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Review: FCN (Semantic Segmentation)

1. From Image Classification to Semantic Segmentation

In classification, conventionally, an input image is downsized and goes through the convolution layers and fully connected (FC) layers, and output one predicted label for the input image, as follows:

Classification

Imagine we turn the FC layers into 1×1 convolutional layers:

All layers are convolutional layers

And the image is not downsized, the output will not a single label. Instead, the output has a size smaller than the input image (due to the max pooling):

All layers are convolutional layers

If we upsample the output above, then we can calculate the pixelwise output (label map) as below:

Upsampling at the last step
Filter Number along layers