Computer Aided Design (CAD) vs Computer Graphics (CG)


The fundamental difference between the Computer Aided Design (CAD) and Computer Graphics (CG) Modelling (CGI) is intent.   The intent of the CAD is to provide manufacturable, dimensionally accurate models, that are capable of generating engineering drawings.  Where as CG programs are focused on creating realistic or artistic imagery capable of communicating purely aesthetic information, such as, surface quality, form and feel.  

Hence, the tools have evolved to suite these needs.  For example the type of modeling adopted for CAD is “Solid Modelling”, where you form the part by applying operations that are very similar to current manufacturing methods.  In CG packages such as Blender and Maya, models are created by manipulating vertices and faces, to develop a mesh. In creating a render/animation the only thing the program requires is where these faces are located, directed and their surface quality.

The good news is adopting these tools are going to be easy, If you are like me, and come from an engineering background.  CAD packages are usually more complicated to learn and handle.  The bad news is that you will have to learn a different form of modelling; “Surface or Mesh modelling”.  As oppose to the “Solid modelling” used in CAD packages.

Here is an example model created in PTC creo.   Notice the workflow required for this model (steps 1-7).  Step 3 to 7 would be the same as actually machining a block.  Where a hole, 4, was added by subtracting material (Boolean operation). And the chamfers and rounds were added later.

Here is the same model created in Blender.  The approach taken to achieve the same result as before is completely different.  I paid more attention to the mesh than simply adding features. 

When creating the model in blender, I had to make sure the number of vertices on the circle matched the number of vertices on the bottom piece. Since the part required round corners I a subdivision surface modifier (Subsurf) was necessary.  This modifier divides the mesh and adds vertices by rounding edges. The adjacent image shows that the mesh looks like after the Subsurf was applied.  With the Subsurf, notice that I rounded the corner, 4, by having a gap between the original edge loops, 1.  Similarly I had to keep edge loops close together if I want a Sharpe corner (e.g. 2).  The hole, 3 was achieved by extruding a circle and connecting those vertices to the top part of the block.