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Bug 3624 - On Linux/X11, dead keys don't produce valid keycode
Summary: On Linux/X11, dead keys don't produce valid keycode
Status: ASSIGNED
Alias: None
Product: SDL
Classification: Unclassified
Component: events (show other bugs)
Version: 2.0.5
Hardware: x86_64 Linux
: P2 normal
Assignee: Sam Lantinga
QA Contact: Sam Lantinga
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2017-04-07 16:00 UTC by Daniel Gibson
Modified: 2017-08-11 18:46 UTC (History)
1 user (show)

See Also:

Attachments
patch for SDL2/X11/keyboard support to fall back to default keycode if X11 couldn't map the scancode to a valid keycode (590 bytes, patch)
2017-04-07 16:00 UTC, Daniel Gibson 		Details | Diff
Translate deadkeys to their normal form (881 bytes, patch)
2017-04-07 23:53 UTC, Mickaël Thomas 		Details | Diff
Translate dead keys to their normal form (1.41 KB, patch)
2017-04-08 01:08 UTC, Mickaël Thomas 		Details | Diff
Show Obsolete (2) View All Add an attachment (proposed patch, testcase, etc.)

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Description Daniel Gibson 2017-04-07 16:00:58 UTC 
Created attachment 2715 [details]
patch for SDL2/X11/keyboard support to fall back to default keycode if X11 couldn't map the scancode to a valid keycode

Starting with 2.0.5, SDL2 finally generates key events for dead keys, which is great!

However, at least when I tested (with German keyboard, Sun dead keys, on ^ key, which is SDL_SCANCODE_GRAVE), it doesn't produce a valid keycode.
The scancode in the SDL_Keysym is SDL_SCANCODE_GRAVE, as expected, but sym is SDLK_SCANCODE_MASK.
The problem appears to be that in X11_UpdateKeymap() X11_KeyCodeToUcs4() returns 0 (which is ok so far) and X11_KeyCodeToSDLScancode() also returns 0, so that switch/case thing goes to the default case and does
  keymap[scancode] = SDL_SCANCODE_TO_KEYCODE(keyScancode);
with keyScancode == 0 => we end up with just SDLK_SCANCODE_MASK

I'd suggest checking for 0 (SDL_SCANCODE_UNKNOWN) and in that case just leave that entry in the keymap unmodified - it's initialized by SDL_GetDefaultKeymap() so it already contains a mapping that's not completely stupid: the keycode matching the scancode, in this case SDLK_BACKQUOTE, which is not really what that key is, but better than nothing.

(I think that this is the behavior I already got when I wrote the first patch attempt for dead keys not generating key events at all, must have broken afterwards somehow)

The attached patch implements this behavior.
Comment 1 Mickaël Thomas 2017-04-07 18:39:50 UTC 
I think the underlying issue could be fixed by adding the Unicode codepoints for the dead keys in video/x11/imKStoUCS.c (in a new table).

I have no idea where those table were generated from in the first place, but I could hack a python script to generate the one we need using imKStoUCS.c and /usr/include/X11/keysymdef.h

That or we could simply add another translation layer that would translate the dead key sym to their non-dead equivalent before calling into imKStoUCS.c

I'd like to hear more from the people experienced with Xlib, in case there is a better way to do this.
Comment 2 Mickaël Thomas 2017-04-07 23:53:33 UTC 
Created attachment 2718 [details]
Translate deadkeys to their normal form

My proposed method was much harder than expected since the key naming scheme isn't really consistent so I went with a simpler way of simply converting the most common dead keys. Hopefully that'll cover most cases (eg French or German keyboard, possibly others).
Comment 3 Daniel Gibson 2017-04-08 00:05:35 UTC 
Yeah, that could help with the most common western dead keys - maybe we could find translations of the other XK_dead_* constants as well?

In any case I'd suggest to also add my change as a fallback (for if the translation fails).
Comment 4 Mickaël Thomas 2017-04-08 00:42:15 UTC 
I don't think all of them are relevant though, since we should only need those having a corresponding keyboard label (not in the shift/altgr group)

I made a list of these using this:

$ echo $(grep -REo 'key\s*<[^>]+>\s*\{\s*\[\s*dead_[a-z]+' /usr/share/X11/xkb/symbols/ | grep -Eo 'dead_.*$' | sort | uniq)

dead_abovedot dead_abovering dead_acute dead_caron dead_cedilla dead_circumflex dead_diaeresis dead_grave dead_iota dead_macron dead_tilde

Extending the list up to XK_dead_iota will include all of them, I just don't know what to replace these with:
- XK_dead_abovering -> ???
- XK_dead_iota -> ???
Comment 5 Mickaël Thomas 2017-04-08 01:08:41 UTC 
Created attachment 2719 [details]
Translate dead keys to their normal form

This should do it, I found the missing codepoints (and traced them back to the matching Keysym) using this:

https://gist.github.com/mickael9/d6378c60c2e509e4d5b05ba2ecc072c9