Attributes and Behaviors of Primitives - Essentials of Interactive Computer Graphics - page 149

for approximating the outline of connected shapes (e.g., approximating the
outline of a circle).

A line segment has two types of attributes: per-vertex and per-drawing command
attributes.

Figure 7.2. Gouraud
shaded line with a black
endpoint and a white end-
point.

Per-vertex attributes. These are attributes that can be different for each vertex.
Examples of per-vertex attributes include the following.

• Color. This defines the color for the vertex. When the endpoints of a line
have different colors, the graphics hardware can perform Gouraud shad-
ing and linearly interpolate the color while drawing the line segment. For Gouraud shading. Linear

color interpolation between
endpoints with different color
values.

example, a line segment can be set to have a white endpoint and a black
endpoint. When Gouraud shading is enabled, the graphics hardware will
draw a line where the color of the line changes gradually from white to
black along the line. Figure 7.2 shows such a line.

• Texture coordinate. Similar to a point's texture coordinate attribute, when
texture mapping is enabled, this attribute allows us to control the color
along the line. Once again, we will study texture mapping in Chapter 12.

• Normal vector. This attribute supports illumination computation for the

Illumination computation.
Compute the color of an
object based on its material
properties

line, that is, how a light source illuminates the line. We will describe illu-
mination models and computation in Appendix A.

and geometric

relationship with light

sources.

Per-drawing command attributes. These are attributes that are associated with
each drawing command. For example, the following attributes will be applied to
all the line segments in a polyline drawing command.

• Line color. When Gouraud shading, illumination computation, and texture
mapping are all disabled, this color defines the color of the entire line (or Flat shading. Filling the inte-

rior of a geometric shape (e.g.,
line or triangle) with the same
color.

line segments for a polyline).

• Material property. When illumination computation is enabled, this at-
tribute defines how the line segments will be illuminated by light sources.
Once again, we will learn about illumination models and illumination com-
putation in Appendix A.

• Line style/width. These define the style (e.g., dotted versus solid line) and
width of a line.