The Infrared Data Association (IrDA) defines physical specifications communications protocol standards for the short-range exchange of data over infrared light, for uses such as personal area networks (PANs).

IrDA is a very short-range example of free space optical communication.

IrDA interfaces are used in medical instrumentation, test and measurement equipment, palmtop computers, mobile phones, and laptop computers (most laptops and phones also offer Bluetooth but it is now becoming more common for Bluetooth to simply replace IrDA in new versions of products).

IrDA specifications include IrPHY, IrLAP, IrLMP, IrCOMM, Tiny TP, IrOBEX, IrLAN and IrSimple. IrDA has now produced another standard, IrFM, for Infrared financial messaging (i.e., for making payments) also known as "Point & Pay".

For the devices to communicate via IrDA, they must have a direct line of sight similar to a TV remote control.

Specifications

IrPHY

The mandatory IrPHY (Infrared Physical Layer Specification) is the first (lowest) layer of the IrDA specifications. The most important specifications are:

Range: standard: 1 m; low power to low power: 0.2 m; standard to low power: 0.3 m

Angle: minimum cone ±15°

Speed: 2.4 kbit/s to 1 Gbit/s

Modulation: baseband, no carrier

Infrared window

Wavelength: 875 ± 30 nm

IrDA transceivers communicate with infrared pulses (samples) in a cone that extends minimum 15 degrees half angle off center. The IrDA physical specifications require that a minimum irradiance be maintained so that a signal is visible up to a meter away. Similarly, the specifications require that a maximum irradiance not be exceeded so that a receiver is not overwhelmed with brightness when a device comes close. In practice, there are some devices on the market that do not reach one meter, while other devices may reach up to several meters. There are also devices that do not tolerate extreme closeness. The typical sweet spot for IrDA communications is from away from a transceiver, in the center of the cone. IrDA data communications operate in half-duplex mode because while transmitting, a device’s receiver is blinded by the light of its own transmitter, and thus, full-duplex communication is not feasible. The two devices that communicate simulate full duplex communication by quickly turning the link around. The primary device controls the timing of the link, but both sides are bound to certain hard constraints and are encouraged to turn the link around as fast as possible.

Transmission rates fall into various categories: SIR, MIR, FIR, VFIR, UFIR, and Giga-IR. Serial Infrared (SIR) speeds cover those transmission speeds normally supported by an RS-232 port (9600 bit/s, 19.2 kbit/s, 38.4 kbit/s, 57.6 kbit/s, 115.2 kbit/s). Since the lowest common denominator for all devices is 9600 bit/s, all discovery and negotiation is performed at this baud rate. MIR (Medium Infrared) is not an official term, but is sometimes used to refer to speeds of 0.576 Mbit/s and 1.152 Mbit/s. Fast Infrared (FIR) is deemed an obsolete term by the IrDA physical specification, but is nonetheless in common usage to denote transmission at 4 Mbit/s. FIR is sometimes used to refer to all speeds above SIR. However, different encoding approaches are used by MIR and FIR, and different approaches frame MIR and FIR packets. For that reason, these unofficial terms have sprung up to differentiate these two approaches. The future holds faster transmission speeds (currently referred to as Very Fast Infrared, or VFIR) which supports a speed of 16 Mbit/s. There are (VFIR) infrared transceivers available such as the TFDU8108 operating from 9.6 kbit/s to 16 Mbit/s. The UFIR (Ultra Fast Infrared) protocol supports a speed of 96 Mbit/s. There, 8B10B coding is used. The Giga-IR protocol supports transmission speeds of 512 Mbit/s (64MB/s) and 1 Gbit/s (125 MB/s) and uses 2-ASK and 4-ASK modulation.

These are distinguished from other similar infrared communications systems that operate outside IrDA specifications. PDA communications formats such as HPSIR and ASKIR are such. This also includes CIR (Consumer IR), commonly used in remote controls, based on a raw protocol which uses sequences of pulse and space. It's possible to manage CIR via software like Lirc.

IrLAP

The mandatory IrLAP (Infrared Link Access Protocol) is the second layer of the IrDA specifications. It lies on top of the IrPHY layer and below the IrLMP layer. It represents the Data Link Layer of the OSI model. The most important specifications are:

Access control

Discovery of potential communication partners

Establishing of a reliable bidirectional connection

Distribution of the Primary/Secondary device roles

Negotiation of QoS Parameters

On the IrLAP layer the communicating devices are divided into a Primary Device and one or more Secondary Devices. The Primary Device controls the Secondary Devices. Only if the Primary Device requests a Secondary Device to send is it allowed to do so.

IrLMP

The mandatory IrLMP (Infrared Link Management Protocol) is the third layer of the IrDA specifications. It can be broken down into two parts. First, the LM-MUX (Link Management Multiplexer) which lies on top of the IrLAP layer. Its most important achievements are:

Provides multiple logical channels

Allows change of Primary/Secondary devices

Second, the LM-IAS (Link Management Information Access Service), which provides a list, where service providers can register their services so other devices can access these services via querying the LM-IAS.

Tiny TP

The optional Tiny TP (Tiny Transport Protocol) lies on top of the IrLMP layer. It provides:

Transportation of large messages by SAR (Segmentation and Reassembly)

Flow control by giving credits to every logical channel

IrCOMM

The optional IrCOMM (Infrared Communications Protocol) lets the infrared device act like either a serial or parallel port. It lies on top of the IrLMP layer.

