Arrows

Section 5.2 Arrows

Arrows can appear in many different contexts within a PreFigure diagram. For instance, a <vector> will be drawn with an arrowhead, usually at its tip, as seen in Figure 5.2.1. Most graphical components that define a path, such as a <polygon> and <parametric-curve>, can be decorated with arrows.

Figure 5.2.1. Arrows that decorate vectors.

In constrast, a <slope-field> may have many arrows, one on the end of each line segment, as shown in Figure 5.2.2 .

Figure 5.2.2. Adapted from Tom Judson’s Ordinary Differential Equations Project
¹
`judsonbooks.org/ode-project/`
.

As a result, it is possible to adjust the relative size and shape of arrowheads. Before describing the possibilities, however, it is important to recognize that the size of arrowheads is scaled by the @thickness attribute so, for instance, the arrowhead on a line whose @thickness=3 will be three times the size as one on a line whose @thickness=1.

Figure 5.2.3 shows a variety of choices for defining the appearance of arrowheads. Notice that there are two attributes, @arrow-width and @arrow-angles, that can be specified and that are described more fully below.

Figure 5.2.3. A collection of arrows demonstrating various options.

Listing 5.2.4. The PreFigure source for Figure 5.2.3.

<diagram dimensions="(400,400)" margins="5">
  <coordinates bbox="(0,0,10,10)">
    <grid-axes labels="no"/>
    <!-- left side arrows -->
    <line endpoints="((1,9),(4,9))" arrows="2"/>
    <line endpoints="((1,8),(4,8))" arrows="2"
	  arrow-width="5"/>
    <line endpoints="((1,7),(4,7))" arrows="2"
	  arrow-width="6"/>
    <line endpoints="((1,6),(4,6))" arrows="2"
	  arrow-width="7"/>
    <line endpoints="((1,5),(4,5))" arrows="2"
	  arrow-width="8"/>
    <line endpoints="((1,4),(4,4))" arrows="2"
	  arrow-width="9"/>

    <!-- right side arrows -->
    <line endpoints="((6,9),(9,9))" arrows="2"
	  arrow-width="5" arrow-angles="(35,60)"/>
    <line endpoints="((6,8),(9,8))" arrows="2"
	  arrow-width="5" arrow-angles="(40,60)"/>
    <line endpoints="((6,7),(9,7))" arrows="2"
	  arrow-width="6" arrow-angles="(45,60)"/>
    <line endpoints="((6,6),(9,6))" arrows="2"
	  arrow-width="7" arrow-angles="(35,80)"/>
    <line endpoints="((6,5),(9,5))" arrows="2"
	  arrow-width="8" arrow-angles="(40,80)"/>
    <line endpoints="((6,4),(9,4))" stroke="red" arrows="2"
	  arrow-width="9" arrow-angles="(40,80)"/>

    <!-- longer line at the bottom -->
    <line endpoints="((1,3),(9,3))" arrows="2"
	  additional-arrows="(0.7,0.5)"
	  arrow-width="9" arrow-angles="(60,90)"/>

    <line endpoints="((1,2),(9,2))" arrows="2"
	  arrow-width="9" arrow-angles="(30,80)"/>

    <line endpoints="((1,1),(9,1))" arrows="2"
	  arrow-width="9" arrow-angles="(10,90)"
	  additional-arrows="0.5"/>

  </coordinates>
</diagram>

Notice that a <line> has an @additional-arrows attribute that allows one to include arrowheads at various locations along the line. The locations are given as a number between 0 and 1 specifying the relative distance along the line segment.

The shape of an arrowhead and how it is attached to the end of a path is shown in Figure 5.2.5. Once again, the size of the arrowhead is scaled by the @thickness of the path. (This style of arrowhead is adapted from Bill Casselman’s PiScript

package.)

Figure 5.2.5. The shape of an arrowhead and its attachment to the path.

Figure 5.2.6 shows the meaning of the @angle-width and @angle-angles attributes. The @angle-width, whose value is in SVG coordinates and whose default is 4, controls how far from the path the arrowhead extends. The @angle-angles attribute is a pair of angles \((A,B)\text{,}\) given in degrees. The default values are \((24,60)\text{.}\)

Figure 5.2.6. The definitions of @arrow-width=w and @arrow-angles=(A,B).

Defining these attributes in a publication file allows them to be applied to all the diagrams in a project.

As we will see later, these attributes are ignored when placing arrowheads in a tactile diagram since the shape of the arrowheads is a convention within the braille community.

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