Implementing SVG attribute "stroke-miterlimit" ( tdf#48066)

[Regina Henschel]

Hi Armin,

since yesterday I have thought about it. I now think, that I will use 
the GDI+ type 'LineJoinMiterClipped'. That makes rendering inside LO 
consistent and that is more important as interoperability with foreign 
applications. It is then still possible to implement the use of the GDI+ 
type 'LineJoinMiter' in addition later on, and use it for pptx.

It is not a problem of scaling, but the kind how the sharp corner is 
clipped is fundamental different.

If you think I should do it different, please tell me.

Next problem: When testing a curve I got bug
https://bugs.documentfoundation.org/show_bug.cgi?id=99165 Rendering of 
mitered curve changes with zoom level
That is independent of SVG, but effects the rendering of SVG too. Do you 
have a code pointer, where I should look for the reason?

The state is now, that I have finished the transport of 
fMiterMinimumAngle for Windows. The rendering is already good with 
decomposition and with anti-aliasing on as well, here on Windows. I have 
started to adapt the headers for the other OS. A MetaXYAction is still 
missing.

Kind regards
Regina

Armin Le Grand schrieb:
> Hi Regina,
>
> yes, the MiterLimits are different in different systems, but all are
> somehow specified using the angle between the two vectors involved. I
> remember to have seen some definitions, most using the same and 15 as
> default value. The ways to be compatible with MS are:
> - find a definition somewhere, mqaybe in forums or newsgroups (I have
> none, sorry)
> - reverse engineer by trying out. Probability is high that there is a
> (linear?) relationship between the values, so it might be a simple scaling
> HTH!
>
> Regards,
> Armin
>
> Am 07.04.2016 um 14:35 schrieb Regina Henschel:
>> Hi all,
>>
>> I have made some progress. But a new problem comes up.
>>
>> I have changed WinSalGraphicsImpl::drawPolyLine [3] so that it gets an
>> additional parameter fMiterMinimumAngle (same meaning as in
>> createAreaGeometryForJoin) and uses it for gdi+. The rendering in
>> edit-mode in Draw and Impress is correct then with gdi+ on Windows.
>> Even Text in SVG works out of the box. [BTW: The current rendering
>> using gdi+ is wrong, see my report bug#99102.]
>>
>> The behavior of gdi+ is described in [1]. To get the same kind of
>> behavior as in LO, when the miter limit is exceeded, the LineJoin type
>> 'LineJoinMiterClipped' has to be used.
>>
>> Being only in LibreOffice and SVG, that would be no problem. But MS
>> seems to use different defaults. The rendering in PowerPoint looks
>> like gdi+ type 'LineJoinMiter'. The specification for 'lim' in
>> ECMA-376, Part 1, chapter 20.1.8.43, has no details, and I have not
>> found details otherwhere.
>>
>> This gdi+ type 'LineJoinMiter' kind of clipping (see the image in the
>> article [1]) will be used in SVG 2 [2] too under the identifier
>> 'miter-clip'.
>>
>> So my question: Ignore interoperability with MS? If not, how to solve it?
>>
>> Kind regards
>> Regina
>>
>> [1]
>> https://msdn.microsoft.com/en-us/library/windows/desktop/ms534148%28v=vs.85%29.aspx
>>
>>
>> [2] https://www.w3.org/TR/svg-strokes/#StrokeShape
>>
>> [3]
>> diff --git a/vcl/win/gdi/gdiimpl.cxx b/vcl/win/gdi/gdiimpl.cxx
>> index 7febbe8..36ecb31 100644
>> --- a/vcl/win/gdi/gdiimpl.cxx
>> +++ b/vcl/win/gdi/gdiimpl.cxx
>> @@ -2031,7 +2031,8 @@ bool WinSalGraphicsImpl::drawPolyLine(
>>      double fTransparency,
>>      const basegfx::B2DVector& rLineWidths,
>>      basegfx::B2DLineJoin eLineJoin,
>> -    css::drawing::LineCap eLineCap)
>> +    css::drawing::LineCap eLineCap,
>> +    double fMiterMinimumAngle)
>>  {
>>      const sal_uInt32 nCount(rPolygon.count());
>>
>> @@ -2064,9 +2065,9 @@ bool WinSalGraphicsImpl::drawPolyLine(
>>              }
>>              case basegfx::B2DLineJoin::Miter :
>>              {
>> -                const Gdiplus::REAL aMiterLimit(15.0);
>> +                const Gdiplus::REAL
>> aMiterLimit(1/sin(fMiterMinimumAngle/2.0));
>> Gdiplus::DllExports::GdipSetPenMiterLimit(pTestPen, aMiterLimit);
>> -                Gdiplus::DllExports::GdipSetPenLineJoin(pTestPen,
>> Gdiplus::LineJoinMiter);
>> +                Gdiplus::DllExports::GdipSetPenLineJoin(pTestPen,
>> Gdiplus::LineJoinMiterClipped);
>>                  break;
>>              }
>>              case basegfx::B2DLineJoin::Round :
>>
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