W3C home > Mailing lists > Public > www-svg@w3.org > January 2010

Re: Tangent Angle At A Path Point

From: Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>
Date: Thu, 21 Jan 2010 16:47:44 +0100
To: www-svg@w3.org, fhemsher@gmail.com
Message-Id: <201001211647.44544.Dr.O.Hoffmann@gmx.de>
I calculated such tangents manually and tested, if they exist.
While tangent(fraction of length) requires some numerics in
general, using the usual parametrisations for path fragments
it is in most cases simple to calculate tangent(path parameter).

However in some cases there is no tangent (SVG has specific rules
for this) - and in my tests it turns out, that all or most viewers have
problems to determine correctly, that there really is or is not a tangent, 
therefore it could be helpful as well to have some implementation 
note with details on how to determine the tangent in such non 
trivial cases. In these cases the path has a tangent, but this cannot 
be determined directly with the common parametrisation, this need 
some more mathematics.
To care about this could improve the quality of implementations
at all already for those cases, where the tangents are already
needed in the current specifications - alignment of markers
and stroke-linecap.

Currently there is one tricky option to get a graphical representation 
of tangent(fraction of length) by using animateMotion and keyPoints 
and related timing (there needs to be no visible motion to get it).
to put the tangent object at the intended position of a path with proper 
alignment, without knowing the position and aligment itself 
(what does not necessarily mean,  that the alignment or position of
objects with animateMotion is directly exposed into the DOM - is this
available? (DOM is not my field  of experiments) if yes, this could be 
the current tricky workaround). And if it is possible to align it without
knowing position and aligment - maybe this informations is not needed
at all for many applications.
And this does not mean, that viewers are doing it correctly for such
problematic and exotic cases I can determine.

Olaf
Received on Thursday, 21 January 2010 15:51:47 GMT

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