2 IMAGE PROCESSING OF WORKS OF ART AT IPL

2.1 Scanning

The first procedure, called scanning, converts the image of an artwork on a film negative to a computer-readable form. Of the several machines in the IPL that can perform this function, the PDS Microdensitometer was used because it is the most accurate. The negative is placed on a glass transport through which passes a narrow beam of light whose brightness is read by a detector. The stage is movable and is under computer control. The negative is scanned by commanding the transport to move, momentarily stopping at each of a number of closely spaced grid points to allow the density of the negative to be read. The scanning process divides the image into a two-dimensional array of data samples or pixels (short for ‘picture elements’) which represent image brightness (grey level). In the typical ‘8-bit’ encoding scheme, there are 256 possible grey levels, ranging from 0 (black) to 255 (white). The eye can only differentiate about 40 distinct shades of grey, but more shades are used in the computer for accurate computation.

The scanned image is stored on computer tape. The typical image in this study had roughly 1000 pixels vertically by 1000 pixels horizontally, so such an image is represented by one million image samples. After scanning the image is processed on an IBM 370/158 computer with the aid of the disc drives which act as temporary storage during processing.

2.2 Digital Image Processing

Digital image processing consists of the application of a series of computer programs to an image to produce a desired result. The image processing computer language in the IPL, VICAR (Video Information Communication and Retrieval),1 is a command-oriented computer language library of parameterized application programs. Complex image manipulations can be executed with the use of a single command. VICAR was designed for ease of use, and does not require intimate knowledge of the machine. Over 300 VICAR applications programs are currently available.

2.3 Playing Back

After an image is processed, it is often converted back into the form of a photograph, a process known as ‘playing back.’ The playback device used in this study, Dicomed, read the pixel grey levels in the processed image and used them to adjust the intensity of a twenty-micron flying electron spot. The spot scans the film negative, stopping momentarily at the location of each pixel and exposing the film to the proper level. The negative is developed and black-and-white prints produced in the usual manner.

For a technical description of IPL hardware the reader is asked to refer to authors.