[BRLTTY] Footsteps towards better accessibility in Linux

[Aura Kelloniemi]

Hi Nicolas and the list,

On 2025-04-10 at 17:40 -0400, Nicolas Pitre <nico at fluxnic.net> wrote:
 > On Wed, 9 Apr 2025, Nicolas Pitre wrote:

 > > On Fri, 4 Apr 2025, Aura Kelloniemi wrote:
 > > 
 > > > The main problem is that there are not enough function key entries in the
 > > > kernel to have a binding for all possible combinations of regular keys and
 > > > modifiers. And because people have different needs (some may want to have
 > > > Shift-F1, while others would prefer Shift-Ctrl-Alt-Tab) there will be no
 > > > universal solution until this limitation is lifted.
 > > 
 > > I'll try to have a look eventually.

Thank you very much!

 > Well, just for the function strings alone (excluding standard letter 
 > mappings) the limit is 256 strings. The default table uses 28 string 
 > entries only.

Yes, exactly.

 > Is this _really_ a limiting factor?

It is not, if everybody comes up with their custom solution, but it is, if we
try to define all combinations supported by xterm and other compatible
emulators. We cannot know what combinations are necessary and for whom.

Let's make a very rough calculation:

PC105 keyboard has 12 F-function keys, 4 arrow keys, DEL, INS, HOME, END,
PGUP, PGDN, space, backspace, TAB, enter, ESC, pause, and optionally 17 keys in the
NumPad. NumPad generally does not count, but some programs, like emacs,
support distinguishing between numpad keys from the other ones on the
keyboard. Together this makes 28 or 45 keys.

Plain variants of these keys are already defined. For some keys there is
already a binding, like CTRL-space and ALT-TAB. For this rough calculation I
ignore the fact that some modifier+key combinations are already defined as
those are very few.

We need to define the following variants: SHIFT, CTRL, CTRL+SHIFT, ALT,
ALT+SHIFT, ALT+CTRL, ALT+CTRL+SHIFT; so seven variants in total.

So, to support all function key combinations we need either 196 or 315
combinations.

But this is not enough, we need CTRL+SHIFT, ALT+SHIFT, ALT+CTRL+SHIFT for all
letters, numbers and symbols. This makes about 3 * 49 combinations = 147
combinations.

Then we would need CTRL+number combinations, but I think this calculation
already makes my point, even though it is not exact.

But I still stick to my opinion that it would be better to support these
directly, not through function string definitions, as these are supposed to be
always the same, not something that every user wants to customize.

-- 
Aura