FRVIEW is a PDP 15 program which enables the contents of an FR80 data tape to be displayed on the VT04 refreshed display. Thus the pictures contained on a tape may be examined before committing it to the FR80 for plotting. FRVIEW was originally produced as a debugging tool for the graphics packages which generate the data tapes. The facilities offered by FRVIEW reflect this origin and render it somewhat less than satisfactory as a general purpose tape-viewing program. Ordinary graphics users who send their FR80 output to one of the SPOOL systems, should normally use one of the utilities associated with those systems in order to preview their output, rather than use FRVIEW.
As well as displaying the data from a tape in graphic form, FRVIEW is also able to dump the data on to the teletype for diagnostic purposes. Statistical information on the frequency of use of each type of FR80 data order may also be collected.
The FR80 "standard data format" tape (7-track, 556 bpi, odd parity) to be viewed should be mounted on tape unit 1. A magnetic tape handler for this unit must be assigned to DAT slot 7. FRVIEW reads its commands from the device associated with DAT slot 3, and prints its output on the device associated with DAT slot 4. The DECwriter (LT2) is a particularly appropriate device to assign to those slots if much printed output is expected. FRVIEW is able to accept lower case input from the DECwriter, which greatly simplifies the use of this device.
In VIEW mode, the program proceeds to decode the FR80 orders from the data tape and to display the resulting pictures on the VT04 screen. At certain intervals, this process will be interrupted in order to allow the user to inspect the output produced so far. The frequency of those interruptions is controlled by the PAUSE LEVEL. In addition to displaying pictures on the VT04, FRVIEW may also print information on the teletype. The amount of information that is printed is controlled by the MESSAGE LEVEL.
The 16K raster of the FR80 is mapped onto the 1 K raster of the main image area of the VT04. The limits of this area are indicated by a square which is drawn automatically. The number of the frame currently being displayed is shown on the top right hand corner or the screen.
All FR80 vector orders are reproduced on the VT04 screen. The other types of FR80 orders, which cannot be simulated directly by the VT04 are either approximated (eg dotted and dashed vectors) or ignored (eg colour orders and picture orders). The 256 intensity levels of the FR80 are mapped onto the 8 levels of the VT04. The box which surrounds the plotting area is drawn at intensity level 3 (the levels go from 0 to 7) as a reference.
