a and b to ensure they are valid:

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std::cout << "Matrix a size: " << a.size() << std::endl; std::cout << "New dimensions: " << newrows << " x " << newcols << std::endl; cv::resize(a, b, cv::Size(newrows, newcols), 0, 0, INTER_CUBIC);
Code Example with Debugging:

Here's a full example of how you might structure the code:

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#include <opencv2/opencv.hpp> #include <iostream> int main() { int rows = 5, cols = 5; int newrows = 10, newcols = 10; // Initialize the input matrix 'a' with random values or zeros cv::Mat a(rows, cols, CV_64F, cv::Scalar(1)); // 5x5 matrix of type double initialized to 1 std::cout << "Matrix a: " << a << std::endl; // Validate new dimensions if (newrows <= 0 || newcols <= 0) { std::cerr << "Invalid new dimensions!" << std::endl; return -1; } // Pre-allocate the output matrix 'b' cv::Mat b; // Resize operation with INTER_CUBIC interpolation try { cv::resize(a, b, cv::Size(newcols, newrows), 0, 0, cv::INTER_CUBIC); } catch (const cv::Exception& e) { std::cerr << "OpenCV Error: " << e.what() << std::endl; return -1; } std::cout << "Resized Matrix b: " << b << std::endl; retu
0; }
FAQ

1. What does the error (-215: Assertion failed) mean in OpenCV?

The error indicates that an internal assertion failed in OpenCV’s matrix handling code. Specifically, OpenCV is trying to access an index that is out of bounds or invalid, often caused by issues with matrix dimensions or improperly initialized matrices.

2. How do I fix the error in cv::resize?

The most common solutions involve checking that:

The input matrix a is not empty.

The output matrix b is correctly pre-allocated.

The resize parameters (newrows and newcols) are valid.

The interpolation method is correctly specified.

3. Why is cv::resize throwing an assertion failure?

This typically happens when one of the matrices involved in the operation is not correctly initialized, or when the dimensions you’re resizing to are invalid. The error can also occur if you're trying to resize a matrix to a non-positive size or an empty matrix.

4. Can I use cv::resize with matrices of type double?

Yes, OpenCV supports resizing of matrices of type double (CV_64F), but ensure that both the input and output matrices are valid OpenCV cv::Mat objects.

Conclusion:

The issue you're facing with the OpenCV cv::resize function is likely due to invalid matrix dimensions, incorrect initialization, or misuse of the parameters. By following the troubleshooting steps and verifying that the matrices are properly allocated and initialized, you should be able to resolve the issue. If the error persists, further debugging might be needed to check how the matrices are being passed and manipulated in the code.