What is pyramid pooling and how does PSPNet use it?

Pyramid pooling aggregates contextual information at multiple spatial scales for segmentation.

PSPNet introduces Pyramid Pooling Module (PPM) to capture both local and global context by applying pooling at multiple grid scales (e.g., 1x1, 2x2, 3x3, 6x6). These pooled features are upsampled and concatenated with original feature maps. This helps the model understand both fine details and global scene layout.