mirror of
https://gitee.com/openharmony/third_party_ffmpeg
synced 2024-11-25 12:19:52 +00:00
110 lines
2.7 KiB
Plaintext
110 lines
2.7 KiB
Plaintext
This is a quick description of the viterbi aka dynamic programing
|
|
algorthm.
|
|
|
|
Its reason for existence is that wikipedia has become very poor on
|
|
describing algorithms in a way that makes it useable for understanding
|
|
them or anything else actually. It tends now to describe the very same
|
|
algorithm under 50 different names and pages with few understandable
|
|
by even people who fully understand the algorithm and the theory behind.
|
|
|
|
Problem description: (that is what it can solve)
|
|
assume we have a 2d table, or you could call it a graph or matrix if you
|
|
prefer
|
|
|
|
O O O O O O O
|
|
|
|
O O O O O O O
|
|
|
|
O O O O O O O
|
|
|
|
O O O O O O O
|
|
|
|
|
|
That table has edges connecting points from each column to the next column
|
|
and each edge has a score like: (only some edge and scores shown to keep it
|
|
readable)
|
|
|
|
|
|
O--5--O-----O-----O-----O-----O
|
|
2 / 7 / \ / \ / \ /
|
|
\ / \ / \ / \ / \ /
|
|
O7-/--O--/--O--/--O--/--O--/--O
|
|
\/ \/ 1/ \/ \/ \/ \/ \/ \/ \/
|
|
/\ /\ 2\ /\ /\ /\ /\ /\ /\ /\
|
|
O3-/--O--/--O--/--O--/--O--/--O
|
|
/ \ / \ / \ / \ / \
|
|
1 \ 9 \ / \ / \ / \
|
|
O--2--O--1--O--5--O--3--O--8--O
|
|
|
|
|
|
|
|
Our goal is to find a path from left to right through it which
|
|
minimizes the sum of the score of all edges.
|
|
(and of course left/right is just a convention here it could be top down too)
|
|
Similarly the minimum could be the maximum by just fliping the sign,
|
|
Example of a path with scores:
|
|
|
|
O O O O O O O
|
|
|
|
>---O. O O .O-2-O O O
|
|
5. .7 .
|
|
O O-1-O O O 8 O O
|
|
.
|
|
O O O O O O-1-O---> (sum here is 24)
|
|
|
|
|
|
The viterbi algorthm now solves this simply column by column
|
|
For the previous column each point has a best path and a associated
|
|
score:
|
|
|
|
O-----5 O
|
|
\
|
|
\
|
|
O \ 1 O
|
|
\/
|
|
/\
|
|
O / 2 O
|
|
/
|
|
/
|
|
O-----2 O
|
|
|
|
|
|
To move one column forward we just need to find the best path and associated
|
|
scores for the next column
|
|
here are some edges we could choose from:
|
|
|
|
|
|
O-----5--3--O
|
|
\ \8
|
|
\ \
|
|
O \ 1--9--O
|
|
\/ \3
|
|
/\ \
|
|
O / 2--1--O
|
|
/ \2
|
|
/ \
|
|
O-----2--4--O
|
|
|
|
Finding the new best pathes and scores for each point of our new column is
|
|
trivial given we know the previous column best pathes and scores:
|
|
|
|
O-----0-----8
|
|
\
|
|
\
|
|
O \ 0----10
|
|
\/
|
|
/\
|
|
O / 0-----3
|
|
/ \
|
|
/ \
|
|
O 0 4
|
|
|
|
|
|
the viterbi algorthm continues exactly like this column for column until the
|
|
end and then just picks the path with the best score (above that would be the
|
|
one with score 3)
|
|
|
|
|
|
Author: Michael niedermayer
|
|
Copyright LGPL
|