In valid convolution, the size of the output shrinks at each layer. So after some point of time additional layers cannot meaningfully performs convolution. For this reason, same convolution is introduced, where where the size of the output remains intact. This is achieved by padding with enough number of zeroes at the borders of input image.
What happens to the size of output feature map in case of full convolution?
If it remains intact then what is the difference between same convolution and full convolution?