Digital image Processing at Los Alamos scientific Laboratory B. R. Hunt and D. H. Janney Los Alamos Scientific Laboratory

Introduction

Nearly everyone has seen the pictures brought back from the moon by the U. S. astronauts. Less remembered nowadays are the television pictures sent back from the moon's surface by the early unmanned lunar vehicles; these pictures were equally spectacular in their time and perhaps more important, since they gave NASA the basis on which to make the judgements about lunar terrain pertinent to the landing of a manned vehicle. Many people remarked on the sharpness and clarity of the television pictures of the moon, and similar ones now coming from Mars. But to the digital image processing community, the quality of the lunar images was no surprise, and was recognized as a result of the work done at NASA's Jet Propulsion Laboratory. JPL had digitally enhanced the incoming tele­

vision images, imparting to the images sharpness that was sometimes missing in the originals.

A less publicized but equally significant effort - one that was inspired in part by JPL's work — has been going on at Los Alamos Scientific Laboratory. Located at LASL is a one-of-a-kind flash x-ray machine used for taking radiographie pictures of rapidly changing events - e.g., shock waves in solids, hydrodynamic phenomena in metals, changes of phase in explosively driven materials, etc. Known as PHERMEX, the machine is a linear accelerator, capable of generating 10 to 20 Mev x-ray beams in pulse intervals as short as 0.1 microseconds. These high energies and short pulse durations do not always lead to optimum image formation conditions, however, and further inter­action with other effects (such as x-ray scatter in the object, finite source size, and object motion) can lead to

May 1974

radiographie images lacking in resolution and visual quality. Typically the images look somewhat "fuzzy" and are usually encumbered with varying amounts of photographic noise. Consequently, the widely publicized success of JPL in improving the sharpness of the lunar television pictures led to an obvious speculation: perhaps digital image processing could sharpen up the radiographie images in question.

One of the authors (Janney) undertook to confirm the speculation. A subcontract was negotiated with JPL for image scanning and image display services on the existing systems at JPL. A minimal amount of programming was undertaken at LASL by the other author (Hunt) to imple­ment the necessary image processing functions. The result­ing demonstrations were so positive in showing how sharp­ness could be restored to flash radiographs that LASL made the decision to acquire the hardware necessary to support image processing. Today, image processing at LASL is functioning in a production environment for enhance­ment of radiographie images, and research and development continues in application of image processing technology to other laboratory problems.

Current Systems and Facilities

The total capability for undertaking digital image proces­sing operations at LASL can be viewed as the sum of three constituent parts: the total computational resources of the Central Computing Facility (CCF) at LASL, specialized hardware systems for digitizing and displaying images, and well-developed software for executing the standard opera­tions employed in image processing. Figure 1 shows the overall view of the specialized image processing hardware and the computer resources.

Central Computing Facility The CCF at LASL consists of several of the largest commercially available scientific com­puters: two CDC 6600 computers, three CDC 7600 com­puters, and one CDC Star-100 computer. In addition, the 6600 and 7600 computers are tied together with a number of minicomputer terminals in a remote job entry network called HYDRA. Other elements of HYDRA include mass-access storage (the IBM photostore), plus the usual comple­ment of peripheral printers, card readers/punches, etc.

Image Processing Hardware All image digitizing is done with a Photometric Data Systems (PDS) scanning micro-densitometer, a specially-built unit with a moving glass stage capable of accepting a full-size radiographie film (35.6-cm by .43.2-cm). Optics focus a scanning aperture on the film to be scanned, which is mounted on the glass stage. Square apertures in sizes from 10-microns to 200-microns on a side are available. The system can take samples at steps as small as 1 micron spacing between raster point centers. The dynamic range of optical densitities on the film accepted by the system is wide, covering a range from 0.0 to 5.0 with repeatable precision of 0.01 assured. The travel speed of the stage is rapid, so that a full 35-cm by 43-cm film can be sampled (on 100-micron centers) in one hour (corresponding roughly to a 4000 by 4000 sample grid). Samples are written onto magnetic tape after being digitized; the magnetic tapes are then taken to the computer center for processing.

Although the PDS system is the only digitizing device, three separate systems are used for redisplaying digital imagery. The PDS system can be used in a film writing mode, in which the precision light source used for illuminat­ing the film in digitization is replaced by a light-emitting diode. All other optics and apertures remain the same. The

Very High Resolution Very High Precision

Image Digitizer

F5 Mag

Jape ,

Central Computing

Facility

Film

Light Emitting Diode Display

ZF5 Very High Resolution Very High Precision

Film Film

Microfilm Image Display

High Resolution High Precision

Low Resolution Low Precision ("Quick Look")

35-mm Film

Figure 1. Overview of LASL's System

58 COMPUTER

light intensity emitted by the diode is modulated to expose the film. Virtually any size film which can be placed on the scanning stage can be exposed. Another film writing system used for output is the FR-80 film recorder, made by Information International, Inc. This system, ordinarily used for microfilm graphics in the CCF, can also be used in an image display mode. Images are created by directly photographing a precision CRT; however, only 35-mm film can be used as the display medium.

