RE: fill-rule problem ..

From: <ronan@roasp.com>
Date: Sun, 5 May 2002 18:19:40 +0200
To: "Goessner / MecXpert" <goessner@mecxpert.de>, "Craig Brown" <cmb@research.canon.com.au>
Cc: <www-svg@w3.org>
Message-ID: <EMENJCDHGFFGOCDJOBLMMEFPDGAA.ronan@roasp.com>
```Please excuse my interjection on this thread. I've been loosely following it
for a while and have had a general question about it in mind since the
beginnnig...

Pardon my naivety on the subject, but I remember in mathematics at
university that this was all well covered using the property of positiveness
of surfaces or faces.
(Please pardon my sketchy memory, I might have this 100% backwards)...

On a plane, the direction of the line (with respect to the normal of the
plane) defining a boundary between two sides of closed boundaries cab ne
defined as a positive (inside) boundary or negative (outside) boundary
depending on whether the line is clockwise or counterclockwise the enclosed
region.

IE
to define a donut, you drew a cw outer circle and a ccw inner circle. This
defines the region between the two faces as 'inside' that boundary pair. If
the directions were reversed, then the 'inside' the entire plane except the
region between the two curves, which was outside.

With this absolute definition of inside and outside, you never have to worry

I realize we have a partial implementation of this in paths but could this
not be generalized for all entities?

I'm confused why SVG did not go with such a model in the first place.

Ronan

§-----Original Message-----
§From: www-svg-request@w3.org [mailto:www-svg-request@w3.org]On Behalf Of
§Goessner / MecXpert
§Sent: Sunday, May 05, 2002 4:49 PM
§To: Craig Brown
§Cc: www-svg@w3.org
§Subject: Re: fill-rule problem ..
§
§
§hi craig,
§
§>
§> The problem with all this stuff is the implementation.
§>
§> Imaging the situation where you have a U...
§>
§> Do you fill inside the U?
§>
§> If the answer is NO - think about the case where the top of the U touch.
§> Now - do you fill?  I assume the answer for a proposed "always" fill is
§> YES.
§>
§> Glitch is now you can't do scan-line rendering.  A scanline at the top of
§> the U effects the result for a scanline in the middle of the U.
§
§.. hmm .. i believe i understand.
§with scan-line rendering, i.e. drawing a ray from any point on the
§canvas to
§infinity in any direction and counting the number of path segments that the
§ray crosses, we cannot implement an fill-rule="always".
§
§> In reality - it is usually very easy to modify the content to get the
§effect
§> required rather than adding 2d complexity to filling.
§
§so an ever working method would be, to draw the U (or any open shape) *and*
§draw it reverse again back to its subpath initial point. then - with both
§rules 'evenodd' and 'nonzero' - the cross-counting works reliable,
§since the
§segments were counted twice.
§
§based on these thoughts following algorithm should be quite easy to
§implement:
§
§1) don't close open subpathes automatically.
§2) count closed subpath segments once and open subpath segments
§twice - with
§different directions.
§
§what do you think?
§--
§stefan goessner
§
§> ...cmb
§>
§> --Original Message--:
§> >It occurred to me that it might be useful to be able to specify
§more than
§> >one color for fill, to alternate the color according to different
§> >fill-rules. For instance:
§> >
§> ><polygon points="100,20 165,196 26,87 176,87 42,198 "
§style='stroke:blue;
§> >fill:yellow,blue; fill-rule:evenodd;'/>
§> >
§> >would give a star with yellow arms and a blue center.
§> >
§> >This might also be used to completly fill an enclosed area, by
§specifying
§> >the same color twice.
§> >
§> >Just an idea-
§> >-Doug
§> >
§> >
§> >> -----Original Message-----
§> >> From: www-svg-request@w3.org [mailto:www-svg-request@w3.org]On Behalf
§Of
§> >> Goessner / MecXpert
§> >> Sent: Friday, May 03, 2002 4:46 AM
§> >> To: www-svg@w3.org
§> >> Subject: Re: fill-rule problem ..
§> >>
§> >>
§> >> A solution to this problem might be an additional fill-rule, perhaps
§> >>
§> >> fill-rule="always"
§> >>
§> >> --
§> >> stefan
§> >>
§> >>
§> >>
§> >> > i simplified the path to a square with a "V" in it.
