Standard Format For The Transmission Of Digital Images

Abstract The emergence of digital display, storage and retrieval equipment has prompted a standard format for transmitting images between different type machines having access to a common channel. The standard assumes that the major cost in image transmission is access to the channel and that computation costs at the sending and receiving machines are small and will continue to get smaller. The standard allows data compression schemes and flexible picture element (pixel) representations. Standard format messages can contain variable numbers of images of different aspect ratios, resolutions, pixel types and compression schemes.IntroductionThis standard format is intended to assist the transmission of digital images over elec­ tronic channels: telephone lines, packet switching networks or in house data lines etc. The standard includes a set of compression algorithms with user definable coding to minimize the message size. It is assumed that as the cost of computation decreases, the dominant cost of image transmission will be the channel, and that significant reductions in transmission cost will be achieved by source coding.The "standard" format presented here is intended primarily for the type of images that have in the past been implemented with photographic film. It is thus suited for "display" purposes. It may be suited for other purposes, but they were not necessarily in mind in the development process. For example, this format may not be direcly suitable for some research applications which require communicating "images" that have negative values, such as some image transforms. In general, though, if the image can be viewed on some kind of two-dimen­ sional display,the image can be adequately massaged to fit the standard with no loss of information.Loss of information will occur in some instances, and this is related to the differences between the image generating equipment and the receivers viewing equipment. For example, if the originating system uses displays with a 1024 x 1024 pixel format, a receiving station that has only 484 x 768 pixel format equipment will have to compromise resolution, field of view or both. The compromise, however, is entirely at the receiving system. If the receiv­ ing system archives the original message, and they one day upgrade to 1024 x 1024 pixel view­ ing equipment, they will be able to review the received image in its original completeness.Another limitation of the standard is that all compression schemes have not been included. Also, a raster (rectangular grid) basis for images was chosen exclusively. The motivation for being this selective was to try to keep the standard simple. After all, a standard is not a standard unless people are using it. If it is a huge task to implement the standard, few people will. At the writing of this paper the standard is not in fact a standard, since it is not in common use. This standard might best have been called, "a recommended starting point for discussion of a standard..." No doubt, in future discussions, the favorite features which are not included in this presentation will be actively petitioned for by those that want them.Previous work on image transmission standards has been done by the IEEE Computer Society MIPA Database Committee3- and NASA (the "Landsat Standard") 2 3 ** . These two projects just cited were designed with magnetic tape in mind as the storage and transport medium. They do not directly address the question of data compression, although they could probably be ex­ tended to include data compression. It seemed simpler to define a compact format to specifically handle image coding for electronic channels than to create an almost equally sized subset standard in addition to an already existing standard.The detection and/or correction of errors that may occur in the message due to the channel are not considered in this standard. Error detection and correction should be the responsi­ bility of the particular channel standard, not the message standard. It is reasonable to expect that each channel standard would provide its own strategy for dealing efficiently with its own particular error modalities.An implementor of the standard format for a particular site/system has to interface his particular site to the channel via standard. Their are two basic tasks; selecting the