September 3, 2014
Columnist Gerald Davis walks the reader through the steps needed to create a 3-D CAD movie that can be used to bring an assembly or product idea to life.
Before grabbing the mouse, we begin our 3-D CAD animation by preparing a written outline—a storyboard—of the scenes that are to be included in the movie. As we doodle on a page like that in Figure 1, we determine how each stage in the play will contribute to the overall impression the animated clip leaves.
Initially the storyboard might simply be a short list of topics that must be covered in the video clip. Those topics might be further outlined as sequences of scenes or perhaps just a single scene. A scene includes the background and camera point of view, as well as components that are animated to fade, appear, or move in some way.
The storyboard also will be used to “program” the 3-D CAD software. A timeline of events—similar to a player piano’s roll of holes—will control the animation. Each event is marked with a key point. Keys are used to set distances and angles between components. Key points are also placed to control cameras and component appearances. The software magically calculates the transitions between the key points. Those transitions are rendered to become individual frames in the video clip.
The 3-D CAD software used in support of this article is well-suited to working on individual scenes. Other software may be more efficient for composing the overall movie.
The events on the timeline depend on prior events. After we edit the keys to refine the movie, the event sequence may require recalculation, sometimes a time-consuming process.
The shorter the clip is, the quicker the recalculation will be. Thus, the experienced CAD jockey tries to anticipate the level of complexity. Perhaps multiple Motion Studies are in order.
In our scenario, the goal—also known as the complexity—is to show the observer how to mark a line at a 1.25-inch location on the workpiece using a square and a scribe. The movie clip produced for this scenario will be incorporated by others into a more comprehensive production.
The following is a refinement of the storyboard shown in Figure 1. Messages that are to be conveyed in the clip are:
This assembly has components that are mated to constrain how they move relative to each other. Relative motion between components can be simulated in two basic ways: real-time mouse drag and calculated Motion Study.
A Motion Study is similar to a scene in a movie in that several Motion Studies can be part of an assembly, just as several scenes can be found in a movie. To decide how many Motion Studies are likely to be needed, you might want to review the CAD tricks required for each scene.
1. Set the ruler to 1.25 in. Turn the nut counterclockwise to demonstrate “loosening” of the nut; slide the ruler to 1.25 in.; turn the nut clockwise to demonstrate “tightening” of the nut.
A set of three of the keys on the timeline of the Motion Study manage the start/stop of the nut’s motion. Figure 2 provides more detail. In this demo, those timeline keys control a distance-mating relationship—or distance-mate—between the nut and the ruler clamp. This model features a screw-mate between those same two components. The result is that the nut turns as the distance changes. After the nut’s distance-mate is changed, another key changes the distance-mate between the end of the ruler and the face of the head casting to 1.25 in.
In addition to component motion, this scene should give the observer a clear view of the ruler scale and the nut before the nut starts turning. The scene then calls for a zoom-in for a clear shot of the ruler scale at 1.25 in.
A few more timeline keys are needed to control the observer’s point of view by moving the camera.
2. Demonstrate the proper positioning of the square on the workpiece (see Figure 3). Zoom in to show the square and workpiece separated a short distance from each other, and move the square toward the workpiece until they touch.
Several keys are needed to change the point of view to follow the action. The goal is to make it clear that the head casting is resting against the workpiece’s face and that the ruler is also in good contact with the workpiece.
A pair of keys to set the starting and ending distance between the workpiece and the ruler takes care of the motion (see Figure 4).
3. Fetch the scribe and scratch a line at the end of the ruler. Demonstrate the tip of the scribe moving against the face of the ruler.
Three path-mates are used to control the flight of the scribe. Path-mates involve some setup, such as 3-D sketching and mating, before launching the Motion Study editor. We’ll use the same 3-D sketch and mate to it three different ways. Any of those three mates simultaneously being active causes mate conflict. To avoid that, we suppress them after creating them; that is the same as “turning them off” in Motion Study speak.
Back in the Motion Study editor we then can add a few timeline keys to turn on only the appropriate path-mate during the desired time period in the video clip (see Figure 5). With one of the three path-mates turned on, another corresponding key pair controls the scribe’s starting and ending distance along the 3-D sketch path.
A docking path-mate flies the scribe in and out of the square’s head. This path-mate has a pitch control setting that keeps it moving horizontally as the distance along the path is changed.
A flying path-mate controls the flight of the scribe into approximate position for scratching. The orientation of the scribe during this stage is not important. This path-mate allows free pitch and roll of the scribe, giving a fluid motion along the 3-D path.
