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Re: [SVGT12 Comment] Comments from Andrew Girow

From: Chris Lilley <chris@w3.org>
Date: Sat, 2 Apr 2005 05:27:01 +0200
Message-ID: <1378016775.20050402052701@w3.org>
To: andrewgirow <andrewgirow@yahoo.com>
Cc: www-svg@w3.org

On Wednesday, September 1, 2004, 5:31:03 PM, Andrew Girow wrote:

AG> From: andrewgirow <andrewgirow@yahoo.com>
AG> To: SVG-Mobile@yahoogroups.com
AG> Date: Wednesday, September 1, 2004, 5:00:04 PM
AG> Subject: [SVG-Mobile] SVG Tiny 1.2 DOM Feedback and Questions

AG> While checking/ implementing SVG Tiny 1.2 DOM
AG>  I got several questions:

Hello Andrew,

The SVG Working Group thanks you for your review of the SVG Tiny 1.2 
Last Call specification draft. The group has reviewed you comments and 
has the following responses.

AG> 1. THE COLOR TRAITS
AG> In the A.2.6 Attribute and Property Access is written "Trait is the
AG> typed value (e.g., a number, not just a string) that gets assigned
AG> through an XML attribute"

AG> The set trait for color uses two functions instead of one:

AG> setRGBColorTrait [null | SVGRGBColor]
AG> setTrait(none | currentColor | inherit)

AG> I see here a problem for application programmers who are not experts
AG> in SVG. Why there are two functions for the same type?

AG> The get trait for color uses just one function: getRGBColorTrait.
AG> So, there is no way to know that the given PathElement, for example,
AG> uses the `currentColor' of the parent GroupElement.

AG> The get trait also does not allow to know that the PathElement fill
AG> attribute is inherited from the parent GroupElement.

AG> If the getRGBColorTrait returns null. What it is suppose to mean:
AG> none, currentColor or inherit?

AG> The set and get traits must be consistent. If the program sets the
AG> fill attribute to `currentColor' it must be a way later on to check
AG> if this attribute equals to the `currentColor'.
AG> As I understand the whole idea behind traits is to provide strongly
AG> typed access to SVG attributes. In a case of color we have two types
AG> instead of one!

1. The Color Traits.

The group has carefully considered different alternatives to get and set 
trait values and feels that the solution it selected (use of typed 
methods for reading and writing trait values) is the best trade-off 
considering the constraints. This is reflected in the latest work done 
on the uDOM specification and the SVG 1.2 Full Working Draft that was 
referenced from the SVG Tiny 1.2 specification that you reviewed did not 
reflect these latest changes.

With the latest changes, typed methods are used to set values of a 
specific type. So, for example, if the user wants to set a blue color on 
an element, it will use the setRGBColorTrait method with an SVGRGBColor 
value parameter. Null values are _always_ disallowed as trait values. If 
special values are needed (such as 'none', 'inherit' or 'currentColor'), 
the values are considered to be string values and set through the 
setTrait() method.

When reading a trait, the user gets the _computed_ value. In the case of 
a color, the returned value is an SVGRGBColor instance or null in case 
the computed value is 'none'. The SVG WG realizes the handling of 'none' 
is asymetrical but the various options were considered (like allowing 
null in setRGBColor or throwing an exception in getRGBColor if the value 
was none) and the group feels the current design is the most natural for 
developers which can do common operations on colors without much 
specific knowledge of SVG.


AG> 2. SVGSVGElement

AG> In the SVGSVGElement there is a new attribute that could not be found
AG> in the previous versions of SVG DOM.

AG> attribute float currentRotate; // raises (DOMException) on setting

AG> The question is how this parameter applied to the current
AG> transformation matrix?
AG> Does it go after the currentTranslate of before? What are the order
AG> of operations now for the current transform? Do we need this at all?
AG> I mean that there is no explanation why it is in the DOM now.

2. SVGSVGElement.

The documentation on currentRotate has been updated to precisely define 
what it means and its relation with the currentScale and 
currentTranslate members. The latest documentation says:


        User Agent Transforms

The DOM attributes currentScale, currentRotate and currentTranslate are 
combined to form user agent transformation which is applied at the 
outermost level on the SVG document (i.e., outside the outermost 'svg' 
element) if "magnification" is enabled (i.e., zoomAndPan attribute is 
set to "magnify"). Their values can potentialy be modified through 
user-agent specific UI. User agent transformation can be obtained by 
multiplying matrix

 [currentScale      0       currentTranslate.x]         [cos(currentRotate) -sin(currentRotate 0]
 [     0      currentScale  currentTranslate.y]  by     [sin(currentRotate) cos(currentRotate) 0]
 [     0            0               1         ]         [         0                  0         1]
 

i.e. (translate, then scale, then rotate the coordinate system). The 
reference point for scale and rotate operations is the origin (0, 0).

Please let us know if you do not find these answers satisfactory.


-- 
 Chris Lilley                    mailto:chris@w3.org
 Chair, W3C SVG Working Group
 W3C Graphics Activity Lead
Received on Saturday, 2 April 2005 03:35:25 GMT

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