<title>Human-Readable/Machine-Readable Imagery</title>

Abstract

AbstractA recording technique has been developed that allows machine-readable digital information to be encoded within human-readable imagery. Each picture element in the recorded image con­ sists of a square matrix of discrete spots, each spot taking one of several possible optical densities. A specific format consisting of a 2 x 2 spot matrix, with 8 quantization levels per spot, has been investigated in detail. This 2 x 2 x (8) format encodes 12 bits of machine-readable digital information per pixel and can exhibit 330 discrete readable pixel densities. This technique can be used, for example, to record human-readable imagery derived from EO sensors with 10 bits of dynamic range. The full sensor data base could be recovered from the imagery, by use of a suitable reader, without loss of digital information. In addition, collateral information such as height data, annotation, etc., can be encoded within the digital data in machine-retrievable format in a way that does not affect the human- readable information.This paper discusses algorithms developed for mapping the digital data into human-readable pixel densities, some techniques for error correction and pixel identification, and examples of collateral data fusion. Results of a digital data retrieval experiment are presented.IntroductionCurrent electronic information-gathering systems are capable of collecting immense amounts of data that must be processed, stored, and reconstructed to allow timely intelli­ gence exploitation and data dissemination. For example, a tactical system with thousands of pixels per line and a data rate of tens of megapixels per second running full time would generate over lO-^ bits of data per week. In an effort to overcome the shortcomings of con­ ventional approaches to the efficient archival storage and retrieval of enormous quantities of digital information, Itek, since 1977, has been developing an alternative technology, human-readable/machine-readable (HR/MR) imagery. This technique encodes the digital value for each pixel of the original data base in a square matrix of variable spots. By dividing a pixel into several parts, each quantized in possible optical density, hybrid encoding of digital information is achieved that yields a one to one spatial correspondence on the film plane between optical and digital pixel storage. HR/MR can be thought of as a hybrid method combining the two earlier recording techniques of variable spot recording and binary matrix recording.1For one particular HR/MR format, in which each spot may take on one of 8 distinct levels, a pixel consisting of four of these spots can encode any of 8 = 4,096 digital values. Image data recorded in the HR/MR format provides both human-guided random access within the image (as in photographic film recording) and full-range digital data base retrieval for subsequent exploitation (as in magnetic tape and optical disc storage). HR/MR recordings of imagery can be easily and inexpensively replicated for dissemination of data to multiple users, can provide a high bit packing density (over 10? bits per square inch has already been achieved), and can be retrieved with low bit error rates. A variety of techniques have been developed to allow the recording and retrieval of extremely large digital pixel word values within an HR/MR pictorial format.Other presently available storage methods do not meet the combined requirements of human readability and machine readability. The recording of wide dynamic range information on photographic film loses information due to the limited number of density levels that can be achieved on film. Thus the full digital data base cannot be retrieved for later exploita­ tion. Magnetic tape and the emerging optical disc technology offer digital data storage, but fail to provide a human-readable image. Computer-compatible magnetic tape has the further disadvantages of bulkiness, poor archival quality, and the considerable investment in storage material and storage space for a data base approaching 10-L5 bits.One advantage of HR/MR over conventional digital data storage techniques is that much information can be retrieved from an HR/MR recording even if a computer or reader system failure occurs. This feature may be particularly important in situations such as tactical reconnaissance, where access to the data must be maintained at all times.