AutoCAD Introduction Lesson 11 Perspective Viewing & Plotting
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If the use of 3D CAD is the process of modelling, then the process of
plotting is one of capturing the most appropriate view of that model.
So far you have only used orthogonal views of the moodel, but AutoCAD
provides numerous toools to create spectacular perspectival views.
To make this process more familiar to the user the analogy of a 35mm
camera is used.
The camera has:
- A target point (the centre of vision)
- A camera point (the location of the view)
- A focal length (the degree of perspective)
- A twist (rotation from horizontal)
Because we are using a virtual camera, we also have access to variables not available in the physical world: Front and Back clipping (a zone where objects are not rendered) and all objects are rendered as transparent until we instruct AutoCAD to make the objects solid.
An unfortunate by-product of using a computer is its speed (or more importantly its lack of speed) at creating the image. As you know, the computer uses a mathmatical equations to display the model on screen, the sad fact is that the maths is far more complicated to display the model in a perspective view than an orthogonal one. However when we take a photo we tend to move around and try a number of different views before we press the shutter. If I had to specify the camera and target locations on a map and then specify the focal length of the lens without being able to see the image I suspect I would give up tommorow.
To counter these two conflicting situations, AutoCAD provides the tools to establish the views interactively, but only with a limited number of objects. The user is required to select a number of objects that are then rendered in real time as you move the mouse to control the variables: position, focal length, twist, etc.
Having created a number of views of a 3D model, all that is left is to
render the images on paper.
Perhaps the most basic CAD paradigms, is that we always model at 1:1, and
I would be the last person to contradict this. However AutoCAD
provides a tool called "Paperspace" that puts a new twist on this paradigm.
1:1 normally means with respect to the object being modelled. With
paperspace however we create the Plot at 1:1 with respect to the paper
the image is plotted on.
The model is created in Modelspace and then ploted in Paperspace, where tools are provided to assemble a number of different views on a virtual page of any size (A1, A0 etc.). It is possible to create a single plot comprised of a mix of perspectival and orthogonal views of specified scale, and then add text and title blocks to produce a composite plot.
You will use both of these techniques to produce a plotfile than can then be opened in CorelDraw! and incroporated into other documents or coloured and treated using its more graphical toools. Remember CorelDraw is a vector drawing package just like all CAD applications.
This tutorial is the copyright of J. Attree
South Bank University, London UK