  Implicit surfaces are known for their ability to represent smooth objects of
arbitrary topology thanks to hierarchical combinations of primitives using a
structure called a blobtree. We present a new tile-based rendering pipeline
well suited for modeling scenarios, i.e., no preprocessing is required when
primitive parameters are updated. When using approximate signed distance
fields, we rely on compact, smooth CSG operators - extended from standard
bounded operators - to compute a tight volume of interest for all primitives of
the blobtree. The pipeline relies on a low-resolution A-buffer storing the
primitives of interest of a given screen tile. The A-buffer is then used during
ray processing to synchronize threads within a subfrustum. This allows coherent
field evaluation within workgroups. We use a sparse bottom-up tree traversal to
prune the blobtree on-the-fly which allows us to decorrelate field evaluation
complexity from the full blobtree size. The ray processing itself is done using
the sphere-tracing algorithm. The pipeline scales well to surfaces consisting
of thousands of primitives.
