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Further reading ⇓

□ Overview
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□ No.11

Graphics Note No 2: Microdensitometer Tape Viewing and Processing Program

W D Shaw

23 April 1974

1. INTRODUCTION

SRC's Photoscan P-1000 microdensitometer at Atlas is capable of scanning a 12.5× 17.5 cm (or smaller) photographic transparency at steps of, 25, 50 or 100 microns and quantizing light transmission at each point into 256 levels. A Computer Automation Alpha-16 minicomputer controls the microdensitometer and may output a 7-track magnetic tape containing the scan data and certain scan parameters. One record is written to the tape for each step in the horizontal direction.

2. PROBLEM

It would. be useful to have a program on the PDP15 to allow microdensitometer users to view portions of tapes output by the Alpha-16 in order to verify their accuracy. A program is available on the 1906A for plotting tapes on the SD4020, but the turn round time is often unacceptable. It would be necessary to view these tapes using certain types of thresholding or filtering operations.

3. PROPOSAL

Because of the large amount of data on the microdensitometer tapes, efficient viewing would be best achieved by selecting small regions of the scanned area for high resoluticn display. The selection of the areas could be done from a very low resolution display of the entire tape. As one is likely to want to process or make hardcopy of these various "views" of regions, it seems useful to allow construction of a new "tape" (on or from disc) in microdensitometer format.

There appear to be many interesting ways of filtering and displaying the data, Flexibility in this area will be maintained by the subroutine structure of the program. The rough flowchart of the package would look like:

Read a record EOT Y N Call filtering routine Call display routine Write modified record to MT or disc New command Start Quit

A command to the package would include the following parameters:

  1. Input device: Magnetic tape unit or disc file containing microdensitometer information
  2. Output device: Any combination of VT04, Tektronix, disc file, or magnetic tape unit
  3. Region: The bottom left and top right-hand co-ordinates of the region to be output
  4. Filtering: The name of a subroutine which performs some transformation (such as averaging or thresholding) on the input data
  5. Display type: A set of parameters specifying the way in which the filtered data is to be displayed on the VT04 or Tektronix

It is likely that PIGS will form the interactive front end of this program. Useful routines internal to the viewing package (such as the magnetic tape-reading routine) will be documented.

REFERENCES

SRC Microdensitometer at ACL, Notes for Users of the Digitization Service, 21 March 1973, M Elder

SRC Microdensitometer at ACL, Notes for Potential Users, 21 March 1973, M Elder

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