AIS. Connecting objects.

(This post was written 2+ months ago but got stuck in my drafts folder. Sorry for that)

The Nokia's once famous motto 'Connecting people' has probably gone forever (especially given recent acquisition by Microsoft), but let us grab its idea. This post will be about a nice 'connecting' concept offered by the AIS (Application Interactive Services) package, part of the visualization mechanism of Open CASCADE. Unfortunately, like many other gems scattered across the product, this one sits virtually undocumented and therefore its powerful features are significantly underutilized in OCC-based apps.

This is about AIS_ConnectedInteractive and AIS_MultipleConnected, which allows you to construct derived visual representations from already computed (or to be computed) ones. Imagine an assembly consisting of various sub-assemblies, which in their turn may consist of further sub-assemblies, and so on. Eventually each sub-assembly is broken down into part(s). Each part or sub-assembly are 'instantiated' in a parent assembly, where an instance is a reference to a referred part or assembly plus an attached transformation. Here is an example of an assembly (from STEP test suite as1*.stp):
The next image is an assembly tree where you can see instances of various sub-assemblies.
Then also goes a sub-assembly l-bracket-assembly consisting of instance of a part named l_bracket and 3 instances of nut-bolt-assembly (each with different transformation), where each is a combination of two instantiated parts (bolt and nut).
AIS_ConnectedInteractive and AIS_MultipleConnected offer efficient way to compute visual representations of such complex assemblies.

AIS_ConnectedInteractive essentially implements the proxy pattern (or alike) and contains a reference to another AIS_InteractiveObject and transformation matrix.

Handle_AIS_InteractiveObject aRefObject =
 CreateRepresentation (...); TopLoc_Location aLoc = ...; Handle_AIS_ConnectedInteractive anInstance =  new AIS_ConnectedInteractive; anInstance->Connect (theObject, aLoc);

Note that you don't need to know details of aRefObject representation, you just attach it to anInstance.
Thus, AIS_ConnectedInteractive is well suited to create visual representation of an instance in the assembly hierarchy.

AIS_MultipleConnected follows the composite pattern and allows to combine multiple representations into one. Therefore it is an efficient way to construct visual representation of the (sub-)assembly.

Handle_AIS_MultipleConnectedInteractive anAssebly =
 new AIS_MultipleConnectedInteractive; anAssebly->Connect (aChild1); anAssebly->Connect (aChild2);

Again, this allows to abstract from internal details of each child object.

I use the above approach to construct visual representations of objects in a new data model designed in CAD Exchanger (currently planned to be made part of public API in version 3.0), and so far it works great.

To further abstract the creation of a final representation of the root object (or any interim one selected in a tree browser), there is a factory method that returns a handle to an object of the base type AIS_InteractiveObject.

Handle_AIS_InteractiveObject anObject =
 ModelPrs_InteractiveObjectFactory::Create (...);

Thus, the callers do not know the real type of the object that will be created – it can be either AIS_MultipleConnected, or AIS_ ConnectedInteractive, or AIS_Shape, or any other subclass of AIS_InteractiveObject.

The key benefits of *Connected* are in reusing representations of referred objects, instead of recomputing them. This allows to:
  • Reduce computation time (as creation of part representations takes the greatest time)
  • Reduce memory footprint (you do not have to create extra objects supporting the representations)
  • Reduce code size (you do not have to design extra classes for composite objects, only parts representations are really necessary)
  • Achieve greater flexibility and abstract approach (internal implementation details of referred objects are hidden and can change independently from instances and assemblies) 

To have a quick test, you might want to try DRAWEXE:
pload ALL
wedge w 1 2 3 0.5
vdisplay w
vconnect iw -5 0 0 1 0 0 0 0 1 w #creates AIS_ConnectedInteractive of AIS_Shape
box b 2 0 0 3 1 2
vconnect a -5 0 0 1 0 0 0 0 1 w b #creates AIS_ConnectedInteractive of AIS_MultipleConnected

At last, some limitations to be aware of:
  • The referred objects must belong to the same AIS_InteractiveContext. This is unfortunate but is not specific to *Connected*.
  • Setting attributes to a referred object (e.g. part) affects the referring representation (which is sort of expected). Setting an attribute to a referring object seems to have no effect.
  • Some glitches with selection (as reproduced in DRAW). 

Currently these are not affecting CAD Exchanger development, so I did not investigate these in greater details.

Anyway, hopefully these hints will be helpful for those dealing with complex data structures and GUI. If you already worked with the *Connected* and have some experience to share, it would be great to hear your comments.