[OpenJDK 2D-Dev] RFR: 8233006: freetype incorrectly adjusts advances when emboldening rotated glyphs

[Sergey Bylokhov]

Looks fine.

On 4/16/20 7:58 am, Philip Race wrote:
> 
> 
> On 4/16/20, 6:31 AM, Sergey Bylokhov wrote:
>> Hi, Phil.
>> I have only the question about the new comment:
>>
>> 340     // Let's not adjust the metrics of any glyph that is zero advance.
>> 341     if (slot->linearHoriAdvance == 0) {
>> 342         return;
>> 343     }
>>
>> The comments said that we do not want to adjust the metrics and return, but we already adjusted it a little bit before:
>> 335     slot->metrics.width        += extra;
>> 336     slot->metrics.height       += extra;
> 
> That is stored in metrics but it is the bounding box of the glyph image.
> So we definitely need to adjust that before we return since we widened the glyph outline.
>>
>> I do not know the exact reason to check linearHoriAdvance at line 341, but then why we skip the check of "linearVertAdvance"?
> 
> A few reasons.
> First, you'd need a font with vertical layout support (very rare) and for JDK to then actually support vertical text layout for it to matter - so not needed.
> Second, it is not clear if we'd want to do it even if we had such support. We weren't previously getting scaled
> linear horizontal advance from freetype and it was "OK". Just a tiny bit more less spaced than ideal because bold glyphs are wider.
> With vertical it is less clear to me that you'd scale the advance in the same way. So until such a day comes it is fine as it is.
> Third, freetype didn't adjust it either, just like it wasn't adjusting the horizontal case.
> 
> Even this line below .. I am not sure is used for anything ..
> +    slot->metrics.vertAdvance  += extra;
> 
> .. but freetype added it, so I did so too to be safe.
> 
> -phil.
>>
>>
>> On 4/15/20 2:00 pm, Philip Race wrote:
>>> Bug : https://bugs.openjdk.java.net/browse/JDK-8233006
>>> Webrev : http://cr.openjdk.java.net/~prr/8233006
>>>
>>> The bug here is that the freetype function for synthesising bold is not ready to handle rotation.
>>>
>>> In the process I noticed it did not adjust the advance used by the fractional metrics case,
>>> even though the outline is bolded.
>>>
>>> Also, in what seems to be a completely wrong thing to do, freetype would
>>> widen the advance of glyphs which have zero advance.
>>>
>>> So I decided that the best thing to do was to write our own.
>>> A chunk of the heavy lifting - widening the outline - is still done by freetype
>>> but there were a lot of details to get right and test.
>>>
>>> I wrote a test to visualise the problem but the actual test checks by looking
>>> at the bounding rectangle of the drawn pixels and compares its height to
>>> the declared metrics of the font, failing if they disagree by too much.
>>>
>>> Note that the code path is only exercised when synthetic bolding is needed.
>>> So real bold fonts don't test this code.
>>> Since there's not an easy way to say which fonts have real bold, I decided the
>>> test should use a BOLD version of every font on the system, which on almost
>>> all systems will test some significant number of such cases.
>>> I kept the UI for visualising as it will be useful for later debugging of failures.
>>>
>>> Also it made me notice that the case where the text was not rotated at all was
>>> drawing shorter than all the other cases.
>>> I traced this back to the fix for 8203485 which added a macro FT26Dot6ToInt
>>> and used it to get the integer advance in the unrotated, integer metrics case.
>>> The idea there wasn't completely wrong, but I don't think it was completely right either.
>>> I got rid of the macro and instead used the same FT26Dot6ToFloat macro as used
>>> in the rotation cases. So we now return the exact floating point value to the calling
>>> Java code. That then can round appropriately as it needs to. This fixed the inconsistency
>>> and the test for 8203485 still passes as do all other tests.
>>> This change will likely lead to some cases where unrotated advances now round up one pixel wider,
>>> but so far it looks correct to me. They'll be restored to something more like what they were
>>> before 8203485, since that removed rounding and added truncation instead to fix a problem
>>> with the rounding being incorrect for rotations because it could round down when it should round up.
>>> Now we just let the Java code handle it.
>>>
>>> I've run these tests on all platforms and they pass. Mac isn't using this freetype path so it is not affected
>>> but it is still good to know the tests pass there ...
>>>
>>> -phil
>>
>>


-- 
Best regards, Sergey.