Date   : Wed, 20 Jul 1994 00:04:40 +0100
From   : jfid@... (James Fidell)
Subject: Re: Teletext graphics question...

Originally sent to me by rtr@... (Richard Russell (KW 36684)).
He also says that the Broadcast Teletext Specification should be
publicly available, but that it has no ISBN.

I'm going to post to uk.??? to see if someone can help, but if anyone else
has ideas on where to get a copy of this, please post them.

James.

> From the Broadcast Teletext Specification:
>
> "Generally all Control Characters are displayed as Spaces, implying at
> least one Space between rectangles with different Display Colours in
> the same Row. The Hold Graphics mode allows a limited range of abrupt
> Display Colour changes by calling for the display of a Held Graphics
> Character in the rectangle corresponding to any Control Character
> occuring during the Graphics Mode. This Held Character is displayed in
> the modes obtaining for the rectangle in which it is displayed, except
> for the Contiguous/Separated Mode which forms part of the structure of
> the Held Graphics Character.
>
> "The Held Graphics Character is only defined during the Graphics
> Mode. It is then the most recent character with b6=1 in its character
> code, providing there has been no intervening change in either the
> Alphanumerics/Graphics or the Normal/Double Height modes. This
> character is to be displayed in the Contiguous or Separated Mode as
> when it was first displayed. In the absence of such a character the
> Held Graphics Character is taken to be a Space."
>
> Note the following implications of this:
>
> 1. The Contiguous/Separated bit must be stored with the Held Graphics
> Character (the Held Graphics Character may be shown separated,
> for example, even if the current display mode has been changed to
> contiguous).
>
> 2. The Held Graphics Character must be cleared to a space at the
> beginning of each line, and on any change between Alphanumerics &
> Graphics or between Single & Double Height.
>
> 3. Because the Held Graphics Character is displayed in the position
> corresponding to a control character, you *must* correctly adhere to
> the "set at" and "set after" rules for all the control characters.
>
> Richard.
>
> P.S. Please post this to the mailing list. I don't know how to!


-- 
 "Yield to temptation --             |
  it may not pass your way again"    |     jfid@...        
                                     |
        - Lazarus Long               |              James Fidell