[HarfBuzz] Backspace and Delete behaviours

Rajeev J Sebastian rajeev.sebastian at gmail.com
Sat Jul 23 09:06:37 PDT 2011

Delete erases logical clusters, whereas backspace erases logical characters.

We don't see any discrepancy in how ICU/Openoffice does deletion (not
yet anyway).

Rajeev J Sebastian

On Thu, Jul 21, 2011 at 11:57 AM, pravin.d.s at gmail.com
<pravin.d.s at gmail.com> wrote:
> On 21 July 2011 11:27, Harshula <harshula at gmail.com> wrote:
>> Hi Shriramana,
>> On Thu, 2011-07-21 at 09:27 +0530, Shriramana Sharma wrote:
>> > On 07/21/2011 09:04 AM, Pravin Satpute wrote:
>> > > 1) In pango we have faced problem of backspace due to decomposition.
>> > > i.e. U+0958  was getting decomposed into U+0915 and U+093C, so for
>> > > entered character was required 2 backspaces to delete.
>> >
>> > That doesn't sound right. The backspace should erase the last character
>> > that was input. Rendering engines should only take care of rendering
>> > whatever sequence of characters has been input and not change the
>> > sequence of characters right?
>> That's a bit simplistic. When a smart input method engine (IME) is
>> activated, it can make sure that a backspace erases only the part of the
>> glyph that the last keystroke inserted. e.g. It takes four keystrokes to
>> generate කෝ (Sinhala syllable) with a particular IM. Another IM requires
>> only two keystrokes. Each of these IMs controls how the backspace key
>> operates and only deletes the part of the glyph inserted by the last
>> keystroke.
>> However, when the smart IME is deactivated, backspace then becomes
>> someone else's responsibility. Here we see behaviour differences. e.g.
>> For කෝ , Pango/gedit deletes in decomposed form (press backspace 4
>> times) and ICU/OpenOffice deletes in composed form (press backspace 2
>> times).
>> Perhaps, there should be a separate discussion on which behaviour should
>> be the default?
>> Also, IIRC, Pango had a separate module that handled the cursor
>> movement. I remember having to modify it for Sinhala.
> Looks like we are going off topic little bit so changed subject line, but i
> would like to ask one thing
> is there any standard define for Backspace and Delete keys behaviour? I
> think Unicode has given this but i have not find it yet
> anyone knows anything?
> Regards,
> Pravin S
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