Klopfcode

2007-09-20T13:26:07Z

138.251.155.236:

The '''Tap Code''' is a [[cipher]], commonly used by prisoners in jail to communicate with one another. The method of communicating is usually by "tapping" either the metal bars or the walls inside the cell, hence its name. It is a very simple cipher, not meant to avoid interception, since the messages are sent in [[cleartext]].

It was reportedly invented in June [[1965]] by four [[POW]]s imprisoned in [[Hoa Lo]], [[Vietnam]]: [[Captain]] Carlyle ("Smitty") Harris, [[Lieutenant]] Phillip Butler, Lieutenant Robert Peel and [[Lieutenant Commander]] Robert Shumaker. Harris remembered an [[United States Air Force|Air Force]] instructor who had shown him a cipher based on a [[Polybius square]], as shown on the graph below. Each letter was communicated by tapping two numbers: the first designated the row (horizontal) and the second designated the column (vertical). The letter "X" was used to break up sentences and the letter "C" replaced the letter "K".

The Tap Code is featured in [[Arthur Koestler]]'s classic work ''[[Darkness at Noon]]'', which was published in 1941. The prisoners refer to it as the knock code, but the technique is still the same.

The Tap Code is outlined in the table below:

::{| border="2" cellpadding="10" cellspacing="0" style="margin: 0 0 1em 0.5em; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse; font-size: 95%;"
|- bgcolor="lightgray"
! !! 1 !! 2 !! 3 !! 4 !! 5
|-
!bgcolor="lightgray"| 1
| A || B || C || D || E
|-
!bgcolor="lightgray"| 2
| F || G || H || I || J
|-
!bgcolor="lightgray"| 3
| L || M || N || O || P
|-
!bgcolor="lightgray"| 4
| Q || R || S || T || U
|-
!bgcolor="lightgray"| 5
| V || W || X || Y || Z
|}

For example, to specify the letter "A", you would tap roughly the following: . .

Or to communicate the word "WATER" the cipher would be the following (the time between each pair of numbers is smaller than the one between two different letters):

..... .. . . .... .... . ..... .... ..
(5,2) (1,1) (4,4) (1,5) (4,2)
W A T E R

Because of the difficulty and length of time required for specifying a single letter, most prisoners devised [[abbreviations]] and [[acronyms]] for common items or phrases, such as "GN" for ''Good Night'', or "GBU" for ''God Bless You''.

There is however the possibility to do with less taps. First one can arrange the letters in a diagonal fashion. The code shown below never needs more than 8 taps per letter opposed to letters with 9 and 10 taps in the "standard" tap code. Second you can exploit letter frequency as in [[Morse code]]. Obviously E should be the code with two taps, not A.

::{| border="2" cellpadding="10" cellspacing="0" style="margin: 0 0 1em 0.5em; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse; font-size: 95%;"
|- bgcolor="lightgray"
! !! 1 !! 2 !! 3 !! 4 !! 5 !! 6 !! 7
|-
!bgcolor="lightgray"| 1
| E || A || N || D || W || V || ?
|-
!bgcolor="lightgray"| 2
| T || I || R || M || B || .
|-
!bgcolor="lightgray"| 3
| O || H || U || P || Z ||
|-
!bgcolor="lightgray"| 4
| S || C || Y || Q || ||
|-
!bgcolor="lightgray"| 5
| L || G || X || || ||
|-
!bgcolor="lightgray"| 6
| F || J || || || ||
|-
!bgcolor="lightgray"| 7
| K || || || || ||
|}

Here is the word WATER with the standard tap code and the modified version.

..... .. . . .... .... . ..... .... ..
..... . .. . . .. . . ... ..

== See also ==
* [[Knock three times (disambiguation)]]

==References==
* [http://www.airsoftgent.be/dbase/tapcode.htm The Tap Code]
* [http://www.braingle.com/brainteasers/codes/tapcode.php Online Tap Code Encoder/Decoder]
* Koestler, Arthur, ''Darkness at Noon'' (1941). Translated by Daphne Hardy. See page 19 of the Bantam Publishing paperback, 1981 printing for more info.

{{Crypto navbox | classical}}

[[Category:Encodings]]

Lichtfeld

2005-11-25T17:59:01Z

138.251.155.236:

The light field, also known as the plenoptic function or radiance field, is a function that describes the amount of [[light]] travelling through every point in 3D space in every possible direction. Light transport is measured in [[radiance]].

The light field varies with [[wavelength]] <math>\lambda</math>, position <math>x</math>, and unit direction <math>\overrightarrow{\omega}</math>, thus it is written:

<math>L_{\lambda}(x, \overrightarrow{\omega})</math>

One can also consider the time-varying light field and the [[polarization]] angle (phase) of the rays in the light field.

Gershun first used the term 'light field', however his function measured [[irradiance]], unlike the modern version that was introduced to [[photometrics]] as the photic field, to [[computer vision]] as the plenoptic function, and to [[computer graphics]] as the [[light field]] and related lumigraph.

The 'light slab' parameterization is often used in computer graphics. This is a 4D parameterization of the light field in free space, i.e. "outside the scene." It defines a point and direction by a pairs of coordinates on two planes.

==External link==

* [http://graphics.stanford.edu/papers/lfcamera/ Light Field Photography with a Hand-Held Plenoptic Camera]

Wikipedia - Benutzerbeiträge [de]

Klopfcode

Lichtfeld