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What are Imagine objects?

                   I.  What are Imagine objects?

When a computer program wants to draw a 3D object, it must have some way of internally representing it. Some modelers store each object as a bunch of 2 dimensional polygons- a 3D object is a formed from a whole bunch of these polygons pasted together. A cube might be defined as six 2D squares arranged in a group. Since our final picture just has to LOOK like it is solid, defining the outer surface is usually all we need to do to make it seem as if the objects ARE solid.

Any object can be defined as a bunch of flat polygons. Curved surfaces like a sphere can use a lot of polygons in order to approximate the surface closely; certain computer tricks (including a very important one called Phong shading) can smooth out the surface even more. Most of the 3D objects, or models, that you've ever seen in any 3D computer graphic were defined as polygons. Sometimes advanced programs define surfaces with a mathematical equation, or by a certain type of curve, and sometimes a computer model will have certain objects it "knows" how they should look (like a mathematically defined sphere or cone) but most use polygons for definition, Imagine included.

All objects in Imagine are defined as a bunch of triangles. Nothing more. It is particularly easy for a computer to decide what a triangle would look like when viewed as a 3D image. Any more complex polygon (like a square or octagon) can be broken down into a bunch of triangles pretty easily. Having only one "shape" to deal with is actually a convenience for us, as we don't have to worry about questions of what type of polygons a certain object is made of, or how to convert one type of polygon into another. The computer likes dealing only with triangles because it can optimize it's renderer, the program that actually draws the pictures, to expect and deal with just one shape simple instead of 246 different ones.

Although an object is made of only triangles (called FACES) it has points and edges which define where these faces go. If you think of a simple triangle, it has 3 defining points at the corners, three edges connecting these points, and one face which actually makes up the body of the triangle. Imagine can better deal with the objects by defining these sub-parts, and it allows us to manipulate the objects much more easily.

Every object has a number of defined POINTS. Imagine understands an EDGE to be a line segment that connects any two of these points. A face is defined by naming the three edges that make it up. Instead of storing nine numbers for each triangle (the X,Y,Z location of each corner) it just names the edges, which in turn name the points. This reduces the size of a description of an object considerably. It also helps in editing objects, since if you move a point, each face that it is part of will adjust itself to the include the new location of the point. The other alternative would be to have each face manually manipulated individually, which is obviously a big pain.

Think of a square. Imagine would store a square as two triangles that share one edge together. The square would actually contain FIVE edges (the four sides and the diagonal) and FOUR points (one at each of the corners.) It would have two faces, or triangles. A cube is stored as twelve faces, formed by eighteen edges, which are in turn defined by eight points.

This definition of objects actually gives us some extra leeway in how we define our model. Imagine doesn't require your object to be connected at all; that is, your object could be two completely separate surfaces that never touch. You might want an object to be a flying logo. The letters don't actually touch and form one solid object; they are independent from each other. Imagine doesn't care; you can call any collection of points, edges, and faces an object. Imagine also gives you tools for splitting off part of an object (like a letter) or joining two parts together.

Since this is a computer model and not a physical one, we can violate physics and have objects self-intersect. You might overlap two spheres half-way and join them together to form one object. You'll only see the outer surface when you render the new double-sphere object.

There actually are two objects that Imagine does not define as a group of points, edges, and faces: a perfect sphere and an infinite plane. These are the only exceptions to the normal definition of objects in Imagine. Well, OK, there's another. An axis containing NO points can still be manipulated as an object. It certainly won't show up in a render, but sometimes it's nice to use a lone axis as an invisible object in certain cases. You can also use the axis as the start of a brand new object.

There are certain "Editors" in Imagine that allow us to view and manipulate objects in different ways. Some editors let you place objects in scenes, or define how the objects change with time. The Detail Editor is where objects are usually created and modified. It allows low-level editing of objects; you can add points and faces by hand, move them, delete old ones and in general be as picky as you like in adjusting every point.

Defining objects point-by-point is obviously not very suited to complex objects, sometimes with THOUSANDS of points. There are more powerful controls that let you modify your object in more global ways. You can add pre-made 'primitive' objects like a cylinder or a torus (doughnut shape.) These primitive objects have the points, edges, and faces that define it already defined. There are certain tools that let you draw an outline, say the profile of a chess pawn, which is converted to a three-dimensional `spun' object, as if it was chiseled out on a lathe. Other tools let you slice off parts of your object using knives that you can build yourself. In general, object creation is done with these powerful tools, and picky touch-ups are the only time you grab and move individual points. A sculptor does not glue sand grains together!

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