April 2016
By Lutz Dieffenbach

Image: © AnnaElizabethPhotography/istockphoto.com

Lutz Dieffenbach is a technical illustrator and technical designer. He has been working as a freelancer for industries, technical publishing houses and agencies since 2004.


ldieffenbach[at]ldti.de
www.ldti.de


 


 

Technical illustrations: choosing the right perspective

Isometry, military projection or vanishing points – technical illustration knows various ways to project an object. Each of them is focused on revealing the object in the most natural view possible and to increase the users comprehension of the product.

Depending on the purpose of the presentation and the selected target group, a product can be presented using a number of projections in technical documentation. This article provides an overview of these projections, as mentioned in the standards series DIN ISO 5456 for technical drawings.

First, we should answer the question why a technical writer should show his readers a projected illustration of an object when the production drawings are already available. The answer is obvious: A projected representation helps the user to better imagine the object and therefore to understand it better.

On the other hand, if the technical writer was to show the object only in the form of a production drawing, the user would have major difficulties in correctly identifying the geometrical body. It is difficult for the user to derive a perspective from the views alone. Moreover, by selecting a certain perspective the technical writer has the opportunity to guide the user’s focus.

Parallel projections

We will first look at parallel projections as stated in the DIN ISO 5456-3 "Technical drawings – Projection methods – Part 3: Axonometric representations". These include isometric, dimetric, trimetric, cavalier/cabinet projections and the military perspectives. Parallel projections are widely used in technical documentation, since the proportions of an object are represented to scale. Moreover, the representation is close to a production drawing.

Isometry

Isometry as represented in Figure 1 is considered a standard perspective in technical documentation since it is easy to create. It is also the commonly used perspective in illustration programs for technical documentation.

All edges are equal in length and both angles are of equal degree in isometric drawings. The unshortened edges convey a certain dimensional accuracy of the object.


Figure 1: Isometry – all edges are equal in length
Source: Lutz Dieffenbach


Dimetric drawings

This form of parallel perspective, as shown in Figure 2, has two different angles, and the side edges are shortened. This perspective is closer to reality. Since one side is shortened, it is possible to represent large objects in less space.


Figure 2: Dimetry – shortened side edges
Source Lutz Dieffenbach

 

Trimetry

Both angles are unequal and all edges are of different sizes in this variant, as shown in Figure 3. It is possible to show an object realistically with the help of parallel perspective. The DIN 5456-3 does not recommend this drawing technique. Nevertheless it can be used in technical documentation.


Figure 3: Trimetry – differing edges and angles
Source: Lutz Dieffenbach

 

Cavalier and cabinet oblique projections

Unlike the cabinet projection, the cavalier oblique projection in Figure 4 does not show a shortened third axis in the DIN standard. In the common literature the cavalier projection has a shortened third axis. Here it is possible to focus on the front of an object.


Figure 4: Cabinet projection – shortened third axis; Cavalier projection – no reduction
Source: Lutz Dieffenbach

 

Military projection

The military projection represented in Figure 5 is called the "planometric projection" as per DIN ISO 5456. The vertical edges are shortened and objects can be shown from above in this projection.


Figure 5: Military projection – good for a view from the top
Source: Lutz Dieffenbach

 

Realistic representation

If we take a look at the central projections according to DIN ISO 5456-4, it is usually the "vanishing point projection" that is mentioned. Therefore, let’s use this technical term here.

Vanishing point projections are used by illustrators, when reality is to be mapped. The true sizes of the edges cannot be measured in that case.

The horizon (field of view), shown in Figure 6 and the horizon line are the deciding factors for creating and positioning the projection. The vanishing points of the edges of the object all end at the horizon line.


Figure 6: Horizon and horizon line of a human being
Source: Lutz Dieffenbach

 

The one-point method

In the one-point method or central vanishing point shown in Figure 7 all edges run to a central point. The front of an object can be highlighted in this manner for instance.


Figure 7: Central projection – Vanishing point in the centre
Source: Lutz Dieffenbach

 

Two- and three-point methods

In the two-point method, the edges run to two vanishing points – see Figure 8. This projection corresponds to our natural view to a large extent.


Figure 8: The three-point method in figure 9 allows representing large objects in a small space.
Source: Lutz Dieffenbach

 


Figure 9: Three vanishing points – large object in a small space
Source: Lutz Dieffenbach

 

Above and below

If the objects are represented above or below the horizon line in central projections, then the objects can be viewed more from above or below – see Figure 10.


Figure 10: Above or below the horizon line
Source: Lutz Dieffenbach


The bird’s eye projection and worm’s eye projection are still mentioned in the general literature, see Figures 11 and 12. Information on these is not available in the DIN ISO 5456-4. In the three-point method, the objects can be placed above or below the horizon line in such a way, that they can be viewed directly from above or below.


Figure 11: Bird’s eye projection – View of an objective from above
Source: Lutz Dieffenbach

 


Figure 12: Worm’s eye projection – View of an object from below
Source: Lutz Dieffenbach

 

Various projections have been introduced with the help of individual objects in this article. The opportunities for their use can be identified from the two examples below. The answer to the question "What is to be conveyed to the person viewing the object?" is the final deciding factor for choosing a projection. How much space is available for the projection and is a real environment required? It is possible to achieve a better understanding of the product with appealingly projected representations.

 

Possible use cases for different perspectives

Parallel projections are often used when real representation is not required. The vanishing point perspective is recommended for a more realistic projection. If space is in short supply – as is the case on a smartphone display – trimetry might be your perspective of choice.

The examples below illustrate how the different perspectives can be put to use.

Figure A shows the required space and the set-up of a confocal microscopy system. The illustrator has chosen a non-realistic environment, using trimetry.


Figure A
Source: Leica Microsystems CMS GmbH


Figure B shows the fictional driving console of a locomotive, where the organization of the displays is presented in a real environment. The driver should see a real image of his workspace, which is why the illustrator has chosen the central vanishing point projection.


Figure B
Source: Lutz Dieffenbach