1405498 - Ligature glyph [was: Font kerning] spans lines when word-wrapping

Status: RESOLVED DUPLICATE of bug 479829

(Core :: Graphics: Text, defect)

Description

[grmat]

Attachment: screenshot

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:55.0) Gecko/20100101 Firefox/55.0
Build ID: 20170918122929

Steps to reproduce:

I was reading an article (https://heise.de/-3849115). Was able to reproduce on FF55+56 on Linux and 58 (nightly) on Windows.


Actual results:

There are pixels rendered in the new line actually belonging to the last character of the previous line.
Looks like font kerning spans multiple lines when long words are wrapped, resulting in distracting pixels in front of the new line. See the attached image from the last paragraph of the liked article. The word of interest is "Beschaffungspraxis" and it looks a bit like "Beschaf-'fungspraxis"


Expected results:

I think kerning should be disabled on the position where words are wrapped around multiple lines.

Comment 1

[grmat]

Attachment: 200% zoom

Comment 2

[Emil Pasca [:emilpasca]]

User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:54.0) Gecko/20100101 Firefox/54.0

I have tested this issue on Windows 10 x64 and Mac OS 10.12 with the latest Firefox release (56.0) and the latest Nightly (58.0a1-20171005220204) and managed to reproduce the issue.
After loading the mentioned article from the provided URL (https://heise.de/-3849115), when scrolling down, you can observe that there are distracting pixels which are resulted from the split between the lines of the word "Beschaffungspraxis" .

Comment 3

[Jonathan Kew [:jfkthame]]

This isn't kerning, it's an "ff" ligature (i.e. in the font the site is using, the two "f" characters are rendered by a single combined "ff" glyph). When a line-break occurs within the range of text that is included in the ligature, we paint half of it on the first line, and half on the second.

(I don't think the issue would occur with kerning, because even if the glyphs are kerned such that they overlap significantly, there's still a clear distinction between the glyph that belongs to the character before the line-break and the glyph that belongs to the character after the break. And so we can paint only the appropriate glyph on each line.)

This is a known issue; I'm pretty sure we have a bug on file somewhere. The trouble is that we do text shaping before line breaking (which is necessary, because until we've done the shaping -- including handling ligatures and kerning -- we don't know the width of the text and therefore can't tell where line breaks should happen). But then after line-breaks are chosen, we don't currently re-shape the text at the break.

Comment 4

[Jonathan Kew [:jfkthame]]

Ah, there it is: see bug 479829.