Page 168 - Learn To Program With Scratch

Line Follower

LineFollower .sb2 Can we make a sprite follow (or trace) a path on the Stage, like the one
shown in Figure 6-24, all by itself? The answer is yes, and in this section,
we’ll write a program to do it. If you look closely at the sprite in the figure,
you’ll notice that we painted the nose and the two ears of the cat with dif-
ferent colors. The figure also shows an enlarged view of the cat’s head.

Left sensor
Middle sensor
Right sensor

Figure 6-24: Sample path for a sprite to follow

The plan is to use the cat’s nose and ears as color sensors for detecting
the black line underneath. Our algorithm for tracing the black line uses the
following heuristics (rules based primarily on logical reasoning and trial-
and-error experimentation):
• If the nose of the cat (pink color) is touching the line, move forward.
• If the left ear of the cat (yellow color) is touching the line, turn counter-
clockwise and move forward at a reduced speed.
• If the right ear of the cat (green color) is touching the line, turn clock-
wise and move forward at a reduced speed.

Of course, the exact speed (movement steps) and turning angles can
be different for different routes and have to be determined by experimenta-
tion. A script that implements the above algorithm and causes the sprite to
follow the line is shown in Figure 6-25.
The script in Figure 6-25 uses a new block: color is touching? (from
the Sensing palette). This block checks whether a color on the sprite (speci-
fied in the first color square) is touching another color (specified in the sec-
ond color square). If the specified color on the sprite is touching the other
color, the block returns true; otherwise, it returns false. The color in a color
square can be chosen by clicking on the color square and then clicking any-
where in the Scratch project to pick the desired color.

146 Chapter 6
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