Calculate the Output Size of Conv. Layer

Say, we want to convolve a 7 X 7 image with a 3 X 3 filter with a stride length of 2. What's the final size of the output?

Before going to the equation, let's lay down some definitions:

  • the image is of size n X n
  • the filter is of size f X f
  • the padding is of size p
  • the stride is of length s

The output size will be:

$$\lfloor \frac{n + 2p - f} {s} + 1 \rfloor \times \lfloor \frac{n + 2p - f} {s} + 1 \rfloor$$

Note: Should the fraction not be an integer, we'll have to round it down.

And since:

  • n = 7
  • f = 3
  • p = 0
  • s = 2

This give us 3 X 3 because:

$$\lfloor \frac{7 + 0 - 3} {2} + 1 \rfloor \times \lfloor \frac{7 + 0 - 3} {2} + 1 \rfloor = 3 \times 3$$

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