Teletext separated mosaic graphics

[Doug Ewell]

Harriet Riddle wrote:

> It's worth pointing out that the control codes for showing mosaic
> characters as separated are also used in at least some formats to
> switch alphabetical characters to underlined display.
>
> See for example the definitions for SPL and STL here:
> https://www.itscj.ipsj.or.jp/iso-ir/056.pdf (that document details the
> C1 control codes for Data Syntax 2 Serial Videotex—which would seem to
> be the Teletext set but as a C1 set, and as such with CSI rather than
> ESC).

Applications of any sort that are compliant with ISO/IEC 6429 (ECMA-48, ANSI X3.64) should understand ESC [ as a synonym for CSI.

> Essentially, the expectation seems to be that an emphasised variant of
> a font would display mosaic characters separated, while a regular
> variant of a font would display them connected.

We still haven't written the Technical Note for using the Legacy Symbols -- that's largely on me -- but as far as teletext is concerned, the recommended practice is to translate teletext control codes directly onto the Basic Latin space. For example:

- "contiguous graphics" becomes U+0019
- "separated graphics" becomes U+001A
- "double height" becomes U+000D
- "end box" becomes U+000A

There is no conflict with the normal meanings of U+000D and U+000A because teletext does not use these to separate lines. In general, a teletext application should treat control codes the way teletext would treat them, and should not try to mix C0 and teletext interpretations. This also means Rob's scenario:

> It also means a simple text-only file of just the characters won't
> recreate a screen as the control codes to switch between contiguous/
> separated won't work.

may not be well-conceived; the file should probably not be "text" in the sense that its lines end with some combination of CR and/or LF, unless there is an intermediate translation step.

--
Doug Ewell, CC, ALB | Thornton, CO, US | ewellic.org