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RE: [SVG12] Issues regarding stroke-dasharray

From: Jon Ferraiolo <jonf@adobe.com>
Date: Mon, 23 Jan 2006 22:31:23 -0800
Message-ID: <6ECA24BE410D994496A2AE995367C5C8642D7B@namail3.corp.adobe.com>
To: "Rick" <graham.rick@gmail.com>, <www-svg@w3.org>


Hiya Rick!
Your suggestion sounds good to me - the SVG spec should state how dash
patterns are supposed to work on shapes such as circles and rectangles.
I am not speaking officially for the working group, but my expectation
is that the working group will agree with you and add appropriate
language to the spec. The most natural thing would be to state in the
description of each of the basic shapes how the dash patterns in
stroke-dasharray must be applied. We have "path equivalents" in the spec
for rect, line, polyline and polygon, which seems like the obvious
method for those elements, but I agree that we need to come up with
non-ambiguous rules for circle and ellipse. (I would think you would
start at zero degrees and sweep in a mathematically "positive" rotate
direction, where "positive" is treated in the same manner as SVG's
transformations.)

Jon


-----Original Message-----
From: www-svg-request@w3.org [mailto:www-svg-request@w3.org] On Behalf
Of Rick
Sent: Sunday, January 22, 2006 8:03 PM
To: www-svg@w3.org
Subject: [SVG12] Issues regarding stroke-dasharray


Dear SVG working group,

Thank you for your hard work in resolving issues on this CR.

This is an issue that I've been aware of this for quite some time, but
had forgotten.  Please forgive me for leaving it to this late date to
bring it up.

Dash arrays can have a dramatic effect on the look of an image,
particularly if they are large.  I'm currently working on a technical
proposal that involves the use of SVG and have some compatibility
problems between major viewers over this feature.

The description of dasharray
http://www.w3.org/TR/SVG/painting.html#StrokeProperties does not
specify in what manner the effect should be applied.  This is
particularly esoteric with closed shapes such as rect or circle
because they have no intrinsic starting point or direction.  The spec
does outline how these objects are to be described, but it does not
specify that dashed lines must follow these rules.  There are
inconsistencies in current implementations in this respect that
produce drastically different results for some markup.

I would suggest appending wording similar to the following to the
description of dasharray to assist implementers with this feature:

 ---
The dash sequence must start at the beginning of a shapes data and
follow the stroke description through to the end of the shape.  For
shapes that do not have obvious starting points, the method for
rendering them is specified in the basic shapes section of this
specification.  <link>
  ---

Please revise the specification to clarify this.

Further to this issue, I believe that the methods for describing of
circles and ellipses needs further clarification.

http://www.w3.org/TR/SVG/shapes.html#CircleElement states:

 ---
The arc of a 'circle'  element begins at the "3 o'clock" point on the
radius and progresses towards the "9 o'clock" point. The starting
point and direction of the arc are affected by the user space
transform in the same manner as the geometry of the element.
 ---

It is unclear whether, unaffected by any transformation, that the
direction of these shapes should be clockwise or counterclockwise.  3
o'clock and 9 o'clock are 180 degrees from each other.

I would suggest changing the wording so that 9 o'clock be changed to
either 12 o'clock or 6 o'clock, or that a CW or CCW direction be
specified.

Please revise the descriptions of the circle and ellipse elements to
clarify this.

--
Cheers!
Rick
Received on Tuesday, 24 January 2006 06:32:18 GMT

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