In traditional projective geometry, projections are created by establishing a picture plane somewhere in space and casting projection rays from points in space toward the picture plane. In computer graphics, projections are created by mathematics applied to xyz co-ordinate triplets. For example, plan projection is created by discarding the z co-ordinate. 

AutoCAD does not support oblique projection. Orthographic, axonometric, and perspective projection methods are fully supported. 

The creation of a perspective projection simply involves choosing the perspective option in one of the menus. However, if a specific perspective is wanted, from a particular viewpoint, the set-up requires that several parameters be provided: 

• location of the viewpoint referred to variously as the eye or camera,
• location of the target point,
• viewing angle, and
• location of the front and back clipping planes.

The view (eye) and target points define the line of sight, which, with the viewing angle, defines the cone of vision or, as it is often called, the viewing pyramid. 

AutoCAD uses a camera metaphor in setting-up a perspective projection. To change the viewing angle, adjust the zoom lens length. The camera can also be twisted and panned. 

Some modellers offer depth clipping to eliminate from the scene those objects that lie in front or in back of the viewing pyramid. This is done by specifying the distance of the front and back clipping planes from the viewpoint.. 

Most modellers can simultaneously display more than one projection. Typically, the program's window is sub-divided into four viewports, each displaying a different projection: top, front, right, and axonometric or perspective. In AutoCAD, the number, content, and configuration of viewports can easily be set-up and changed. From the View menu, choose Tiled Viewports, then Layout, to subdivide the graphics area into several viewports, and then set-up the desired projection in each viewport.

Wireframes can be difficult to interpret because they are transparent; however, they are easier for the computer to display so they are AutoCAD's default method of displaying a 3D model. If the model was always displayed on the screen as a solid it would take considerable time for the computer to display changes you make in the model because of the number of calculations the computer would have to make to regenerate the figure. This would slow down the fuctioning of the program to a crawl.

A far better approach is to generate a hidden-line or shaded view of the model. The hidden-line procedure computes which edges are cut or hidden by opaque surfaces. 

Be aware that hidden-line drawings produced by modelling programs, however, often need to be corrected and improved to achieve acceptable graphic quality. A common problem is that hidden-line algorithms do not create new edges implied by the model's geometry, as in intersecting objects. Such problems can be corrected by avoiding intersecting objects or adding the missing lines later by editing the 2D projection drawing. 

In AutoCAD, hidden-line and shaded views of the model can be created by choosing Hide or Shade (and one of the four shading modes) from the View menu. 
 

	Cube in wireframe mode
	Version 1 – cube with hidden-line removal
	Version 2 – cube with hidden-line removal
	Intersecting cubes
	Intersecting cubes with hidden-line removal

Hold down left mouse button. Drag window from right to left (direction is IMPORTANT!) across the plan, crossing all vertical lines, but no horizontal lines. The right to left selection window, selects everything it crosses even objects not fully contained by the window. 

Base point:

Ignore location of cursor and simply click left button. Polar coordinate method of defining displacement will be used in next response. 

Second point of displacement: @13000<90 (i.e., move up 13 metres) 

Relocation using the AutoCAD polar coordinate method is based on distance from an origin and angle relative to the X axis. AutoCAD's normal mode of angle measurement is anti-clockwise. Your drawing should resemble the figure below. 

The grid lines from the Plan are longer than necessary. Autocad's stretch command is ideal for changing the length of many things at once. 

Select objects:

Hold down left mouse button. Drag window from right to left across the tops of the grid lines.

Base point:

Simply click left button 

Second point of displacement: @3000<270 (i.e., move down 3 metres) Command: qsave (save your work)

To draw the bottom edge of floor: 

Command: line

From point: end (or  from the Snaps toolbar) 

Endp of: Pick bottom end of the left elevation grid line drawn above. 

To point: end

Endp of:

Pick bottom end of right grid line

To point: (to exit) 
 

To create top edge of floor, 500mm above (use bottom edge): 

Command: offset (or ) 

Offset distance or Through: 500

Select object to offset:

Pick bottom edge of floor 

Side to offset?

Pick side above 

Select object to offset: (to exit; See figure, below) 

Then convert the long sides to the floor layer and trim them at floor level: 

Command: change

Select objects: end

of

Pick both vertical edges 

Properties/: p (for Properties) 

Change what property ?: la (for layer) 

New layer : floor

Change what property ?: (to exit) 

Now trim the excess:

Command: trim

Select cutting edge(s) 

Select objects :

Pick top line of floor

<Select object to trim>: 

Pick both edges extending above the floor line. See figure below.

<Select object to trim>: (to exit) 

Command: qsave (save your work frequently)

Command: end (to save and exit) 

Remember to log off before leaving.