Geometric Shape Geometries (IM: XG)

Inside Macintosh: QuickDraw GX Graphics /: Chapter 1 - Introduction to QuickDraw GX Graphics / Geometric Shapes

Geometric Shape Geometries
Each type of geometric shape uses the geometry property of its shape object in a slightly different manner. For example, empty shapes and full shapes store no information in their geometry, because they require no further geometry information--their shape type says it all.
However, for other types of geometric shapes, the shape type does not contain all the geometry information necessary to define the shape. The geometries of these shapes contain (x, y) coordinate pairs called geometric points--points that specify the location, dimension, and form of the geometric shapes:

Point geometries contain one geometric point--an x-coordinate and a y-coordinate--to specify the position of the point shape.
Line geometries contain two geometric points--one point to specify where the line starts and one to specify where the line ends.
Rectangle geometries also contain two geometric points--one point to specify one corner of the rectangle, and another point to specify the opposing corner.
Curve shapes store three geometric points in their geometry--one to specify where the curve starts, another to specify where the curve ends, and another, called the off-curve control point, to specify the tangents used to define the curve.
Polygon geometries are made up of zero, one, or more polygon contours. Each polygon contour is series of geometric points connected by straight edges.
Path geometries are similiar to polygon geometries, but path geometries also store information about which geometric points are on-curve and which are off-curve control points. Therefore, path contours can have curves as well as straight lines.

For more information about the geometries of each geometric shape type, see the chapter "Goemetric Shapes" in this book.
Figure 1-3 shows a polygon shape with a single polygon contour made up of three geometric points. This figure shows three views of the polygon geometry: as a list of (x, y) coordinate pairs, as three geometric points plotted on a geometric grid, and as three points connected by three straight lines. This third way of viewing geometries is used frequently throughout this book, as it shows not only the geometric points, but also the implied edges that connect them. Notice that geometric points have fixed-point coordinates--you can specify fractional positions.
Figure 1-3 A polygon shape with a single polygon contour containing three geometric points