§> >> >
§> >> > <svg width="300" height="200">
§> >> >   <path id="A" transform="translate(20,20)" stroke-width="2"
§> >> stroke="blue"
§> >> > fill="lightgray" fill-rule="nonzero"
§> >> >         d="M0,0 H100 V100 H0 Z M10,50 50,90 90,50" />  <!--
§clockwise
§/
§> >> > counterclockwise -->
§> >> >   <path id="B" transform="translate(140,20)" stroke-width="2"
§> >> stroke="blue"
§> >> > fill="lightgray" fill-rule="nonzero"
§> >> >         d="M0,0 H100 V100 H0 Z M90,50 50,90 10,50" />  <!--
§clockwise
§/
§> >> > clockwise -->
§> >> > </svg>
§> >> >
§> >> > 1) if a (sub)path is not closed, an invisible closepath from the
§> >> (sub)path's
§> >> > current point to its initial point is used for filling.
§> >> > 2) if one path contains the other completely the fill-rule
§determines
§if
§> >> the
§> >> > subpath has to be filled or not.
§> >> > 3) fill-rule="nonzero" (the default) will
§> >> >    a) fill the inner subpath, if it has the same (angular) direction
§as
§> >> the
§> >> > outer subpath.
§> >> >    b) not fill the the inner subpath, if its angular direction is
§> >> different
§> >> > to the outer subpath's.
§> >> > 4) fill-rule="evenodd" will never fill the inner subpath
§> >> >
§> >> >
§> >> > it is conform to the SVG 1.1 spec, that there is *no way*
§to fill the
§> >> square
§> >> > in path (id="A") completely.
§> >> > the path can be modified (id="B")to match point 3a) above. But
§> >> in case of
§> >> an
§> >> > "N"-shaped subpath instead of a "V" that wouldn't help either.
§> >> > --
§> >> > stefan
§> >> >
§> >> >
§> >> > ----- Original Message -----
§> >> > From: "Doug Schepers" <doug@schepers.cc>
§> >> > To: "Goessner / MecXpert" <goessner@mecxpert.de>
§> >> > Sent: Wednesday, May 01, 2002 2:24 PM
§> >> > Subject: Re: fill-rule problem ..
§> >> >
§> >> >
§> >> > > Could this be a bug in ASV3?  I got it to render as solid once,
§after
§> >> > > refreshing a couple of times, and once got a blue artifact
§> >> line straight
§> >> > > from (50,50) to (90,50). [WinMe/IE6/ASV3]
§> >> > >
§> >> > > What is the logic behind it not filling the square with gray? No
§lines
§> >> are
§> >> > > being crossed. Do I misunderstand the fill rule?
§> >> > >
§> >> > > -Doug
§> >> > >
§> >> > > ----- Original Message -----
§> >> > > From: "Goessner / MecXpert" <goessner@mecxpert.de>
§> >> > > To: <www-svg@w3.org>
§> >> > > Sent: Wednesday, May 01, 2002 5:48 AM
§> >> > > Subject: fill-rule problem ..
§> >> > >
§> >> > >
§> >> > > > i have a simple square with two quadratic bezier segments
§> >> in it. it is
§> >> > > easy
§> >> > > > to define this by a single path element.
§> >> > > >
§> >> > > > <svg>
§> >> > > >   <path transform="translate(20,20)" stroke-width="2"
§stroke="blue"
§> >> > > > fill="lightgray"
§> >> > > >         d="M0,0 H100 V100 H0 Z M10,50 Q30,30 50,50 T90,50" />
§> >> > > > </svg>
§> >> > > >
§> >> > > > but i cannot find a fill-rule to completely fill the
§> >> square. even the
§> >> > new
§> >> > > > winding-count rule wouldn't work.
§> >> > > >
§> >> > > > --
§> >> > > > stefan goessner
§> >> > > >
§> >> > > >
§> >> > >
§> >> >
§> >>
§> >>
§> >
§> >
§> >
§> >
§> --
§> Craig Brown      Principal Software Engineer
§> Canon Information Systems Research Australia     Phone: 61 2 9805 2649
§> 1 Thomas Holt Drive,  North Ryde,  NSW 2113.     Fax:   61 2 9805 2929
§>
§
§
```
Received on Sunday, 5 May 2002 12:20:49 UTC

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