The two systems discussed in the previous paragraph both operate off-line; magnetic tapes generated by digital image processing in the CCF are carried out to these display systems. A third system, presently being designed for on-line use, is a color CRT with rotating disk refresh memory. The display is being interfaced to the HYDRA network by a PDP-11 minicomputer, and ultimately will allow on-line interaction with image processing operations via the HYDRA network. The CRT/refresh display was built by Comtal Corporation.

The three displays discussed above represent a range of capabilities. The PDS film writing display possesses the highest precision and greatest flexibility; it is also the slowest. The FR-80 system possesses precision and flexibil­ity, but its resolution is not as good as the PDS; on the other hand, it is faster and provides quicker turnaround. The Comtal Corporation on-line CRT display will provide a virtually instantaneous "quick-look" display; its overall flexibility and precision will not be competitive with the PDS or FR-80. However, taken together, these three systems provide a capability for image display at virtually any level desired.

Image Processing Software Associated with the CCF com­puters and the specialized image processing hardware is a library of software, the Los Alamos Digital Image Enhance­ment Software (LADIES). LADIES is a package of sub­routines, callable from FORTRAN programs. Most of the subroutines are written in FORTRAN, with certain key operations that consume excessive amounts of computing time (data format manipulations, special I/O, etc.) written in machine assembly language for maximum code efficiency. The basic philosophy followed in the development of the library was to make each subroutine as much of a stand­alone entity as possible. Thus, a user may carry out a digital image processing operation, such as contrast stretch­ing or digital filtering, by writing a CALL statement and supplying a FUNCTION subroutine which describes the characteristic operation to be used in the processing. All reading and writing of data tapes, conversion of data from microdensitometer to floating-point formats, and linkage to other subroutines are contained within each subroutine that the user selects to carry out his operations. Sub­routines currently in the library fall into several classes: subroutines for convolution and digital filtering; subroutines for manipulation of image contrast by density scaling, histogram equalization, etc.; utility subroutines for fast transforms, data packing and unpacking, image labelling, computation of image statistics, etc.; and subroutines related to specific PHERMEX radiography problems, such as film characteristics and x-ray angular intensity variations.

Recent efforts in software have been directed toward the development of a command language for image pro­cessing work. The use of FORTRAN subroutines has three undesirable aspects. First, the user may desire to do only

one operation on a picture, such as stretch the contrast, but this operation cannot be performed with a single-card com­mand; other statements must be supplied to do such things as file opening and closing, file positioning, subroutine parameter definitions, etc. Second, FORTRAN and the associated CALL statement syntax are unforgiving. That is, the omission of a parameter or a comma in the CALL will abort the job. Third, experience shows that many image processing subroutines require an extensive parameter list, most of which can be set to standard values and are retained as parameters only for the sake of flexibility in extreme cases.

The response to those objections at LASL has been to design a command language that will possess forgiving syntax, allow complete default options for unspecified parameters, and make automatic many of the routine file definitions and manipulations. This system is organized on the basis of command keywords and parameter keywords. All critical parameter keywords are in a COMMON block referenced by all image processing subroutines. The param­eters will be initialized to their default values upon program loading, and the default values of the parameters are then available to the user without explicit statements. The syntax will be made forgiving through the use of a broad class of delimiters and the option of a keyword being set by default. Thus, the following statements would be equivalent to the command language processor;

FILTER (INP = 1, 0UT = 2, FILTFCN = 4HSUBF) FILTER OUT = 2 FILTFCN 4HSUBF INP = 1 FILTER

In the first statement the input is on I/O unit 1, output on unit 2, and the subroutine named SUBF has the filter frequency domain characteristic. The second statement has the same effect, the order being unimportant since the processor sets parameter keywords by name and not by the sequence in which they are encountered. The third statement is the same since the system default options are the same as the ones originally stated explicitly.

The command language development discussed above is called FLIP (Free-form Language for Image Processing). Coding for the language is partially complete with the imple­mentation being by preprocessor, which reads FLIP com­mand statements and generates FORTRAN object code references to the existing LADIES library. The generated code will then be compiled and executed. This implemen­tation route was chosen for the obvious reasons of simpler preprocessor design and utilization of existing library software.

