bugThe FreeType Project - Bugs: bug #33215, umlaut on top of u shifts upwards

 
 

bug #33215: umlaut on top of u shifts upwards

Submitted by:  bram tassyns <bramt>
Submitted on:  Tue 03 May 2011 01:41:18 PM UTC  
 
Severity: 3 - NormalItem Group: Incorrect behaviour
Status: InvalidPrivacy: Public
Assigned to: Werner LEMBERG <wl>Open/Closed: Closed
Planned Release: None

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Thu 30 Jun 2011 05:52:16 AM UTC, comment #4:

Ok, thanks.

Anonymous
Wed 29 Jun 2011 06:31:07 PM UTC, comment #3:

Sorry for the long delay.

For this particular font, using units_per_EM is a bad choice, unfortunately.

What you experience is expected behaviour. Within the `prep' table, the code tests whether the ppem value is > 2047. If this is true, execution of bytecode within the `glyf' table is switched off (for all glyphs). I consider this a bug in the font itself, since the down-shifting of the umlaut dots should always be done, regardless of the used ppem value.

I suspect that there are more fonts which switch off certain features if the ppem value is equal or larger than the design size.

Since I need hinting at the design size's resolution by myself for the ttfautohint library, I wonder whether and how I shall provide this information (i.e., the fact that there is bytecode for a given glyph which has been deactivated by the font itself for the current resolution)...

Werner LEMBERG <wl>
Project AdministratorIn charge of this item.
Wed 29 Jun 2011 06:17:26 PM UTC, comment #2:

Bram has answered:

I did some further testing on this issue in our software. Looks like I was wrong about the used dpi. We call:

FT_Set_Pixel_Sizes( inFace, inFace->units_per_EM, inFace->units_per_EM )

and not:

FT_Set_Pixel_Sizes( inFace, 1000, 1000 )

Units_per_EM for this font is 2048. When switching between 2048 and 1000 in ftgrid the difference is visible. Could you verify that this is the expected behavior of the truetype hinting and not a bug in the hinter?

Werner LEMBERG <wl>
Project AdministratorIn charge of this item.
Sun 15 May 2011 04:50:33 AM UTC, comment #1:

If you use the ftgrid program from the current git repository, you can easily see how the umlauts jump if you either switch on or off hinting, or if you switch on or off forced autohinting – even at 300ppem or more! Thus I suspect that you are rendering either with forced autohinting or with hinting switched off.

Debugging with FontForge shows that bytecode function 41 is responsible for this effect, aligning the diaeresis vertically to a CVT value.

The `umlaut u' glyph is one of the rare examples which really need bytecode hinting for correct rendering, since the logic to shift the diaeresis gets applied after the subglyphs have been hinted. FreeType can't do anything here if autohinting is active.

I've marked this report as invalid, since I can see the jumping behaviour with current FreeType demo programs. Please reopen it if necessary.

Werner LEMBERG <wl>
Project AdministratorIn charge of this item.
Tue 03 May 2011 01:41:18 PM UTC, original submission:

Attached is a pdf file containing a textline and that same textline outlined using freetype. You'll notice that when compared to acrobat's rendering the umlaut has shifted upwards.

We use hinting and the size is <units per em> x <units per em> dpi for the hinting. It might be caused by a difference in the dpi used for hinting. But I'm not that fluent in reading truetype hinting code :).

bram tassyns <bramt>

 

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Attached Files
file #23340:  PRN57957.pdf added by bramt (39KiB - application/pdf)
file #23341:  PRN57957.ttf added by bramt (48KiB - application/octet-stream)

 

Depends on the following items: None found

Items that depend on this one: None found

 

Carbon-Copy List
  • -unavailable- added by wl (Posted a comment)
  • -unavailable- added by bramt (Submitted the item)
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    Follow 5 latest changes.

    Date Changed By Updated Field Previous Value => Replaced By
    Sun 15 May 2011 04:50:33 AM UTCwlStatusNone=>Invalid
      Assigned toNone=>wl
      Open/ClosedOpen=>Closed
    Tue 03 May 2011 01:41:18 PM UTCbramtAttached File-=>Added PRN57957.pdf, #23340
      Attached File-=>Added PRN57957.ttf, #23341

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