1
00:00:00,000 --> 00:00:00,530


2
00:00:00,530 --> 00:00:06,050
Obscuring edges, convex
edges, and concave edges.

3
00:00:06,050 --> 00:00:09,690


4
00:00:09,690 --> 00:00:12,120
Physical constraints
limit the number of ways

5
00:00:12,120 --> 00:00:15,210
that scene features
can form vertices.

6
00:00:15,210 --> 00:00:18,660
Most combinations of line labels
simply cannot occur at any

7
00:00:18,660 --> 00:00:21,240
junction.

8
00:00:21,240 --> 00:00:25,020
Furthermore, labeling for
adjacent junctions must match

9
00:00:25,020 --> 00:00:28,800
because the nature of a line
cannot change along its length.

10
00:00:28,800 --> 00:00:30,450
This makes the
selection of labeling

11
00:00:30,450 --> 00:00:33,270
as the assembly
of a jigsaw puzzle

12
00:00:33,270 --> 00:00:37,800
because previous selections
strongly limit each new choice.

13
00:00:37,800 --> 00:00:40,080
In our sample scene
the L junction

14
00:00:40,080 --> 00:00:44,010
starts with the complete
set of 123 possible labels

15
00:00:44,010 --> 00:00:46,680
and this number is
immediately reduced to 76

16
00:00:46,680 --> 00:00:49,110
because the relative region
brightnesses precludes

17
00:00:49,110 --> 00:00:52,800
certain shadow labelings.

18
00:00:52,800 --> 00:00:55,140
Next the mutual constraints
between junctions

19
00:00:55,140 --> 00:00:57,480
have an effect the
hourly feelings

20
00:00:57,480 --> 00:01:00,930
are reduced from the initial
79 to the 52 compatible

21
00:01:00,930 --> 00:01:03,360
with the neighboring L.

22
00:01:03,360 --> 00:01:06,690
Similarly, only 32 of
the options for the L

23
00:01:06,690 --> 00:01:09,060
are now compatible with the
remaining arrow labeling.

24
00:01:09,060 --> 00:01:14,700


25
00:01:14,700 --> 00:01:17,880
Sometimes the analysis
of a particular junction

26
00:01:17,880 --> 00:01:19,560
can cause and
effect to propagate

27
00:01:19,560 --> 00:01:24,490
for a considerable
distance paths propagate

28
00:01:24,490 --> 00:01:26,560
through the drawing
until activity

29
00:01:26,560 --> 00:01:28,960
dies at junctions where
no new changes are forced.

30
00:01:28,960 --> 00:02:07,700


31
00:02:07,700 --> 00:02:10,190
By the time the last
junction is considered

32
00:02:10,190 --> 00:02:11,900
the overall
arrangement typically

33
00:02:11,900 --> 00:02:14,960
forces a single interpretation
for the entire line drawing.

34
00:02:14,960 --> 00:02:23,620


35
00:02:23,620 --> 00:02:26,650
After determining basic
facts about the lines

36
00:02:26,650 --> 00:02:28,510
other specialist
programs attempt

37
00:02:28,510 --> 00:02:32,370
to find the identity,
location, and dimensions.