Keywords: gildas-prog

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# Programming

- Initialisation
  The GTVIRT library is initialized by a a call to
  `SUBROUTINE INIT_GTVIRT`
  followed by a call to `SUBROUTINE GTINIT(LX,LY,LUNG,LUNH,NAME,USER_ROUTINE`
  where
  - `LX,LY` are the page dimensions
  - `LUNG,LUNH` are logical unit numbers for graphic output and hardcopies, to be supplied by the caller
  - `NAME` is a character string giving the name of the top directory of the drawings. `NAME` should start with a sign.
  - `USER_ROUTINE` is a user-supplied subroutine called when moving from a sub-directory to the other. It must provide to the GTVIRT information about coordinate systems. The only suitable routine is available in the GREG program, and is named `GREG_USER`

- The `GTVIEW` subroutine

  The `GTVIEW` subroutine is one of the main control routine in the library. It is used to send drawing commands from the internal metacode to the graphic device, but also to performs other actions on the metacode.

  `SUBROUTINE GTVIRT(Mode)`

  where `Action` is a character string which can be

  - 'Append'
    Plot all metacode starting from last drawn vector
  - 'Rewind'
    Clear the screen, and plot the whole metacode from first vector
  - 'Update'
    Update the screen (all windows) if needed
  - 'Limits'
    Recompute the plot limits (Bounding Box)
  - 'Purge'
    Delete images associated to all windows
  - 'Delete'
    Delete images associated to the current window
  - 'Zap'
    Delete cache-bitmap associated to images
  - 'Sleep'
    Set screen update off
  - 'Wake'
    Set screen update on

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