CAP 201a - Computer Animation I

Lesson 8 - Using Compound Objects


This lesson discusses using multiple shapes to create an object. Objectives important to this lesson:

  1. Boolean operations with shapes
  2. The Loft tool
  3. The Scatter tool

The lesson begins by defining Boolean set operations as they are applied to shapes. It will be good to think about standard Boolean diagrams when thinking about these definitions:

  • Subtraction - an object made by removing a shape from another shape. In the image on the right, the overlapping portion of object B has been subtracted from object A. The result is only the red portion of object A in this case.

  • Intersection - the common volume formed by two objects that overlap in some way. In the image on the right, only the intersection of the two objects is shaded. The result is only the blue portion of the objects in this case.

  • Union - the total volume of two objects, including any intersection. In the image on the right, the union of the two objects includes each object and the intersection. No volume is represented twice. The lines defining the intersection have been lost.

  • Merge - like a Union, but all edges of the objects are retained.

Obviously, you have to create multiple objects in a scene to combine them in shapes like this.

The text moves on to describe the ProBoolean tool, an improved version of the earlier Boolean tool. You access it in the first exercise by going to the Create panel, choosing Geometry, choosing Compound Objects from the dropdown box, and clicking the ProBoolean button.

Boolean operators can be applied to objects and managed in a stack, like modifiers. Also like modifiers, they can be reordered in their history stack and deleted. In addition, an operator can be changed into a different operator.

Exercise Notes and Questions

Work through the exercises and turn in your answers to all questions below as part of the homework for this assignment.

Exercise 1: This exercise illustrates using set operations to make openings in a building. Work through it carefully, making sure you see what objects exist before you make any changes.
Question 1: After subtracting an object from another in steps 1 through 7, what does the text mean when it says we have a negative space?
Question 2: What is the effect of applying a Union operator to the upper and lower wall of the building?

The lesson continues with new material about Lofts.

  • A loft creates a 3D object from one or more shapes and a path.
  • Paths may be be open or closed.
  • The shape must be able to flow along the path without overlapping the space that it sweeps out at different points in the path. The text explains this by saying that the path should not have corners that are sharper that the shape that flows along it. Think of a pipe: you cannot bend a pipe so that the walls of the pipe overlap.
  • A loft can flow from one shape to another, or can flow through multiple shapes.
  • Shapes in a loft can be misaligned. Look for misaligned first vertices in the shapes as the most likely problem. Use the Compare tool to correct this problem. (This problem will occur in exercise 2.)
  • The Skin parameters rollout has two particular controls useful with lofts: The Shape steps control sets the number of faces around the shape. The Path steps control sets the number of faces along the path. Both controls affect the number of faces in the loft.

The lesson lists five deformation grids used with lofts, but demonstrates only one of them in the next exercise.

  • scale
  • twist
  • teeter
  • bevel
  • fit

Exercise 2: Exercise 2 walks you through a series of steps to create a screwdriver blade.
Question 3: You start with two shapes and a path. What are the two shapes?
Question 4: In step 14, you will create a fast transition between the two shapes. What makes the transition faster than the one shown in the image for step 12?
Question 5: In step 27, you see a twist in the object with the Compare tool. How do you recognize the first vertex of each shape in the Compare tool window?

The text briefly introduces a new tool before the third exercise: the Scatter tool. Its purpose is to create a number of duplicates of an object within a defined area. The area could be a large surface (like a landscape) or a specific part of your scene (like a narrow band on a helmet).

Exercise 3: This exercise has you use the Scatter tool to make lots of cacti (yes, that's the plural of cactus) in the landscape surrounding a version of the gas station.
Question 6: The exercise has you change the scatter (distribution) type to Random Faces to simulate a natural environment. What would happen if you were to use Even distribution instead?
Question 7: What setting is used to allow variety in the height of the duplicated cacti?