DECDLD usage -good enough for minimal graphics?

[Paul Koning]

>>>>> "John" == John Allain <allain at panix.com> writes:

 >> ...transfer the custom fonts to the terminal.  ...this method
 >> ...can redefine and line up the characters so that they form a
 >> picture,

 John> recalling...  I thought the problem with that was you could
 John> only define 256 characters at a time, and when you tried to
 John> load a new set of bitpatterns, all instances old and new
 John> onscreen would change, making only a picture with 256 unique
 John> cells (no more) possible.  Don't know if this is a fact, but
 John> that's what the spec seemed to say to me at the time.  A full
 John> screen could be up to more like 24x80=1920 characters.  A lot
 John> could still be done with even that.  Another possible problem:
 John> fixed inter-character gaps.  Perhaps someone here has actually
 John> Done this.

That sounds right.  Given that the VT220 is a character display
terminal, it's refreshing from a character buffer via the character
bitmap memory.  If the bitmap memory changes, so does the display.  If
you don't want that, you need a terminal with a bitmap display memory
-- the VT240, or the PRO for that matter...

Ignoring the issue of inter-char gaps (don't know about that, easy
enough to test), you should aim for a graphic design where all the
elements together can be encoded in however many fixed and
programmable character bitpatterns there are.  That way you load
things once, and the display then becomes a changing pattern of
characters.  Again, that was common on PLATO, which had (62) loadable
characters.   For example, the display for PacMan could be done that
way.

	paul