Research

Much of LASL's interest in image processing arises from the production problems in radiography discussed above, and consequently their research in image processing is biased toward the support of production programs. This might have been a detriment to the research effort, were it not for the nature of the flash radiography in typical experi­mental situations. A realistic model of the PHERMEX flash x-ray system, in fact, would require solution of many of the

May 1974 59

outstanding problems in image processing. For example, the nature of flash radiography processes at very high energies leads to a model for image formation which is non­linear, dependent on the data being imaged and with a space-variant point-spread-function. Further, the limita­tions imposed by flash radiography lead one to areas where image restoration or enhancement has to function in low signal-to-noise ratio environments. The complexity of these problems means that the research necessary to fulfill the production demands of PHERMEX is also research in some of the most challenging problems in image processing. Only a restricted subset of these problems can be tackled, of course.

Today, LASL is concentrating its research in the areas of image restoration and enhancement, with a simplified linear model of PHERMEX image formation (the complete non­linear model is still considered too complex). Past research efforts in constrained least-squares estimates1 have been implemented in user-oriented codes. New research efforts have begun in the use of multi-step contrast stretching, and histogram equalization in overlapping sections for image enhancement. Recent image restoration research centers around the use of non-linear processing. Almost all image restoration in current applications is based on linear pro­cessors and digital computer implementation of processes that are fundamentally optical. Most recent work in image restoration at LASL has been directed toward breaking the linear processor/optical analogy mode öf thinking with non­linear estimation procedures. It is believed that by making greater use of the real power of the digital computer for implementing processes that have no optical analogies, better results in restoration will be achieved.

Examples

The following figures present some typical examples of the work going on at LASL in enhancing radiographie images. Figure 2 shows a radiograph of a section of a hollow metal sphere. Even though the sphere is hollow, the inner surface cannot be seen in the original radiograph, due to a phenomenon which the radiographer refers to as the "inner tangent problem." The x-rays passing just tangent to the inner surface encounter the greatest length of material in the object, and therefore are the most heavily absorbed and the most scattered. The effects combine to give very poor image contrast. Figure 3 shows the enhanced image, achieved by high-pass digital spatial filtering of the original. The inner surface is clearly visible in this image, as indicated. Note also the general sharpening of the outside of the sphere, making it much easier to observe the outer sphere edge.

Figure 4 shows a radiograph of a nuclear reactor fuel rod. The fuel rod is composed of an outer cylinder and an inner cylinder. The inner cylinder contains individual fuel pellets, composed of the fissile material used in the reactor core to generate heat. Figure 5 shows the enhanced image, with much greater sharpness of all features. The fuel rod was part of experimental reactor studies, and of great importance was the question of the stability of the fuel rod dimensions in the high-temperature, high-neutron-flux en­vironment of the nuclear reactor. It is considered easier to make measurements of the edges, thicknesses, and angles in Figure 5 than in Figure 4, owing to the overall increased sharpness. Other examples are reported in a previous paper.2

60

SLEEVE Figure 5. Enhanced Image of Figure 4

Acknowledgement

The work associated with this article was conducted under the auspices of the Atomic Energy Commission.

REFERENCES 1. B. R. Hunt, "The Application of Constrained Least-Squares

Estimation to Image Restoration by Digital Computer," IEEE Transactions on Computers, Vol. C-22, pp. 805-812, September 1973.

2. B. R. Hunt, D. H. Janney, and R. K. Zeigler, "Radiographie Image Enhancement by Digital Computers." Materials Evaluation Journal of the ASNT, Vol. 31, pp. 1-5, January 1973.

B. R. Hunt is alternate group leader for image processing in the Computer Science Division at Los Alamos Scientific Laboratory. While em­ployed at Sandia Laboratories, Albuquerque, New Mexico, he was involved in systems analysis of strategic weapons. He received a BS in aero­nautical engineering from Wichita State Univer­sity in 1964, an MSEE from Oklahoma State University in 1965, and the Ph.D in systems engineering from the University of Arizona in

1967. He has published many technical articles on image processing and associated research.

D. H. Janney is a group leader for image pro­cessing at Los Alamos Scientific Laboratory, working in flash radiography, extraction and analysis of data, and image enhancement appli­cations. Earlier he was employed at the Micro­wave Laboratory at Stanford. He received a BS in engineering physics from the University of Illinois in 1952, an MS in physics from Stanford University in 1953, and a Ph.D in applied physics from Stanford in 1957.

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Continued f rom page 16 E. U. Condon, Former Director of NBS, Dies

Dr. Edward U. Condon, fourth director of the National Bureau of Standards died March 26 in Boulder, Colorado. He was 72.

Appointed director of the Bureau in 1945 by President Truman, Condon served as director until 1951, resigning to become director of research at the Corning Glass Works. During his years as director of NBS, Condon was faced with the challenge of reorganizing the Bureau's programs to meet the meas­urement needs imposed by the tre­mendous technical advances made dur­ing World War II. In the course of carrying out this task he instilled new life in many of the Bureau's traditional areas of research, as well as leading it into such new areas as semiconductor materials, statistical engineering, and electronic computers.

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