Operation Manual

A beginner’s guide to Scratch
39
Once the program can detect two colours touching, then we can tell it how to
respond when this happens. The colour event becomes a trigger, causing the
program to do something.
To set the colours in your block, click on one of the little coloured squares. Your
mouse-pointer will turn into an “eye-dropper”. Use the eye-dropper to click on the
colours you want to use in your block.
But how can we use the ability to detect colour to help make our LFV follow a line
on the road? Well, let’s start by importing our “line_background” and the sprite
“yellow_car”.
Notes:
Tip...
If you can’t stop
your car for long
enough to sample
a colour, hit the
red “stop scripts”
button at the
top-right of the
screen.
Look closely at the car, and you’ll see that there’s a patch of green on its bumper.
We’re going to use that green as our first “sensor”. Build the script that you can
see in the next screenshot.
Now, click on the green flag and see what happens. Oh dear, everything is fine as
long as the line curves to the left but as soon as it curves to the right the car
wanders off. What’s going on?
The problem is that the car needs two sensors. Currently, if the line bends to the
right, the black line will touch the green sensor – the program will detect this and
tell the car to turn to the right.