Data model highlights – Parasolid, ACIS, Open CASCADE

by - 19:39

Working on the Parasolid importer of CAD Exchanger I had a new chance to observe specificities of another modeling kernel used to describe a B-Rep (Boundary Representation) model. This post highlights a few high-level differences between Parasolid, ACIS and Open CASCADE related to data models.

Comparing to ACIS, Parasolid seems to use a smaller set of primitives. Of course, there are typical elementary curves and surfaces (lines, circles, planes, cylinders, ...), NURBS, basic swept surfaces (revolution, extrusion), offsets. Parasolid defines geometrical object orientation right at the geometrical level by specifying a boolean flag at each curve or surface. An original surface normal is defined by cross-product of U and V derivatives, and can be confirmed or reversed by that boolean flag. ACIS defines normals by using special values of some key parameters, ignoring U and V parameterization. For instance, a sphere with a normal oriented inward would be defined in ACIS as having a negative radius. If you remember, Open CASCADE defines orientation as attached to the topology entity (face for surface, edge for curve).

Transformations in Parasolid can only be applied to component instances in assemblies. ACIS uses transformations only attached to bodies (the top-level entity which is a collection of solids). Open CASCADE can use location at any topology level, from vertex to arbitrary compound. This flexibility allows to reduce the model memory footprint – for instance, a solid box can be represented by single face with 6 different locations. The downside, however, is that you must take extra pre-cautions to not unintentionally modify shared objects, e.g. by creating copies as needed.
Unlike Open CASCADE, both ACIS and Parasolid often share underlying geometries – faces referring to the same surface are quite common in each. Open CASCADE discourages this to prevent modifications. Another benefit, which I realized recently – reduce data race risks in multi-threaded applications.

Among Parasolid geometrical types, I found just a couple of most interesting. The first one, which does not present in ACIS and Open CASCADE, is an intersection curve. An intersection curve is defined by a set of points, two surfaces being intersected and a law to define tangents and parameterization of the curve.
Another one is a rolling_ball_blend surface defined by two support surfaces, spine curve and a ball radius (see image below). Although, ACIS also has such type, the kinds of supports are often different. Whereas ACIS often uses support curves (which it calls ‘spring curves’), Parasolid most often uses surfaces. This added extra challenge to implement conversion as Open CASCADE’s GeomFill_Pipe (I described in the past) does not offer such combination.

Parasolid does not offer that broad set of procedural surfaces that ACIS does (variable_blend, net, skin, tube, law, etc). However it still has a larger set than Open CASCADE which is based on the STEP ISO010303-42 standard. Open CASCADE set is really a subset of what two other offer, except maybe support of Bezier curves and surfaces, and the fact that pcurve can have any type, not just B-Spline as two kernels require.

All 3 kernels have a concept of 3D and 2D representations for face boundaries. Parasolid has a notion of ‘fin’ or ‘half-edge’, and ACIS uses ‘coedge’ which is an entity referred by face’s loops and in its turn, refers to edge and optionally a p-curve. Interestingly, in Parasolid it can have either 3D or 2D representation but not both (though I did encounter one file in 1000+) that violated this requirement.

As far as tolerances are concerned, Parasolid, like Open CASCADE, does use local tolerances attached to the topological entities. ACIS introduced local tolerances in some intermediate versions only.

Similar to ACIS-SAT, the Parasolid-XT file format has evolved from version to version. Unlike ACIS, it is virtually human-unreadable due to use of numbers as type identifiers. Parasolid uses a concept of schemas that define entities type number, field layout, etc. To support backward compatibility, they introduced dynamic extension by embedding schema definitions into the file itself. Open CASCADE file format has never changed for last 15 years at least.

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  1. Hi Roman.
    I've long been following for your work.
    I am sure you have amazing results.
    I remember that you start playing with ACIS-SAT, approximately at the same time when I.
    Now I see that you already "tamed" X_T.
    Could you please to help me - where you get info about Parasolid-X_T specification?


  2. Hi jazzros,
    There is some public description of the Parasolid-XT format. Just Google for it. However, as usual, the devil is in details. There are about 80+ schema files defining the format from v0.15 to v22; various configurations of how entities can really be encoded (e.g. cliff blended_edge defining a rolling ball surface), etc. It is only possible through intensive and deep testing (there are currently 1000+ test X_T files in the database). Again, the approach that pays off from day 1 is to have a full unit testing framework - create a test before you write a code.

  3. hi,
    you said
    "An original surface normal is defined by cross-product of U and V derivatives, and can be confirmed or reversed by that boolean flag."

    u and v in geom_surface is real numbers,
    how can i get their derivatives?

    as i remember from calculus 2, u and v should be functions of lines, am i wrong, but they are represented as reals in geom_surface?

    as you can see i am a bit confused could you advise something?