This is a page to document bounding box clipping in the Mapnik codebase.
Clipping is used primarily to ensure
valid and appropriate coordinate bounds are sent to layer datasource plugins when fetching features to render, and is necessary when checking for intersection between a map bbox and its layers when projections differ (since proj4 will fail if invalid extents are used for a given projection).
Clipping can interact in subtle and important ways with a variety of other map and layer level parameters, and became more error prone (but powerful) with the introduction of support for Proj4 in r330.
Discussion of fixing clipping after addition of map_buffer to avoid truncated text at tile edges: