Pat Hanrahan's Tapestry Conference 2013 short story slide presentation

1. Showing is not Explaining
Pat Hanrahan
Stanford and Tableau
Tapestry

2. Euclid’s Algorithm*
*First known algorithm

3. Greatest Common Divisor
2, 3, 4, 6 divide 12
2, 3, 6, 7 divide 42
6 is the GCD

4. Euclid’s Algorithm
def gcd(a,b):
while b != 0:
if a > b:
a = a-b
else:
b = b-a
return a

5. Demonstration

6. Problems with Animation [Tversky et al.]
 Motion is fleeting and transient
 Cannot simultaneously attend to multiple motions
 People try to parse motion into events, actions and
behaviors
 We strive to infer causality and intention
 Anthropomorphizing: physical motion may cause
confusion or lead to incorrect conclusions
 Difficult to estimate paths and trajectories

7. Euclid’s Algorithm
def gcd(a,b):
while b != 0:
if a > b:
a = a-b
else:
b = b-a
return a
a=15, b=6
a=15
b=6

8. Euclid’s Algorithm
def gcd(a,b):
while b != 0:
if a > b:
a = a-b
else:
b = b-a
return a
a=9, b=6
a=15
b=6

9. Euclid’s Algorithm
def gcd(a,b):
while b != 0:
if a > b:
a = a-b
else:
b = b-a
return a
a=9, b=6
a=3, b=6
a=15
b=6

10. Euclid’s Algorithm
def gcd(a,b):
while b != 0:
if a > b:
a = a-b
else:
b = b-a
return a
a=9, b=6
a=3, b=6
a=3, b=3
a=15
b=6

11. Euclid’s Algorithm
def gcd(a,b):
while b != 0:
if a > b:
a = a-b
else:
b = b-a
return a
a=9, b=6
a=3, b=6
a=3, b=3
a=3, b=0
a=15
b=6

12. ?

13. Why Greatest Common Divisor?
Fact 1
Consider the answer r
a
b
r

14. Why Greatest Common Divisor?
Fact 1
Consider the answer r
r divides b
a
b
r

15. Why Greatest Common Divisor?
Fact 1
Consider the answer r
r divides b
Consider b
a
b
r

16. Why Greatest Common Divisor?
Fact 1
Consider the answer r
r divides b
Consider b
a
b
r

17. Why Greatest Common Divisor?
Fact 1
Consider the answer r
r divides b
Consider b
r divides a a
b
r

18. Why Greatest Common Divisor?
Fact 1
Consider the answer r
r divides b
Consider b
r divides a
Therefore, r divides a & b
a
b
r

19. Why Greatest Common Divisor?
Fact 1
Consider the answer r
r divides b
Consider b
b divides a
Therefore, r divides a & b
Hence, r <= GCD
a
b
r

20. Why Greatest Common Divisor?
Fact 2
Suppose d divides a
a

21. Why Greatest Common Divisor?
Fact 2
Suppose d divides a
Suppose d divides b
b

22. Why Greatest Common Divisor?
Fact 2
suppose d divides a & b
a
b

23. Why Greatest Common Divisor?
Fact 2
suppose d divides a & b
a
b
r

24. Why Greatest Common Divisor?
Fact 2
suppose d divides a & b
then d must divide r
a
b
r

25. Why Greatest Common Divisor?
Fact 2
suppose d divides a & b
then d must divide r
Any divisor d of a & b
must divide r a
b
r

26. Why Greatest Common Divisor?
Fact 2
suppose d divides a & b
then d must divide r
Any divisor d of a & b
must divide r
Therefore, GCD <= r
a
b
r

27. Why Greatest Common Divisor?
Fact 1: r <= GCD
Fact 2: GCD <= r
Therefore r = GCD
Cool math reasoning! a
b
r

28. Recording and Showing
Getting Easier
with
Computers

29. “Showing is not Explaining”
Thanks to Barbara Tversky
for explaining this maxim to me

31. Aftermath of Hurricane Katrina

35. By Emmett Mayer III

36. Principles
1. Perceiving is hard, graphics design to the
rescue
2. Need to add the invisible forces of cause
and effect, intention and consequences
3. People produce better visuals if they are
told to explain
Software needs to support these principles