Summary:

    To make the most of the blast SOP, you'll want to understand how to utilize group expressions.  For information on this visit:  https://www.sidefx.com/docs/houdini/model/groups.html

    It's also wise to specify the Group Type whenever you use this node.  I've found that the blast sometimes fails to work unless you specify exactly what kind of geometry components you'd like to remove.

Main Parameters:

Group Type:

    *  Breakpoints:
        --  This relates to NURBS breakpoints.  For more information about NURBS and breakpoints, visit this video:  
https://www.youtube.com/watch?v=iv51A4syohU&ab_channel=hipflask

    *  Edges:
        --  Deletes points based on which edge they belong to.  Keep in mind that "edges" are not considered to be a geometry component in Houdini because they are not responsible for the generation of a primitive.  Here, we are using edges to ultimately affect points which then get deleted.

    *  Points:
        --  Deletes points

    *  Primitives:
        --  Deletes primitives


Recompute Normals:

--  The removal of geometry can change the effective normals on the surface. This option will recompute any point normals to ensure they reflect the result of the deletion.  This is especially noticeable towards the edges of where you deleted primitives.  Because point normals rely on surrounding faces for their orientation, it's important to recalculate them if they're next to a face which just got deleted. 

Keep in mind!  Point normals are not the same thing as vertex normals.  The blast SOP will seek to correct point normals rather than vertex normals, and if you have vertex normals, then it will convert them into point normals.


Delete Non Selected:

--  Inverse the selection


Fill Simple Holes:

--  This is useful when you want to delete points, but don't want it to cause a hole in your mesh.  Be sure that the group type is set to points.  


Delete Unused Groups:

--  Lets say you delete some polygons that are part of a group, and that group no longer has any polygons in it.  Delete Unused Groups will delete that group which as no polygons left in it anymore.