[osg-users] osgText inconsistant size

[Robert Osfield]

Hi Romain,

Thanks for investigating and characterising this bug.  With your
results in mind I've just done a code review of FreeTypeFont.cpp and
Font.cpp and see that design/implementation both assume that kerning
is invariant of font resolution, which you have found it isn't, so
this is most definitely a flaw.

>From my current review the correct way to solve this would seem to be
modify the Font::getKerning(..) and
FontImplementation/FreeTypeFont::getKerning(..) methods to take a
FontResolulution parameter in the same way that the getGlyph(..)
method does, this would allow the kerning implementation to reset the
font resolution if required as it does during the getGlyph().

Unfortunately such a change would break the API so isn't something we
can back port to OSG-3.2 or OSG-3.4, but is something we could merge
with OSG-master/OSG-3.5.x dev series.

Thoughts?
Robert.

On 12 January 2016 at 16:44, Romain Ouabdelkader
<romain.ouabdelkader at gmail.com> wrote:
> Hi,
>
> I have tracked down the issue, the difference in size come from the kerning
> being calculated in different font resolutions.
>
> If we trace the execution:
>
>   osg::ref_ptr<osgText::Text> text0 = new osgText::Text;
>   text0->setFont(font_path);
>   text0->setFontResolution(128, 128);
>   text0->setText("V");
>   text0->setText("a");
>
> Here we generate the glyph for char 'V' and 'a' with the resolution (128,
> 128).
>
>   osg::ref_ptr<osgText::Text> text1 = new osgText::Text;
>   text1->setFont(font_path);
>   text1->setText("p");
>
> We generate a dummy glyph that set FreeType face resolution to (32, 32)
>
>   text0->setText("Va");
>   float first_call = text0->getBoundingBox().xMax();
>
> (1) Since the glyph for char 'V' and 'a' are already generated for
> resolution (128, 128), we use the glyph in the cache (see Font::GetGlyph)
> *without modifying FreeType face resolution*.
> We also get kerning from FreeType but we *don't* change the face resolution,
> thus the kerning is calculated in (32, 32) resolution.
>
>   text0->setText("c");
>
> We generate a dummy glyph (that isn't in the cache) that set FreeType face
> resolution to (128, 128).
>
>   text0->setText("Va");
>   float second_call = text0->getBoundingBox().xMax();
>
> Same as (1) but FreeType face resolution is (128, 128) while calculating the
> kerning.
>
>
>
> This isn't a big issue, but it breaks our tests depending of the order of
> execution...
>
> The issue can be resolved if Font::getKerning ask for font resolution, or if
> we set font resolution at the start of Text::computeGlyphRepresentation (not
> exposed in Font).
>
> What do you think ?
>
> Regards,
> Romain Ouabdelkader.
>
>
> 2016-01-06 19:16 GMT+01:00 Romain Ouabdelkader
> <romain.ouabdelkader at gmail.com>:
>>
>> Hi,
>>
>> I have discovered an issue with osgText: under certain conditions,
>> Text::getBoundingBox() returns different values with the same text, font,
>> font resolution, etc.
>>
>> You can reproduce the issue with this example:
>>
>> int main() {
>>   const char *font_path = "museo500.ttf";
>>
>>   osg::ref_ptr<osgText::Text> text0 = new osgText::Text;
>>   text0->setFont(font_path);
>>   text0->setFontResolution(128, 128);
>>   text0->setText("V");
>>   text0->setText("a");
>>
>>   osg::ref_ptr<osgText::Text> text1 = new osgText::Text;
>>   text1->setFont(font_path);
>>   text1->setText("p");
>>
>>   text0->setText("Va");
>>   float first_call = text0->getBoundingBox().xMax();
>>
>>   text0->setText("c");
>>
>>   text0->setText("Va");
>>   float second_call = text0->getBoundingBox().xMax();
>>
>>   std::cout << first_call << std::endl;
>>   std::cout << second_call << std::endl;
>>   if (first_call != second_call) {
>>     std::cout << "Error!" << std::endl;
>>     return 1;
>>   }
>>
>>   return 0;
>> }
>>
>> Output:
>> 36.4961
>> 35.9961
>> Error!
>>
>> The issue seems to be very specific, if I remove any of these lines the
>> issue doesn't appear.
>>
>> I've attached the font if you want to reproduce it.
>>
>> Regards,
>> Romain Ouabdelkader.
>>
>
>
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