CNN: Why do we first resize the image to 256 and then center crop to 224?

Question

The transformation for Alexnet image input is below:

transforms.Resize(256),
transforms.CenterCrop(224),

Why do we first resize the image to 256 and then center crop to 224? I know that 224x224 is the default image size of ImageNet but why we can't directly resize the image to 224x224?

Answer 1

Perhaps this is best illustrated visually. Consider the following image (128x128px):

Say we would resize it to 16x16px directly, we'd end up with:

But if we'd resize it to 24x24px first,

and then crop it to 16x16px, it would look like this:

As you see, it's getting rid of the border, while retains details in the center. Note the differences side by side:

The same applies to 224px vs 256px, except this is at a larger resolution.