2D Convolution with Python and NumPy for Image Processing8 min read

In the realm of image processing and deep learning, acquiring the skills to wield Python and NumPy, a powerful scientific computing library, is a crucial step towards implementing 2D convolution. In this journey, we’ll delve into the sequential approach, enabling you to execute image processing tasks with precision and effectiveness.

2D Convolutions stand as a cornerstone in the evolution of convolutional neural networks and an array of image processing filters, including blurring, sharpening, edge detection, and more. These operations offer a multitude of advantages, from capturing spatial hierarchies to enabling efficient learning through parameter sharing. They bring translation invariance, effective feature extraction, and computational efficiency to the table. As a result, 2D convolutions have firmly established themselves as an integral component in various deep learning architectures, especially those dedicated to computer vision tasks.

The essence of 2D convolution lies in using a kernel to traverse an input image systematically, resulting in an output image that reflects the kernel’s characteristics. If you’re new to the world of convolutions, I strongly recommend exploring the convolutional neural networks playlist by for a comprehensive introduction.

In the upcoming sections of this article, we’ll demonstrate the practical application of 2D convolution. We’ll explore the process of applying an edge detection kernel to an image using the same technique.

Import Required Libraries

To embark on the journey of implementing 2D Convolution, we must first equip ourselves with the essential tools. Two libraries will be our trusty companions on this path:

OpenCV (cv2) will be our go-to resource for image preprocessing, while NumPy (numpy) will play a pivotal role in the actual implementation of the convolution process.

These libraries will serve as the foundation upon which we’ll build our understanding and mastery of 2D convolution. With the tools in place, we’re ready to dive into the world of image processing and convolution.

Pre-process Input Image

For achieving optimal results with 2D convolutions, it’s advisable to preprocess the input image in grayscale. To streamline this process, we’ll create a method. Let’s begin:

This method takes the path of an image file as a parameter and ensures that it’s in grayscale for our convolution operations. It’s worth noting that the image file should either be located in the same directory as your Python script, or you can specify the complete path to the image file.

To convert the image to grayscale, include the following code within the method:

The cv2.COLOR_BGR2GRAY code parameter is essential to convert the default BGR image mode to grayscale when working with OpenCV. After the conversion, the grayscale image will be returned.

So, with this method at your disposal, you’ll be well-prepared to preprocess images in grayscale for effective 2D convolution operations.


For optimal results with 2D convolutions, it is advisable to process the image in grayscale. To achieve this, a method can be created. Let’s start with:

This code defines the processImage function, which reads an image from the specified path, converts it to grayscale using OpenCV, and saves the resulting grayscale image. Make sure to specify the correct image path when using this code.

Create 2D Convolution

To commence the 2D Convolution process, we’ll begin by defining the method header:

In this method, the user specifies the image and kernel for the convolution operation, with optional parameters for padding (default set to 0) and strides (default set to 1).

The subsequent step involves utilizing NumPy to perform cross-correlation on our kernel. To achieve this, it’s straightforward to horizontally and vertically flip the kernel matrix:

This flipping step is essential to perform convolution effectively.

To initiate the convolution, we’ll execute operations in both the x and y dimensions. First, we gather the dimensions of the image and kernel in the x and y directions:

Next, we apply the size formula to each output dimension:

Subsequently, we utilize the determined dimensions to generate a fresh matrix:

The effectiveness of this approach relies on having equal padding on both sides. To ensure this, we must confirm that the padding value is zero. If it is, we should avoid any unnecessary operations to prevent potential errors. Consequently, we proceed with the following conditional statement:

Now, you’re prepared to move forward with the conditional statement and the subsequent steps in the convolution process. If you have any specific code or details you’d like to include or if there’s anything else you’d like to continue with, please let me know.

To achieve this, we create a new array with extended dimensions, entirely filled with zeros:

It’s important to note that the padding value is doubled to ensure uniform application of padding on all sides. Therefore, if the padding value is set to 1, the dimensions of the padded image will increase by 2.

The subsequent action involves replacing the central portion of the padded image with the actual image:

This step ensures that the padded image matches the dimensions of the original image, with the actual image placed in the center.

To handle scenarios where there’s no padding, we include an else statement:

Now, you’re ready to proceed with the subsequent steps in the convolution process. If you have any specific code or details you’d like to include, or if there’s anything else you’d like to continue with, please let me know.

Delving into Convolution

Now, let’s dive into the core concept of convolution. The process entails traversing the image, conducting element-wise multiplication, and summing the results. The outcomes will be assigned to the corresponding elements in the output array.

We start by writing our initial loop to iterate over all elements in the y dimension:

Following this, we implement a break statement to check if the image has reached the end in the y direction. The convolution process will halt when we reach the bottom-right corner of the image matrix:

This statement ensures that we don’t attempt to convolve beyond the bounds of the image.

To consider the stride, we employ a conditional statement:

The designated stride value is used to maintain a consistent step size during convolution.

Next, we use a loop to iterate through each element in the x dimension:

The next step is to check if the kernel is positioned at the far right of the image. If it is, the x loop is exited, and the convolution process restarts in the downwards y direction:

To summarize, the primary convolution operator plays a pivotal role in executing convolution by performing element-wise multiplication and aggregating the results in the output matrix.

The heart of the convolution process lies in the following code snippet:

Within this snippet, we conduct element-wise multiplication between the kernel and the corresponding portion of the padded image. The results are summed up and assigned to the corresponding element in the output array.

The try-except block ensures that the convolution process adheres to the specified strides and breaks when it reaches the end of the image matrix.

Finally, the outcome is returned:

And with this, you’ve completed the implementation of the entire convolution method.

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