GED Transactions

Transactions are discrete events that change data from one state to another. In BRL-CAD, geometry editing transactions refer to operations that change geometry or metadata from one state to another - say, for example, renaming a region from region1.r to region2.r.

Our new geometry editing library libged is not currently based on transactions, but simply acts on the data. This means that after an action is applied, there is not convenient way to remember what was done or how to reverse it.

This project would introduce into libged a paradigm of functions generating transactions, which are then applied. So instead of a translate command simply applying a matrix operation that moves a sphere 10mm in the -x direction directly, the command would return a transaction that is then applied to the sphere. The advantage of this is that the transaction paradigm allows for both transactions and reverse transactions to be generated, allowing for a powerful undo capability. Take the sphere example. If an application is maintaining an ability to undo operations, it can note that a transaction event has been applied to sphere and store the inverse of that transaction in its undo log. Then, an undo operation simply applies the transaction at the top of the undo stack (in the case of the sphere, the inverse transaction would be moving 10mm in the +x direction) and restores the previous state without having to store the entire state of the sphere prior to and after editing. Admittedly when editing a single sphere this is not a large problem, but in more complex cases the savings quickly become significant.

A student proposal should outline an API for transaction generation and application, and a proposal on how to integrate it into libged gradually (libged is large and such a conversion shouldn't be an all-or-nothing proposition) For some libged transactions the inverse transaction may not be obvious, and the API details and design for implementation also need careful consideration. A student proposing this topic should discuss it on IRC or email with BRL-CAD developers (preferably IRC).

References

• src/libged
• src/mged
• include/ged.h

Requirements

• Familiarity with C