Line Rendering — Use of Variable Line Weights
Geometry and accurate representation of visual information play an important role in technical illustration. Proper use of line weights can help in communicating the visual data effectively to the end user. This is achieved by a technique known as — Line Rendering. Line Rendering is achieved by selecting consistent line weights to indicate intersection, edges, and the extremities of surfaces.
Line rendering is vital for black and white line art illustrations. It is cheaper and cost-effective to publish information for documents having short revision life-cycle. Line rendering is based on giving an object’s features different weights so that an appearance of solidity can be formed without tone or use of grayscale and shadows. Thus, saving color and reducing printing costs.
Look at the line art below, any variables such as color, materials, lighting, and environment are absent. This makes the illustration easier to read and, in many cases, more effective than photo-realistic images. Also, it is cost-effective and easier to revise and create.
A good line rendering is to take a minimalist approach to line work. As seen below, this is an illustration of an aircraft towbar. The towbar is usually a lightweight aluminum alloy construction to facilitate ground movement of aircraft. It acts as a large lever to rotate the nose landing gear while connected to push-back tug. The image below gives a good idea of the concept.
I will be using the towbar as an example for demonstrating the line rendering concept. Below is an isometric view vector graphic of the towbar.
The towbar is rendered using selective line work. As an idea, a real towbar would have much more complexity than is actually illustrated. For example, fittings, and all fasteners not distinguishable at the drawing scale are omitted. Likewise, actual components not necessary to the purpose of the drawing are also left out (e.g. tow truck or landing gear). The simplified graphic can be used to show enlarged details, as is the case here. The ability to distinguish which lines are crucial to an illustration is developed through experience. A general rule of thumb is to start simple and edit line work as you progress.
Line rendering is based on giving an object’s features different line weights so that an appearance of solidity can be formed without tone or gradients. For this, you should analyze object geometry for the line work necessary to show the shape and describe the function. In order to demonstrate the line rendering technique, the same object is used for the following examples.
Basically, lineart illustrations follow 2 basic rendering techniques.
- Single-Line-Weight Rendering
This is the simplest technique. It is pretty easy as all we will need is a consistent vector trace or CAD-generated vector (e.g. *.CGM, *.DXF, *.DWG etc.) After you have the vector geometry opened in the illustrating program, move and copy that geometry to a new layer (so you have access to the original construction if need be). Edit the duplicate geometry for visibility and detail, and make sure that the anchors (end points) are in fact coincidental. Allocate a line weight to all the lines appropriate for the scale of the object. I have used 0.180 mm line-weight. Although this method is fast and effective, it has a serious drawback: it doesn’t really look like an illustration. It’s actually just a pictorial drawing, ready to be turned into a technical illustration.
2. Two-Line-Weight Rendering
By using a single line weight, the visual distinction of the object from the background is lost. A line on the perimeter that separates the object from the background is just as visually dominant as a thin interior intersection line. Below illustration is rendered using two line weights 0.18 mm and 0.35 mm, it demonstrates the result of assigning a heavier line to the outside of the object. Now, the object stands out from the background and projects a sense of solidity. As you can see from the example, this simple change adds significant clarity to the illustration and is appropriate for a large percentage of line illustrations.
By using a single line weight, the visual distinction of the object from the background is lost. A line on the perimeter that separates the object from the background is just as visually dominant as a thin interior intersection line. Below illustration is rendered using two line weights 0.18 mm and 0.35 mm, it demonstrates the result of assigning a heavier line to the outside of the object. Now, the object stands out from the background and projects a sense of solidity. As you can see from the example, this simple change adds significant clarity to the illustration and is appropriate for a large percentage of line illustrations.
For a continuous smooth outline set the Cap and Corner Stroke Styles to rounded. Set outline style to ‘Outside outline’. Make sure that all anchors are coincidental so that the outline blends well.
It is important to achieve sufficient a difference between the outline and other object lines. Usually, it is good is to make the outline 2.5 times thicker than the inside lines. However, extremely bold outside lines signify weight and mass and are applicable only for large, heavy objects ( aircraft, heavy construction vehicles, buildings, etc.). Hence, you can imagine, light, transparent, or soft objects require lighter perimeter lines.
Line weight rendering is the place that most technical illustrators start because if they have done a good job at construction, an effective line weight rendering is already there. This technique forms the basis for more sophisticated rendering methods. A line rendering is a least-expensive method for constructed or projected technical illustrations. It can also be selective in that 3D Tessellated lines or obscure detail can be deleted. Also, it’s easy to make versions of a line illustration, reducing future illustration revision and editing time. Line weight rendering remains an essential rendering technique for creating awesome technical iIllustrations.
References:
Adobe Photoshop CS3 WOW!. Peachpit Press. Berkeley. 2003
Photoshop 7 Bible. John Wiley & Sons. New York. 2002.
