Direct Editing and Direct Modeling

Paul Hamilton’s comment made me wonder why there may be some confusion regarding what I have been calling “Direct Editing” on this blog. Alibre and its customers claim that they already have direct editing, and some think that it is the same as what SpaceClaim, Siemens and others are offerring. The problem has been the term “Direct Editing”. I should have been using the term “Direct Modeling” instead and I will begin to do so from this post onwards.
This post will also serve as a comparison (as I see it) between the Direct Editing capabilities offerred by Alibre Design and SolidWorks.
Alibre Design has a sub-menu called Direct Edit in it’s Feature menu that contains a bunch of commands to directly edit something. We will find out exactly what in a while.


Now lets see how direct editing works in Alibre Design. I sketched a 1″ x 1″ square and extruded it by 2″ to get a box.
I then used the Push Pull Face or Sketch command from the Direct Edit sub-menu and used the mouse to push the dark colored face (in the figure above) into the solid by 1″ to arrive at a cube.


I then used the same command to pull the same face outward by 1″ to return to the original geometry.



Now let’s see what happenned to the feature tree.


After the Extrusion feature we now have two Move Face features. So although the geometry is exactly the same as before I started direct editing, we now have two extra features. So if I were to sit all day and push/pull a million times I would have a feature tree a million features long and would still have modeled a 1″ x 1″ x 2″ box. This is called Direct Editing in Alibre Design. But what is it directly editing? The Extrusion feature? No. If it were directly editing the Extrusion feature then the extrusion distance parameter would have been modified from 2″ to 1″ and then back to 2″ and we would have just one Extrusion feature in the tree. Direct Editing in Alibre Design means adding features to the feature tree.
Now lets see how SolidWorks does the same thing. I created a similar 1″ x 1″ x 2″ box in SolidWorks by extruding a square and turned on Instant3D, their direct editing feature. I picked the square face, pushed it into the solid and then pulled it back. When I looked at the feature tree I found that there is still the single Extrude feature and no other features. So it appears that SolidWorks is smarter then Alibre Design because it recognized that the face I was pushing and pulling was created by an extrusion and modified the extrusion distance instead of adding Move Face features to the tree. Direct Editing in SolidWorks means that you directly edit the parameters of a feature, whereas in Alibre Design you directly edit the entire model by adding features to the end of the feature tree.
So you might think that SolidWorks has a better Direct Editing solution than Alibre Design. Well, according to me, it is quite the opposite. I bet you will find this interesting. Lets go back to Alibre Design and push one of the rectanglular (not square) faces into the solid. As expected, the face is pushed in and a third Move Face feature is added to the feature tree. If you try and to the same thing in SolidWorks, the rectangular face does not get pushed into the solid. Rather the entire model is pushed in space. This is because SolidWorks cannot find an Extrusion feature corresponding to the rectangular face and hence cannot change its value to push the face into the solid. SolidWorks does have a separate Move Face command, which is not tied into the Instant3D feature, which can be used to move the rectangular face into the solid. But just like Alibre Design it adds a Move Face feature to the feature tree.
So if you compare the two in terms of directing editing a model using the mouse, I would prefer Alibre Design to SolidWorks, simply because they have managed to let the user use their Move Face command (they have one as well) with the mouse. SolidWorks has not yet managed to tie their Move Face command to Instant3D. However, it would be nice if Alibre Design would not add Move Face features if the user was moving a face linked to an extrusion. However, both end up messing up their feature trees. And this is Direct Editing when it comes to history based parametric modeling systems like Alibre Design and SolidWorks.
As you can see, it is quite a messy affair. I call it messy because in order to make a modification, a user has to study the history tree and figure out which sketch or parameter of which feature he needs to modify in order to effect his change. And now if users can arbitrarily move faces using Direct Editing commands, the feature tree is bound to get only more and more complicated and difficult to understand and work with. No model is perpetual. It undergoes several changes thoughout its lifetime with the changes being made by different people. As a programmer, I know how hard and frustrating it is to modify someone else’s code, especially if the programmer has not left comments in the code. In SolidWorks you can add comments to a feature. In Alibre Design, I don’t believe you can.
Direct Modeling is an entirely different ball game. Direct Modeling systems are not history based, although some like Solid Edge ST have features, parameters and constraints. In others like Spaceclaim, a model is simply a bunch of closed solids and open surfaces. These systems give the user the ability of directly modify the geometry of a model without being constrained by the limitations associated with history based systems. They differ in the way they offer this functionality and the outcome of a modeling operation, but essentially they are all about making the geometry creation and modification processes quick and easy.
The fundamental differ
ence between Direct Modeling systems and history based systems is that Direct Modeling systems make or allow geometric changes by taking a bird’s eye view of the entire model, whereas history based systems make or allow geometric changes by performing modeling operations in a sequential mannger. Hence they lack the flexibility of Direct Modeling systems. Its like deciding your trip by looking at a map as opposed to reaching a place and then deciding where to go next.
Alibre and SolidWorks seem to be of the opinion that their customers need Direct Editing and not Direct Modeling. I know for sure that Alibre feels that way. In a comment, Paul Grayson, the Founder and CEO of Alibre said, “Alibre already has direct editing and the non-history based editing is simply a feature, not the basis for a product.” Clearly, Alibre and SolidWorks feel that Direct Editing, of the kind I showed above, coupled with the benefits and power of history based parametric modeling outweigh the value proposition offerred by Direct Modeling systems. I tend to think otherwise. As Direct Modeling technologies evolve to more closely integrate parametrics, features, constraints and all the things that make history based systems powerful and also difficult to use, I believe we are going to see the sense in what some consider to be a rush towards Pushmi-Pullyu CAD.
I hope this explains a few things. And I promise never to use the phrase “Direct Editing” when talking about non-history based systems.