Context
Visualisation as an engineering tool
Visualisation has long been recognised as an effective means of communication, and engineering leans on it heavily. Ordinary CAD display already offers a graduated set of visual modes — wireframe, hidden line removed, shaded, shaded with edges — and each answers a different question about the model. Rendering sits at the far end of that range: it generates images close to photographs.
Rendered images serve two audiences. Externally they support sales and marketing, brochures and proposals. Internally they convey design ideas to teams, reviewers and non-technical stakeholders who read a photorealistic image far more fluently than an orthographic drawing. Renders can be produced back into the modelling window, sent to a printer, or written to a file.
Anatomy
The scene and the rendering model
Digital rendering uses two things: a scene, which describes the lighting environment, and a rendering model, which describes how light behaves when it meets a surface.
Light types and what they do
| Light | Character | Use it to |
|---|---|---|
| Ambient | Uniform, non-directional illumination with no defined source. | Lift overall brightness and prevent unlit faces from going completely black. |
| Directional | Parallel rays from an infinitely distant source. | Simulate sunlight; establish a consistent shadow direction. |
| Point | Radiates equally in all directions from a position in space. | Represent a bulb or local source; falls off with distance. |
| Spot | A cone from a position, with a defined direction and beam angle. | Draw attention to a feature; create dramatic product-shot lighting. |
The reflection model
Reflection is evaluated with respect to the surface normal vector at the point where the light ray meets the surface. The angle of incidence and the angle of reflection are measured from that normal, and the material determines how much of the incident light is reflected specularly (as a highlight), how much diffusely (as surface colour) and how much is absorbed. This is why material assignment changes an image far more than adding another light does.
Surface treatment
Decals, textures, materials and appearances
Four related mechanisms that are frequently confused. The distinction matters because two of them affect engineering results and two do not.
- Decal
- An image displayed on one or more faces — a logo, a rating plate, a warning label. Decals have a display order, so overlapping decals resolve predictably. Used almost entirely for marketing and product realism.
- Texture
- Also called surface finish. A texture map is an image file wrapped over the model, in the way that gift wrap covers a box. Position and orientation relative to the model must be chosen before applying it, and the choice is visible in the result.
- Material
- An engineering property set. Materials drive mass property calculations, finite element analysis and dynamic analysis; in manufacturing they inform cutting speed, feed rate and cutting conditions; and in rendering they determine how the model responds to light. A material is the only one of these four that changes numerical results.
- Appearance
- The visual characteristics of a material without its physical properties. Appearances can be applied to parts, features, faces or surfaces, and include transparency — zero being fully opaque and maximum being fully see-through.
Applying an aluminium appearance makes a part look like aluminium and leaves its mass, stiffness and thermal behaviour untouched. Applying an aluminium material changes all of them. Reviewing a mass property report on a model that was only ever given an appearance is a genuine and recurring source of wrong answers.
Point of view
Cameras and camera sleds
Once a scene has lighting and a background, a camera can be added at a chosen position. From there, model and camera can move relative to one another: the camera can orbit a fixed model, or the model can rotate in front of a fixed camera. Moving the camera is generally the more useful of the two, because it allows the object to be examined from any angle without disturbing its position in the modelling space.
A floating camera has to be guided, and it is guided by target points or by a path. Such a path is called a camera sled — conceptually the camera is mounted on a carriage that runs along a defined curve, looking at a defined target as it goes. The technique is exactly what a film crew does with a dolly track, and it produces markedly more controlled results than free-hand orbiting.
Motion over time
Animation
Animation lets an observer watch continuous motion in order to understand dynamic behaviour. Two kinds exist. Real-time animation follows a physical object as it actually moves and records that motion. Playback animation — which is what CAD systems provide, and what the film industry uses for animated features — builds motion from key frames.
- Step 1 Define key frames at significant points in time.
- Step 2 Set the state of the model and camera at each key frame.
- Step 3 Let the system interpolate between them, linearly or non-linearly.
- Step 4 Play the sequence back continuously to create the illusion of motion.
- Step 5 Record to a video file for distribution.
The interpolation choice is not cosmetic. Linear interpolation gives constant velocity and abrupt starts and stops; easing produces motion that reads as physical. Where the animation is meant to communicate a mechanism rather than sell a product, constant velocity is often clearer.
Photorealistic frames are expensive to compute, and an animation multiplies that cost by the frame count. Establish resolution, frame rate and quality settings against the intended use before starting: a review clip and a trade show loop have very different requirements, and rendering the former at the latter's settings is a common way to lose an afternoon.
Best practice
Getting a credible image
Light in layers
Start with one dominant directional source to establish shadow direction, add fill to open the shadows, then add a rim or spot for separation. Adding lights without a plan produces flat, shadowless images.
Assign materials before appearances
Set the engineering material first, so mass properties are correct, then adjust the appearance if the default render does not suit.
Mind the scale of textures
A texture applied at the wrong scale is the fastest way to make a render look synthetic. Check brushed-metal and knurl patterns against a known dimension on the part.
Ground the object
A shadow and a floor plane do more for believability than an increase in ray depth. Objects floating in a void read as computer graphics no matter how good the material is.
Key takeaways
- Rendering simulates the response of a model to a lighting environment; the scene and the material do most of the work.
- Ambient, directional, point and spot lights have distinct characters — light in layers rather than adding sources at random.
- Materials change engineering results; appearances change only the picture. Assign material first.
- Camera sleds give controlled, repeatable motion where free orbiting gives noise.
- Playback animation is built from key frames plus interpolation; the interpolation choice affects how physical the motion reads.
- Set render quality against the intended use before starting, not after.
Series
Continue the pathway
The SOLIDWORKS Design Approach series works through computer aided design as an engineering discipline, from first principles to manufacture.