OBEX

The optional OBEX (Object Exchange) provides the exchange of arbitrary data objects (e.g., vCard, vCalendar or even applications) between infrared devices. It lies on top of the Tiny TP protocol, so Tiny TP is mandatory for OBEX to work.

IrLAN

The optional IrLAN (Infrared Local Area Network) provides the possibility to connect an infrared device to a local area network. There are three possible methods:

Access Point

Peer to Peer

Hosted

As IrLAN lies on top of the Tiny TP protocol, the Tiny TP protocol must be implemented for IrLAN to work.

IrSimple

IrSimple achieves at least 4 to 10 times faster data transmission speeds by improving the efficiency of the infrared IrDA protocol. A normal picture from a cell phone can be transferred within 1 second.

IrSimpleShot

One of the primary targets of IrSimpleShot (IrSS) is to allow the millions of IrDA-enabled camera phones to wirelessly transfer pictures to printers, printer kiosks, flat panel TV's.

Popularity

IrDA was popular on PDA's, laptops and some desktops during the late 90s through the early 2000s. However, it has been displaced by other wireless technologies such as WiFi and Bluetooth, favored because they don't need a direct line of sight, and can therefore support hardware such as mice and keyboards. It is still used in some environments where interference makes radio-based wireless technologies unusable. IrDA popularity is making a comeback with its highly efficient IrSimple protocols by providing sub 1 second transfers of pictures between cell phones, printers, and display devices. IrDA hardware is still less expensive and doesn't share the same security problems encountered with wireless technologies such as Bluetooth. In addition, some Pentax DSLRs (K-x, K-r) use IRsimple for image transfer and gaming.

See also

List of device bandwidths

Consumer IR

RZI

External links

Infrared Data Association website

IrPro IrDA Protocol Stacks

ZMD IRDA

Linux Infrared HOWTO

Linux Infrared Remote Control

Linux status of infrared devices (IrDA, ConsumerIR, Remote Control)

Latest IRDA developments including IrSimple, VFIR and UFIR (2005)

IrDA project of Universidad Nacional de Colombia SIE board

References

Charles D. Knutson with Jeffrey M. Brown, IrDA Principles and Protocols, 2004, ISBN 0-9753892-0-3

Category:Standards organizations

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The correspondence problem tries to figure out which parts of an image correspond to which parts of another image, after the camera has moved, time has elapsed, and/or the objects have moved around.

Overview

Given two or more images of the same 3D scene, taken from different points of view, the correspondence problem is to find a set of points in one image which can be identified as the same points in another image. To do this, try to match points or features from one image with the same points or features in another image. The images can be taken from a different point of view, at different times, and with objects in the scene in general motion relative to the camera(s).

The correspondence problem typically occurs when two images of the same scene are used, the stereo correspondence problem. This concept can be generalized to the three-view correspondence problem or, in general, the N-view correspondence problem. In the general case, the images can either come from N different cameras which depict (more or less) the same scene or from one and the same camera which is moving relative to the scene. The problem is made even more difficult when the objects in the scene can be in general motion relative to the camera(s).

A typical application of the correspondence problem occurs in panorama creation or image stitching — when two or more images which only have a small overlap are to be stitched into a larger composite image. In this case it is necessary to be able to identify a set of corresponding points in a pair of images in order to calculate the transformation of one image to stitch it onto the other image.

Basic Methods

There are two basic ways to find the correspondences between two images.

Correlation-based - checking if one location in one image looks/seems like another in another image.

Feature-based - finding features in the image and seeing if the layout of a subset of features is similar in the two images. To avoid the aperture problem a good feature should have local variation in two directions.

Use

In computer vision the correspondence problem is studied for the case when a computer should solve it automatically with only images as input. Once the correspondence problem has been solved, resulting in a set of image points which are in correspondence, other methods can be applied to this set to reconstruct the position, motion and/or rotation of the corresponding 3D points in the scene.

Very Simple Example

To find the correspondence between set A[2,4,6,1,5] and set B[8,0,2,4] find where they overlap and how far off one set is from the other. Here we see that the first two points in set A correspond with the last two point in set B. This shows that B is offset 2 to the left of A.

Simple Correlation-based Example

A simple method is to compare small patches between rectified images. This works best with images taken at roughly the same point of view and either at the same time or with little to no movement of the scene between image captures; such as stereo images.

Using a left and right image, pass a small window over each position in the left image. For each position check how well it compares with the same location in the right image. Also compare against several locations near that, for the objects in one image may not be at exactly the same image-location in the other image. When you find the best fit the difference in image-location is the correspondence of that feature/point. It is possible that there is no fit that is good enough. This may mean the feature is not present in both images, moved farther than your search accounted for, changed enough between images that the method you use to match is not working, or are being hidden by other parts of the image.

Further Issues

Many methods discard any information that is not present in both images. In A Bayesian Treatment of the Stereo Correspondence Problem Using Half-Occluded Regions, P. Belhumeur and D. Mumford attempt to use this information to incorporate them from the start as a strong clue to depth discontinuities.

See also

Stereoscopy

Parallax

Photogrammetry

Depth perception

Stereopsis

Computer vision

Fundamental matrix

The Joint Compatibility Branch and Bound algorithm

References

External links

Middlebury Stereo Vision page

Papers

D. Scharstein and R. Szeliski. A Taxonomy and Evaluation of Dense Two-Frame Stereo Correspondence Algorithms. (PDF)

Category:Geometry in computer vision Category:Stereoscopy

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