Open CASCADE notes: 2010

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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Sometimes you might want to use a template class that would inherit Standard_Transient (or its subclass). As you need to pair it with a handle counterpart (which would have to inherit respective Handle_Standard_Transient subclass), you can't do this with the DEFINE_STANDARD_HANDLE macro directly. This post explains how you could do this.

ACIS and Parasolid formats (like IGES or STEP for instance) represent a file as a list of entities that have id's. To represent an ACIS or a Parasolid file in memory, the Base_FileModel class is used. It basically provides access to the file header and manages a set of entities where an entity is of course specific to each format. To ease sharing Base_FileModel it should be manipulated by handle (i.e. inherit Standard_Transient). Here is how this is done:

template class Base_FileModel : public Base_Transient
{
public:

//! Defines a type to index entities.
/*! @todo It must be signed (e.g. to address -1 in ACIS. Should it be consistent with size_t (for 64bit port)?
*/
typedef int Index_t;

//! Defines an entity type.
/*! Equals to the template parameter.*/
typedef En Entity_t;

//! Returns a number of entities.
/*! An empty model returns 0.*/
size_t NbEntities() const { return myRevEntMap.Size() - 1; }

//! Adds an entity.
/*! Returns a rank number with which \a theEntity has been added into the model. By
default, a new rank number is maximum rank number + 1.
Null entities are ignored.

This method is not thread-safe.

\sa Bind().
*/
Index_t Add (const Entity_t& theEntity)
{
Index_t aRank = myNullRank;
if (!theEntity.IsNull()) {
aRank = myEntVec.Size() + myNullRank;
Bind (aRank, theEntity);
}
return aRank;
}
...
protected:

//! Constructor.
/*! \a theNullRank defines a null rank used to designate a null entity (e.g. 0 for
Parasolid or -1 for ACIS).
*/
Base_FileModel (const FileHeader_t& theHeader, const Index_t theNullRank) :
myHeader (theHeader), myNullRank (theNullRank)
{
const Entity_t aNull;
myEntVec.SetValue (Index (theNullRank), aNull);
myRevEntMap.Bind (aNull, theNullRank);
}

...

NCollection_Vector myEntVec;
NCollection_DataMap myRevEntMap;
const Index_t myNullRank;

private:
Base_FileModel (const Base_FileModel&);
Base_FileModel& operator= (const Base_FileModel&);
};

Here is how a particular class (for ACIS) is defined:

DEFINE_STANDARD_HANDLE(ACISBase_Model,Base_Transient)

class ACISBase_Model : public Base_FileModel
{
public:

//! Constructor
/*! Creates a model with a new ACISBase_FileHeader.*/
ACISBase_Model() : Base_FileModel (
new ACISBase_FileHeader(), EntityNullRank) {}

...
};

Similarly in ACISBase_Model.cxx:

IMPLEMENT_STANDARD_HANDLE(ACISBase_Model,Base_Transient)
IMPLEMENT_STANDARD_RTTIEXT(ACISBase_Model,Base_Transient)

That's it !

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