What technique is used to maintain the dimensions of an image when applying a 3x3 filter?

Padding the outside of the input with zeros is the correct technique used to maintain the dimensions of an image when applying a 3x3 filter. This process involves adding extra pixels, typically set to zero, around the original image to ensure that the filter can be applied to every pixel, including those at the edges.

When a filter moves over an image, it computes the output for the pixel in the center of the filter. Without padding, the edges of the image would not have enough neighboring pixels available for the filter computation, which could lead to a reduction in image size after filtering. By adding a layer of zeros around the image, you allow the filter to process all the pixels in the image correctly, keeping the output dimensions the same as the input dimensions.

Using padding is a standard practice in convolutional operations within neural networks and image processing, as it helps maintain spatial dimensions and ensures that features at the edges of the image are still taken into account.