Date   : Tue, 03 Jul 2018 12:41:00 +0100
From   : soruk@... (Michael McConnell)
Subject: Announce: Brandy Basic - with teletext?!

Bad form for replying to my own post, but as it's now another day, I've 
uploaded V1.21.1 - which includes working flash (and MUCH faster 
background colour changes).

The implementation I'm following is that of a BBC Micro MODE 7 (and 
testing against the version emulated by RISC OS in RPCEmu), rather than 
that of a Viewdata / broadcast Teletext display.

Now I'm wondering what it would take to extend the bank switching stuff 
to the other 16-colour modes (e.g. MODE 2) to implement flashing colours 
there....

On 02/07/18 21:49, Michael McConnell wrote:
> I'm thinking about double buffering too, but that is a project for
> another day!
> 
> On 02/07/18 20:59, Rob wrote:
>> If you can emulate an entire BBC Micro in *javascript* within a browser
>> window, then I wouldn't expect a compiled native application to have any
>> problems at all.
>>
>> FWIW, teletext graphics, and the way the various control codes interact,
>> can be quite tricky to get right. Hold Graphics is the one that trips a
>> lot of people up, as it can show up mistakes in the coding of other
>> control codes (some take effect on the cell with the code, some only on
>> the cell after!) And then you have to decide if you are implementing a
>> "proper" viewdata/teletext display, where a double height code maps the
>> line onto the line below, and ignores (but still remembers!) whatever is
>> stored in memory for the second line, or a BBC Micro mode 7 where it
>> uses that second line and can therefore produce funky effects. It is
>> very satisfying when you get it all right!
>>
>> Even on a Beeb it's not that easy: Back in the 80s, I was contacted to
>> write a viewdata terminal emulator for the Beeb. It was actually quite
>> hard to get double height working "properly" using the Beeb's hardware.
>> I.e. What happens if you overwrite the double height control code on the
>> first line? All the terminal emulators I had tested got it wrong, as
>> they had already discarded the second line (in order to cater for the
>> Beeb's hardware, which needs both to be the same) and which should now
>> be visible, and is allowed to contain something completely different to
>> the first!? My approach was to write the data into a separate buffer,
>> and copy into screen memory on the vertical sync timer. That way I could
>> deal with double height, and Reveal (Conceal being another code that the
>> Beeb obeys, but then has no way of revealing the hidden characters.)
>>
>> Rob.
>> -- 
>> Sent from a mobile device. Please excuse brevity or spoiling mistooks.
>>
>>
>> On Mon, 2 Jul 2018, 19:38 Pete Turnbull, <pete@...
>> <mailto:pete@...>> wrote:
>>
>>      On 02/07/2018 15:59, Michael McConnell wrote:
>>       > I guess, with the way the screen is currently done (including the
>>      other
>>       > modes) that it would be computationally far to expensive to scan the
>>       > screen for what should be flashing pixels. For Mode 7 at the very
>>      least
>>       > to get a 100% true emulation it would need to maintain a 1000 byte
>>       > screen memory buffer (somewhat akin to the BBC Micro screen memory at
>>       > &7C00) and re-render the entire screen periodically
>>
>>      Hmm.? Sometime in the late 1990s I wrote a Viewdata client for Unix (or
>>      more specifically, for IRIX on an SGI Indy running at 180MHz).? I
>>      needed
>>      it for HOBS, the Bank of Scotland online banking system, which I had
>>      previously used on a Beeb and then an A440.? I had graphics, flash,
>>      double height and separate/contiguous etc working properly then, so I
>>      find it hard to believe it can't be done two decades later on 20x
>>      faster
>>      processors, though I guess it might depend on what it's written on top
>>      of.? I've forgotten exactly how I did it but AFAIR I did use a double
>>      buffer.
>>
>>      --
>>      Pete
>>      Pete Turnbull
>>
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