Answers

4.4 Colour

Vision · Robot club · about 20 min

What this lesson is about

The blob detector: one colour at a time, area and box width as distance.

• Pick a colour
• Which side?
• How far?
• Follow the colour

Questions 7 marks in all

1. [1 mark]A blob is [cx, cy, area, x0, y0, x1, y1, aspect]. What is blob[2]?

   1. AThe area, in pixels
   2. BThe distance, in cm
   3. CThe left edge of its box
   4. DIts colour
   Answer: A. Index 2 is the area: how many pixels it covers. There is no distance in a blob.

2. [1 mark]What does this program print?

   blob = [220, 110, 900, 205, 95, 235, 125, 1.0]
   width = blob[5] - blob[3]
   print(width)
   print("left" if blob[0] < 160 else "right")

   Answer:

   30
   right

   The box width is x1 - x0, 235 - 205 = 30. cx is 220, over 160, so the blob is on the right.

3. [1 mark]The program has read the red blobs. How does it read the blue ones?

   1. ACall set_cv("blob", "blue"), then blobs() again
   2. BCall blobs("blue")
   3. CLook further down the list from blobs()
   4. DCall set_cv("blue")
   Answer: A. The blob detector looks for one colour at a time. Switch colour with set_cv, then read again.

4. [1 mark]Why is the box width a better guide to distance than the area?

   1. AArea grows with the square of the size, so it changes much faster
   2. BArea is always 0 for a ball
   3. CWidth is measured in cm
   4. DWidth does not change as the ball gets closer
   Answer: A. Twice as close looks twice as wide but four times the area. Width goes up steadily, which is easier to use.

5. [1 mark]What does this program print?

   red = [100, 120, 400, 90, 110, 110, 130, 1.0]
   blue = [250, 118, 1600, 230, 98, 270, 138, 1.0]
   balls = {"red": red, "blue": blue}
   nearer = max(balls, key=lambda name: balls[name][5] - balls[name][3])
   print(nearer, "is nearer")

   Answer:

   blue is nearer

   Red is 20 pixels wide and blue is 40. Same size ball, looks bigger, so blue is the nearer one.

6. [1 mark]A blob's box is 60 pixels wide and 20 pixels high. What is its aspect?

   Answer: 3. Aspect is width divided by height: 60 / 20 = 3. A ball is about 1, so this looks more like a stick lying down.

7. [1 mark]Why does a blob have no distance, when a tag does?

   1. AThe detector does not know how big the red thing really is
   2. BThe camera cannot measure distance at all
   3. CColours are always far away
   4. DYou have to call distance() inside blobs()
   Answer: A. A tag has a known printed size, so the detector can work out distance. For a blob, only you know the real size.

The task: red or blue

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

# camera: look for red blobs
set_cv("blob", "red")
print(blobs())

from bugbot import *
connect()
set_cv('blob', 'red')
red = blobs()[0]
set_cv('blob', 'blue')
blue = blobs()[0]
if red[0] < blue[0]:
    print('left: red')
    print('right: blue')
else:
    print('left: blue')
    print('right: red')