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FORMS EDITOR

   * Subject: Forms Editor Tutorial (was: Imagine WWW site)
   * Date: Sun, 12 Feb 1995 23:11:01 -0500 (EST) [59]
   * From: jgoldman@acs.bu.edu

I posted a small Getting to Know the Forms Editor type of thing a while back. Let me see here... Ahhh...

---- Forms Editor thing...----

This is my advice to anyone interested in the forms editor. A lot of this may be elemantary, but some info might actually be useful... This might definitely (good grammar?) get confusing... It's also rather long...

The forms editor is one of Imagine's greatest features. I find it's easier to create incredibly realistic things in the forms editor than with NURBS (ooohhh, imagine a spline based forms editor)...

Basically, when you create in the forms editor you edit an object's profile. It's sort of like MRIs (Magnetic Resonance Imaging)

A Basic Form Example-

Take a simple example, your thumb. Treat your thumb as if it were an object on an axis. Say that the Z-axis runs from the base of your thumb to the tip. The X-axis runs across the top of your thumb. The Y-axis goes through your thumb. In other words, if your thumb were an Imagine object it would be oriented as if you were viewing a painting with your thumb out in front of you...

First trace the right profile of your thumb. If you want to do this the extra step way, lay your thumb down on a piece of paper and trace it for all three views (you'll have troubles with the top view, so just eye it.). Then, just copy what you see on paper to the screen. Do this for all three views. What you get is a pretty crude looking thumb. To get a better looking thumb you have to go in and edit individual cross-sections. But first...

Remember that if the object is solid you must have a totally enclosed profile (the endpoints are separated normally. You have to place the endpoint points at the same location. An easy way to do this is to select the cursor lock and lock the endpoints to similar grid locations).

In the case of the heart, do the same thing as the thumb...

What the Forms Editor Really Does-

It's important to understand what exactly you're doing when manipulating points in the forms editor, so go back to a simple sphere (new object and new defaults). Turn on the solid wireframe option and view the sphere object from the front. Notice how many points are on your front view profile. You will see that they equal the number of vertical sections in the wireframe object divided by two. Every point in an editor view has a twin on the opposite side of the object (except for the top view). If you add a point in the front view you will notice that two points are actually added. One on one side of the profile, and one on the other side. When altering an object's profile you actually directly manipulate a specific point, but because of the forms editor's nature it changes other points around it to tween and smooth the change.

Editing Individual Cross-Sections-

Look at a forms object like several horizontal cross-sections (which are circular by default) connected like skin. Think of the top view as the cross-section editor. By default only one cross-section can be manipulated, the top one. It can be considered a controlling, or key, cross-section. Because it is the only key cross-section defined on the object then whatever you change will affect every cross-section below it.

To have ultimate control of the individual cross-sections you have to define the cross-section you want to manipulate as a key cross-section. You do this by selecting '+ KEY' and selecting the cross-section you want to manipulate (in the front or right view). When you perform this operation (or 'Select') Imagine will display which cross-sections have been defined as key cross-sections in red. At first, you will notice that only the top cross-section is highlighted in red because no others have been selected to act as key cross-sections. Make the middle cross-section a key cross-section. The object now has two cross-sections which can be separately manipulated as keys. Use 'Select' to pick the cross-section you want to alter.

Think of key cross-sections as animation frames. In an animation you can set an initial postion for an object on one frame and a final position on another frame. These two frames are now key frames. When the animation is generated the object's position will tween between the two key frames. The same process occurs with key cross-sections. All non-key cross-sections tween between key cross-sections.

Take an example of a salt shaker with a circular top and square bottom. If you create a circular key cross-section as the top of the shaker and a square key cross-section for the bottom all the cross-sections between the top and bottom will blend to form a smooth transition from cirle to square.

So now you know how to alter the individual cross-sections. Remember that you can also delete key key cross-sections. Just be sure you select the appropriate cross-section. BTW, Imagine tells you waht cross-section you're manipulating in the upper menu bar. Also, remember that Imagine's cross-section tween goes from key to key in a downward order. If there is no key cross-section at the object's base Imagine will use the next highest defined key section.

So Where do the Front/Right Views Come into Play?-

If the top editor edits individual cross-sections then the front and side view editors locate the cross-sections in space. The default sphere has all the cross-sections parallel to each other. You can change the orientation of a cross-section (so that it is not parallel to the ground) by moving points on the front and right views. Basically, the front and side view alter an objects overall profile shape.

This particular explanation only really works with the default sphere, but the results can be translated to any object. The front view controls all the points along the object's x-axis. The side view controls all the points along the object's y-axis. This effectively splits the sphere into four sections; a front profile, back profile, left profile, and right profile.

In the front view select the right-most point and you'll notice that it's twin point will be highlighted on the left. These two points define the right-most and left-most points of one cross-section. Specifically, these two points represent two outermost x-axis points on the middle cross-section.

Similarly, the same two points in the side view control the farthest points on the y-axis of the middle cross-section.

You will find that by altering these you alter the orientation of the cross-section. If you raise the right-most point in the front view and lower the left-most point the middle cross-section now tilts.

What Does It All Mean?-

It's tough to explain exactly how the forms editor operates, and in particular what alters what exactly. So my greatest advice is to experiment.

The easiest way to create object was illustrated in the first section. Create profiles for the object you want to create. If you need better form edit the individual cross-section as if you were manipulating spars on and airplane wing or bulkheads on a boat.

I can tell you now that only experience can show you what will go wrong, and believe me something will go wrong. I can only say, keep things simple. You probably won't understand this now, but try and keep cross-sections relatively parallel to each other. If this means adding more cross-sections then do it. You'll find that some really complex things can't be done with small numbers of cross-sections, and it's important to realize when you need more...

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