Re: GNOME Shell Magnifier Advanced Settings and Testing

[Joseph Scheuhammer <clown alum mit edu>]

Hi Robert,

Thanks very much for your input!
> I saw some screenshots of a more advanced dialog for magnifier 
> settings on the GNOME Shell magnification page [1].

I assume you are talking about the " Magnifier Preferences Dialog" 
section [1].  You might also want to track the work on the proposed 3.4 
features [2].  There is a page about the preferences dialog [3], 
inversion/brightness/contrast effects [4], and focus tracking [5].

> I also read some references to a testing dialog here [2], but it seems 
> that this has been discontinued (?).

Yes, it has been discontinued even though I still use if for testing (I 
wrote it).  I'm not sure exactly when the control panel is going to be 
released, but if you can't wait, and you really want a copy of the test 
dialog, I could probably put something together for you.  Or, get the 
control panel to you before it's released.  Note, I said "probably", 
meaning I'd want to proceed with caution as I don't want to muck up your 
system accidentally.

> I have been reading through this file:
>
> /usr/share/gnome-shell/js/ui/magnifier.js
>
> and, unfortunately, I really would not know where to begin as far as 
> the settings are concerned ...

Yes, that's the source code, but it's not really the thing to read to 
find out the settings.  That's like looking for a needle in a haystack 
:-).  The preferences (the GSettings) are listed at the beginning [6] of 
the GNOME  Shell Magnifier page that you cited, although it is a 
technical description.  But, for now, take a look at the "User Features" 
section [6] and see if you can make sense of it.  I'll happily answer 
any questions you might have.

> const MOUSE_POLL_FREQUENCY = 50;
>
> So I contemplated whether I should change this value from 50 to 1. I 
> gave in and did it!
>
> I made the changes from within Unity2D, so I logged out and then 
> logged into GNOME Shell. I crossed my fingers and then hit CTRL+ALT+M, 
> the keyboard shortcut I set to activate the magnifier...and...VOILA! 
> Super smooth panning!

I'm not sure if you changed the magnifier code or something else in 
Unity2D  (or both).  I'll try changing just the magnifier.js code to use 
a poll frequency of "1" as you did to see what happens.  FYI, I'm 
developing on Fedora-16 -- no Unity there.

Secondly, in a perfect world there shouldn't be any polling at all.  The 
magnifier is polling the mouse to detect when it moves and to determine 
where it is on the screen.  There should be a way for the magnifier to 
ask to be notified when the mouse moves (a callback scenario).  Alas, 
that capacity does not exist within X11, which is locus of that information.

Thirdly, you wrote:
> How likely is it that decreasing this value from 50 to 1 will cause 
> possible instabilities? Is it set to 50 for any given reason (I am 
> sure there has to be a reason)? I ask this because it is the same way 
> with Compiz, but it seems (in either case) that the lower the Poll 
> Interval (Compiz) or Poll Frequency (GNOME shell Magnifier) the more 
> smoothly panning is in fullscreen magnification mode. 

Poll too often, and the system slows down since it's spending a too much 
time asking "where's the mouse?".  That probably explains the panning 
lag you are seeing.  Poll too infrequently and the position information 
is out of sync with the actual mouse.  A consequence is that the the 
magnified mouse cursor image is positioned incorrectly (it lags behind 
the actual mouse), and that could lead to clicking on something that you 
didn't mean to click on.  The polling frequency was set to 50 since that 
was a reasonable compromise.

But, it almost sounds like the frequency should be a user preference. I 
say "almost" since there should be a value that represents an adequate 
compromise.  And, I'm not sure what other factors are at play -- for 
example, with a faster graphics system, perhaps polling more frequently 
would not lead to any detriment.  Hmmm....

> Is there a need for testers as concerning the magnifier?

Absolutely!  Especially user testing.  The magnifier was developed based 
on a number of inputs.  The older gnome CORBA service -- "gnome-mag"  -- 
needed to be replaced.  The GNOME Shell magnifier represents a partial 
duplication of gnome-mag's functionality.  Also, others were asked for 
their input along the way.  For example, one of my co-workers is an 
occupational therapist who works with low vision clients, and I've asked 
her for guidance on numerous occasions.  Having actual users out there 
saying what they need or want is another important source.

> Thanks for taking the time to read through my tons of e-mails, ...

All I can say to that is:  "keep them coming".

Thanks, and I hope this helps.

[1] 
http://live.gnome.org/GnomeShell/Magnification#Magnifier_Preferences_Dialog
[2[ https://live.gnome.org/ThreePointThree/Features
[3] https://live.gnome.org/ThreePointThree/Features/ZoomOptionsDialog
[4] 
https://live.gnome.org/ThreePointThree/Features/LightnessBrightnessContrastEffects
[5] https://live.gnome.org/ThreePointThree/Features/FocusCaretTracking

--
;;;;joseph.


'A: After all, it isn't rocket science.'
'K: Right. It's merely computer science.'
             - J. D. Klaun -