Re: Starange behavior of bkgd when called after bkgdset

[Bill Gray]

On 6/25/22 17:41, Thomas Dickey wrote:
> On Sat, Jun 25, 2022 at 05:06:54PM -0400, Thomas Dickey wrote:
> > On Sat, Jun 25, 2022 at 04:27:58PM -0400, Thomas Dickey wrote:
> > > On Sat, Jun 25, 2022 at 07:18:58PM +0200, Anton Vidovic wrote:
> > ...
> > > > Compare that to the second situation:
> > > >
> > > > Calling bkgdset('+') sets only the bg char of the single line (because
> > > > of the "\n" subsequent call to addstr):
> > > >
> > > > call+1+++++++++
> > > >
> > > > and the subsequent call to bkgd('-') only affects that line and the
> > > > second line and leaves the rest of the screen without any background
> > > > char:
> > > >
> > > > call-1---------
> > > > call-2---------
> >
> > Interestingly enough, Solaris (SVr4) curses does this.
> > The colors may differ (no bce), but the +'s and -'s match up.
> >
> > > sure - if I conclude that it's correct.
> >
> > yes... more documentation is needed, to explain this.
>
> Actually it's there, in the second bullet:
>
>     bkgd
>         The bkgd and wbkgd functions set the background property of the current
>         or  specified window and then apply this setting to every character po‐
>         sition in that window:
>
>         •   The rendition of every character on the screen is  changed  to  the
>             new background rendition.
>
>         •   Wherever  the former background character appears, it is changed to
>             the new background character.

   Apologies in advance if I'm being a bit thick here.  I think I grasp 
what the background _character_ ought to do,  but am unclear about how 
the background _colors_ should behave in this case where you've used 
bkgdset() and then bkgd().

   I made minor modifications to run this in ncurses 6.2,  PDCurses, 
and PDCursesMod,  and to first show 'call 0   ++++',  for reasons that 
will become clear :

https://www.projectpluto.com/temp/bkgd_t.c

   With ncurses 6.2,  running Anton's test program got me

6.2
call 0    ++++
call+1+++++++++++++++++++

   with only the 'call 1' line with a red background.  Hit a key,  and 
that becomes

6.2
call 0    ++++
call-1-------------------
call-2-------------------
(no dashes except on those two lines)

   and the entire screen now has a blue background.  Screenshots at

https://projectpluto.com/temp/before.png
https://projectpluto.com/temp/after.png

   Are you saying that the intended behavior (demonstrated in 6.3) is 
that only the call-1 and call-2 lines should have a blue background, 
with the rest of the screen left white on black?

   Trying it with PDCurses and PDCursesMod got the same 'before',  but 
the 'after' becomes :

https://projectpluto.com/temp/after_pdcm.png

   Similar to ncurses 6.2,  except only the call 1 and 2 lines change 
color,  and the '++++' for call 0 becomes '----'.  I _think_ that second 
bullet point means it should change just like that.  But it seems a 
little strange.

Thanks!          -- Bill

> I'll add a few sentences to explain this special case.
>
> > What's happening is due to this check:
> > https://github.com/ThomasDickey/ncurses-snapshots/blob/6bf016303929e7f49758b7ed27e3b255310b959f/ncurses/base/lib_bkgd.c#L222
> >
> >                 if (CharEq(*cp, old_bkgd)) {
> > #if USE_WIDEC_SUPPORT
> >                     if (!narrow) {
> >                         if (Charable(new_bkgd)) {
> >                             SetChar2(*cp, CharOf(new_char));
> >                         } else {
> >                             SetChar(*cp, L' ', AttrOf(new_char));
> >                         }
> >                         memcpy(cp->chars,
> >                                new_char.chars,
> >                                CCHARW_MAX * sizeof(cp->chars[0]));
> >                     } else
> > #endif
> >                         SetChar2(*cp, CharOf(new_char));
> >                 }
> >
> > In the logic which I adapted from Solaris, that comparison against
> > old_bkgd is checking each cell to determine if the cell used the
> > same background-character as the old value for the background-character.
> > If it did not, it leaves that untouched.
> >
> > Setting the background character to "+" without filling the screen with +'s
> > produces this special case.