A scratching path-mate moves the scribe along the end of the ruler. The scratch scene matters. The scribe must tilt its tip to make realistic contact with the ruler. This path-mate locks the Z axis of the probe at an angle to keep it away from the face of the ruler. In addition to credible motion for the scribe, a scratch is to appear in the model of the workpiece.
4. Demonstrate the emerging scratch in the workpiece. As the scribe moves along its path, extend the length of the scratch in the workpiece.
The scene for the emergence of a scratch is shown in Figure 6. The technique is to model a cut-extrude up to a plane in a featureless model and to control the position of that invisible model with distance-mate timeline keys. It might be difficult to spot in Figure 6, but an emerging scratch is visible in the body of the workpiece model.
5. Demonstrate what a pristine workpiece looks like (see Figure 7a), as well as emphasize the completed scratch (see Figure 7b). To hide unwanted edges and faces, model yet another component to parametrically match the volume of the scratched groove.
Not only does it hide the not-zero-length cut-extrude, the timeline keys make this component’s appearance glow brightly to emphasize the existence of the scratch.
6. The finale changes the appearance of the square to become fully transparent (see Figure 8), which effectively hides it. A pair of timeline keys allows that transparency. This scene also zooms in to emphasize the lovely new scratch in the workpiece, which the last couple of keys enable.
The Final Cut
This scene/CAD review is summarized as six scenes spanning approximately 40 timeline keys. On the average, that’s about 10 timeline keys per scene. From personal experience, a 50-key limit on an old and slow laptop is about as complex as a single Motion Study needs to be. Motion Studies are really limited only by the hardware resources of a workstation. The amount of time spent waiting for the clip to recalculate can be significant.
How long does it take to populate a timeline with keys? In general, this sort of project takes at least a half-day and probably consumes a full day’s attention.
Although it is a simple and easy drag-and-drop mouse procedure to place or edit timeline keys, a CAD jockey requires discipline to stop reviewing the animation and re-editing the keys once they exist. Here’s a tip for speeding the key-placement process: Before launching the Motion Study editor, open the Mates folder and use the mouse to drag the angle- and distance-mates that are to be controlled dynamically in the Motion Study to the top of the list in the Mates folder. Give them useful names, too. For example, rename “Distance8” to “Nut Distance.” (See Figure 4 for an example of reordered and renamed mates.)
The more specific the planned storyboard is, the easier it is to set up the Motion Study. It is helpful if the storyboard addresses details such as the observer’s point of view, scale of view, background and shadow settings, component visibility, component motion, and the rate of change for these animated elements.
Returning to our scenario, the CAD jockey/movie director has refined the storyboard into six scenes, but might now employ the mouse to deploy a few experimental keys on the timeline to evaluate changes in camera aim.
Let’s start with a scene showing only a pristine workpiece:
When played, this preliminary storyboard clip appears to fly the observer around a scene. The visible components do not change appearance or move. This is a good opportunity to work out the timing and transition between these major points of view.
The next layer of key points to be added to the timeline will control the motion and appearance of components. When an object appears out of nowhere, the event helps to draw the observer’s attention. It is therefore important to consider the impact of simultaneous motion with fade-in. Components that remain stationary while materializing give the observer time to assimilate the arrival of something new. Subsequent motion of the object will help to maintain the viewer’s attention to that object.
This recommendation is made in the theme of expository cinema, not action/drama. A dramatic animation might feature a materializing part that is already flying along a path. The observer might be startled. Timing and sequence of keys on the timeline make all the difference in what impression is left with the observer.To control the observer’s point of view, the director has the opportunity to employ as many cameras as are desired. The default graphics window is simple and has controls to turn the perspective view on. CAD jockeys/movie directors are encouraged to set up additional software cameras that allow photorealistic control, including depth of field and aiming. These cameras can be conveniently selected and controlled precisely with timeline keys.
In review, we started with a jotted storyboard and used it to place keys on the timeline to sequence the main scenes in the clip. We then revisited each of those scenes and changed the appearance and motion of components to comport with the storyboard. We’ve previewed the animation and are satisfied with the sequence and timing.
The next step is to generate a video file for distribution to our observers (see Figure 9a). Decisions about video file type (default AVI), frame rate (usually 30 frames per second), and graphics rendering are made at this step. If AVI format is selected, the AVI compression defaults are generally useful for playback on Microsoft operating systems (see Figure 9b). Other file types and export options may be better-suited for use with video editing software. For example, the uncompressed raw frames produce large file size, but possibly better import into the video editor.
Gerald would love to have you send him your comments and questions. You are not alone, and the problems you face often are shared by others. Share the grief, and perhaps we will all share in the joy of finding answers. Please send your questions and comments to email@example.com.
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