Receptive Fields on ConvNets (Receptive Field size confusion)
<p>I was reading this from a paper: <em>"Rather than using relatively large receptive fields in the first conv. layers we use very small 3 × 3 receptive fields throughout the whole net, which are convolved with the input at every pixel (with stride 1). It is easy to see that a stack of two 3 × 3 conv. layers (without spatial pooling in between) has an effective receptive field of 5 × 5; three such layers have a 7 × 7 effective receptive field."</em></p>

<p>How do they end up with a recpetive field of 7x7 ? </p>

<p>This is how i understand it: Suppose that we have one image that is 100x100.</p>

<p>1st layer: zero-pad the image and convole it with the 3x3 filter, output another 100x100 filtered image.</p>

<p>2nd layer: zero-pad the previous filtered image and convolve it with another 3x3 filter, output another 100x100 filtered image.</p>

<p>3nd layer: zero-pad the previous filtered image and convolve it with another 3x3 filter, output the final 100x100 filtered image. </p>

<p>What am I missing there ?</p>