13712 - [PP] Small differences in font metrics between platforms

Status: VERIFIED FIXED

(Core :: CSS Parsing and Computation, defect, P3)

Description

[Pierre Saslawsky]

This bug has been detected while fixing #11862. To reproduce:
- Open the following link on a Win32 AppRunner:
  http://bugzilla.mozilla.org/showattachment.cgi?attach_id=1237
- Open the attached jpg image on the same machine. This jpg has been captured on
a Macintosh after #11862 had been fixed
==> Look how the red boxes have a slightly different height between platforms. It
looks like the Mac is just one pixel smaller on each box.

In fact, unlike what I wrote in the summary line, I'm not absolutely sure that
the problem comes from the font metrics. It's most likely a problem with the font
ascent but it may also come from somewhere else. For instance, I know that at
some point, and because of differences between toolboxes on the different
platforms, the Mac wasn't drawing the exact same squares and polygons as Windows.

Comment 1

[Pierre Saslawsky]

Attachment: Image captured on Mac

Comment 2

[Pierre Saslawsky]

Changing component to Style System

Comment 3

[Pierre Saslawsky]

Setting QA Contact to <[e-mail]>, like on #11862

Comment 4

[Pierre Saslawsky]

I'm having a second thought about it and I'm wondering now whether this is really
a bug. CSS allows us to have a much finer control over the display but are
we supposed to get the exact same display across platforms? After all, maybe the
fonts we are using on Mac and Win32 have a one pixel difference somewhere.

CCing peterl.

Comment 5

[Peter Linss]

No, nobody should expect the EXACT same font metrics accross platforms. The
fonts themselves will vary by platform, vendor, platform, and font rendering
technology. A 1 pixel difference here and there is expected. Especially dealing
with font-specific metrics like 'ex' units.

Now, 'em' units are currently defined as being the 'computed value' of the font
size, not the 'actual value', so they should be consistent accross platforms, as
should units like inches. Note that the ratio of "real world" units to pixels
will vary, be sure that is taken into account.

This issue however, looks like it may be due to rounding errors in the rendering
contexts.

Comment 6

[dcone (gone)]

Alot of rounding errors were fixed.. I think all of them.

Comment 7

[Christine Hoffman]

Based on comments in bug, verifying this bug fixed. Please reopen if you
disagree.