William Sutherland Architect

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Rydal Road
Ambleside
LA22 9BA

015394 34489

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How CAD works  ... or doesn't some days

Rubber banding and snaps
The fundamentals of CAD have not changed for many years. You click your mouse or pen or whatever pointing device you are using at a point on screen which starts say, a line. The pointer can then be moved around with a 'rubber band' line (as though you have one end of a rubber band pinned to a board and you are moving the other end away from it). This is where 'snaps' come in. All CAD programmes feature a method of fastening the other end of the 'rubber band' line to a specific point. If no 'snap' is switched on then you can click your mouse or pointing device anywhere on screen and voila! you have a line drawn on the screen. There are many different 'snap' methods (or 'modes') - the commonest are 'end point snap' where the on-screen cursor will jump to the end-point of a line already drawn and 'intersection snap' where the on-screen cursor jumps to the junction of say, two lines that cross. All these 'snaps' only operate when your on-screen cursor is within a set distance (on the screen) of the point or intersection. Switching between 'snap' modes is usually done by clicking on a menu option - it varies from programme to programme. Once you have mastered these basics the rest should follow....slowly. I found it useful to remember secondary school geometry lessons about 'tangents' and 'arcs' etc. It is no coincidence that the number of curved buildings has increased since CAD became commonplace!

Beyond simple lines and circles
Most CAD programmes have features included or available as an extra (£££!) which will take some of the labour out of drawing repetitive or awkward tasks. Automatically inserting a window into a brick wall for example. This is done by little computer programmes known as 'Macros' or 'Scripts' specially written by the CAD package authors or sometimes by a 3rd party company.

Bitmaps
If Vectors are lines then Bitmaps are images made up of dots. Software packages that allow you to edit digital or scanned photographs on your computer screen are Bitmap editing programmes. Adobe Photoshop, Corel Photopaint, Paintshop Pro all fall into this category. To confuse things some advertised Vector based drawing software is not CAD at all but are Art Drawing programmes. Corel Draw! and Adobe Illustrator fall into this category. The difference being that with a CAD package you are working to a scale and measurements whereas with an art package you are using your 'eye' to judge proportion - OK, it's not a perfect definition but you get the idea.

That was elementary...
If you have grasped the basics that will do really. The advent of 3D CAD exemplified by Nemetschek Allplan FT has moved the concept of CAD to the other side of the planet and understanding how it works is pointless for the average user - just enjoy the features and admire the skill of the programmers. That said here are a few beginner's notes on the subject...

"It's drawing Jim, but not as we know it... "
For Architects only used to drawing with a pen and pencil there are two conceptual leaps to make in a change over to 3D CAD. The first is to grasp the basic idea of how CAD works (see above) in 2D, replacing the physical pencil with the virtual, and the second is to understand the object orientated nature of 3D CAD systems.
'Object orientated' just means that you assemble your building from a set of components rather than by drawing lines alone. These components have a certain amount of 'intelligence' in that they know that they are part of say, a wall and the wall can interact with windows, doors and roof - OK, so you have to see this in action to understand why it matters but essentially it keeps the building design 'live' so that changes can be made at any time. Traditional 2D views are linked to the 3D model interactively also for the production of working drawings (plan, elevations, sections etc.).
Unfortunately many Architects are locked into the idea still of producing a 3D view as a special service to clients (senior partners used to enjoy doing these illustrations I seem to recall!) whereas client expectation now is often for virtual-reality type animation walk-throughs ('virtual reality' sounds good but just means that on screen it looks as though tv's 'Through-the-keyhole' are going around your building with a hand held camera!).
With Allplan FT real-time 'virtual reality' comes as standard - a good balance between realism and the computing power available to the average Architect. (Real-time is yet more jargon indicating that the animated perspective view is always available onscreen while designing the building - some packages you need to export the model to render it adequately - 'Render' just means apply a surface finish to a model with lighting, textures etc.)

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