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Re: SVG 1.1 Spec Rec - 7.6 The Transform Attribute

From: <pegan@gresham-forms.co.uk>
Date: Wed, 26 Jan 2005 16:43:01 -0000
Message-ID: <41F7C895.70605@gresham-forms.co.uk>
To: Thomas.DeWeese@Kodak.com
Cc: ianbrown@istar.ca, www-svg@w3.org
Hi Thomas, Ian,

I think I started this.
Back in December of 2004 I read a web tutorial on SVG ( 
http://www.perfectxml.com/LearnSVG.asp )
and found the *Concatenation of Transformations* section confusing
eg.
    "The elementary transformations of the transform attribute are 
evaluated from *right* to *left*".

I have since been trying to de-confuse myself. 

You  might be able to determine whether I am still confused :-
What I did not know was that at one level transformation matrices were 
indeed being evaluated right to left and at the same time at a different 
conceptual level the evaluation of the transform attribute content was 
left to right (order provided).

I think that the SVG 1.1 spec 7.6 The Transform Attribute explains 
things clearly, correctly and at an appropriate level, and the user 
agents that I am familiar with  (Batik, ASV6) have implemented the spec 
correctly.

Thomas, your explanation makes perfect sense to me.

Thanks,
Patrick

Thomas DeWeese wrote:

>
> Hi Ian,
>
>     The basic problem is that the transform attribute describes the
> transformation from one coordinate system to another not the
> transformation of the geometry (this would be problematic as it
> wouldn't work for groups).
>
>     I'll go through your example using the coordinate system not
> the geometry.
>
> Ian Brown wrote:
>
> > Take a rectangle at (0,0) of width and height 30.
>
>     Take an inital coordinate system (C1)
>
> > Translate it to (200, 0)
> > We then have a rectangle at (200, 0) of width and height 30.
>
>     Create a new coordinate system (C2) translated by 200,0.  We now
> have a coordinate system with it's origin at location 200,0 in
> the original coordinate system (C1).
>
> > Now rotate that rectangle by 45 degrees (about 0,0).
>
>     Now create another new coordinate system (C3) by rotating the
> cooridinate system (C2) by 45 degrees around it's origin, which is
> at location 200,0 in the original coordinate system (C1).
>
> >(remember the rectangle now has an upper left at 200,0 and lower 
> right at 230,30)
> > What do we expect? We expect the upper left and upper right points of
> > the square to lie along the 45 diagonal passing through 0,0
> > i.e. the rotation is about 0,0 not 200,0. The upper left conner will be
> > a distance of 200 from 0,0.
>
>     Now define a rectangle in the final coordinate system (C3), such
> that it starts at 0,0 and has a width/height of 30.
>
> > That is not what Adobe SVG or Amaya does.
>
>     The above is what Adobe, Amaya, Batik, and Mozilla do.
> Please see the attached SVG that illustrates the coordinate
> systems involved.
>
> > The spec should not be compromised by the implementation.
> > Interpreting the spec according to "matrix order rules" (backwards to
> > English?) is interpreting the spec based on using a matrix based
> > implementation. This is highly confusing, and inappropriate.
> > Further since the matrix order for pre-concatenation is opposite to
> > post-concatenation the spec would have specify pre or post rules.
>
>     The problem is that you are trying to apply the transformation
> to the geometry not to the coordinate system.  The transform attribute
> defines the coordinate system that the rectangles coordinates are
> evaluated in it does not define a transformation of the rectangle.
>
> ----
>
> <svg width="400" height="400"
>       xmlns:xlink="http://www.w3.org/1999/xlink"
>       xmlns="http://www.w3.org/2000/svg"
>       viewBox="-20 -20 390 390">
>
>     <symbol id="axis" stroke-width="2"
>             viewBox="0 0 100 100" overflow="visible"
>             stroke-linecap="round" >
>            <line x1="0" y1="0" x2="90" y2="0"/>
>            <line x1="0" y1="0" x2="0"  y2="90"/>
>            <path d="M0 100 l3-10 h-6z" stroke="none"/>
>            <path d="M100 0 l-10,3 v-6z" stroke="none"/>
>     </symbol>
>
>     <g stroke="black" fill="black">
>        <use x="0" y="0" width="300" height="300"
>             xlink:href="#axis"/>
>        <text x="10" y="30" font-size="20" stroke="none">C1</text>
>     </g>
>
>     <g transform="translate(200, 0)" stroke="blue" fill="blue">
>        <use x="0" y="0" width="300" height="300"
>             xlink:href="#axis"/>
>        <text x="50" y="30" font-size="20" stroke="none">C2</text>
>     </g>
>
>     <g transform="translate(200, 0) rotate(45)"
>        stroke="green" fill="green">
>        <use x="0" y="0" width="300" height="300"
>             xlink:href="#axis"/>
>        <text x="50" y="30" font-size="20" stroke="none">C3</text>
>
>        <rect x="0" y="0" width="30" height="30"
>              fill="purple" fill-opacity="0.5"/>
>     </g>
> </svg>
>



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Received on Wednesday, 26 January 2005 16:43:26 GMT

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