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Mix-FFN is a feedforward layer used in the SegFormer architecture. ViT uses positional encoding (PE) to introduce the location information. However, the resolution of $\mathrm{PE}$ is fixed. Therefore, when the test resolution is different from the training one, the positional code needs to be interpolated and this often leads to dropped accuracy. To alleviate this problem, CPVT uses $3 \times 3$ Conv together with the PE to implement a data-driven PE. The authors of Mix-FFN argue that positional encoding is actually not necessary for semantic segmentation. Instead, they use Mix-FFN which considers the effect of zero padding to leak location information, by directly using a $3 \times 3$ Conv in the feed-forward network (FFN). Mix-FFN can be formulated as: $$ \mathbf{x}\_{\text {out }}=\operatorname{MLP}\left(\operatorname{GELU}\left(\operatorname{Conv}\_{3 \times 3}\left(\operatorname{MLP}\left(\mathbf{x}\_{i n}\right)\right)\right)\right)+\mathbf{x}\_{i n} $$ where $\mathbf{x}\_{i n}$ is the feature from a self-attention module. Mix-FFN mixes a $3 \times 3$ convolution and an MLP into each FFN.

SegFormer is a Transformer-based framework for semantic segmentation that unifies Transformers with lightweight multilayer perceptron (MLP) decoders. SegFormer has two appealing features: 1) SegFormer comprises a novel hierarchically structured Transformer encoder which outputs multiscale features. It does not need positional encoding, thereby avoiding the interpolation of positional codes which leads to decreased performance when the testing resolution differs from training. 2) SegFormer avoids complex decoders. The proposed MLP decoder aggregates information from different layers, and thus combining both local attention and global attention to render powerful representations.