3D Graphics Programming with QuickDraw 3D 1.5.4

About Transform Objects

A transform object (or, more briefly, a transform ) is an object that you can use to modify or transform the appearance or behavior of drawable QuickDraw 3D objects. You use transforms to reposition and reorient geometric shapes in space. Transforms are useful because they do not alter the geometric representation of objects (that is, the vertices or other values that define a geometric object); rather, they are applied as matrices at rendering time, temporarily "moving" an object in space. Thus you can reference a single object multiple times with different transforms and can place an object in many different locations within a model.

A transform is of type TQ3TransformObject , which is a type of shape object. QuickDraw 3D defines these basic types of transforms:

matrix transforms
translate transforms
scale transforms
rotate transforms
rotate-about-point transforms
rotate-about-axis transforms
quaternion transforms

No matter how you specify a transform, QuickDraw 3D maintains its data in that form until you begin to render an image, at which time it converts the data to a temporary matrix that is applied to the objects it governs. Because transforms are a type of shape object, you apply a transform by drawing it into a view or by putting it into a group. If you draw a transform in a view, you can use either retained or immediate transforms.

When you apply several transforms to a vector, the transform matrices are premultiplied to the vector. For example, in the multiplication v [A][B]...[M] of the vector v by the matrices A, B,..., M, matrix A is first applied to the vector, then B, and so forth. Accordingly, you should specify transforms to be concatenated in the reverse order that you want to apply them. This scheme is consistent with the application of matrices in a hierarchy, in which matrices at the top of a hierarchy are applied last.

For example, consider the very simple model illustrated in Figure 2 , which consists of three separate groups. A geometric object is first grouped with a scale and a translate transform (the translate transform was added to the group before the scale transform was added); the resulting group is then grouped with a rotate-about-axis transform, and that group is finally grouped with a second translate transform.

Figure 2 A simple model illustrating the order in which transforms are applied

When this model is rendered, the transforms are applied to the geometric object in this order: scale, translate (group 1), rotate-about-axis (group 2), translate (group 3). Your application should add transforms to a group in the reverse order they are to be rendered. That is, in the example, you would first add the translate transform to Group 1 and then add the scale transform.

For information about creating groups of QuickDraw 3D objects, see the chapter "Group Objects."

Spaces

Types of Transforms